Twitter, Inc. v. Taamneh

(Slip Opinion) OCTOBER TERM, 2022 1
Syllabus
NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
being done in connection with this case, at the time the opinion is issued.
The syllabus constitutes no part of the opinion of the Court but has been
prepared by the Reporter of Decisions for the convenience of the reader.
See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.
SUPREME COURT OF THE UNITED STATES
Syllabus
TWITTER, INC. v. TAAMNEH ET AL.
CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR
THE NINTH CIRCUIT
No. 21–1496. Argued February 22, 2023—Decided May 18, 2023
In 2017, Abdulkadir Masharipov carried out a terrorist attack on the
Reina nightclub in Istanbul, Turkey, on behalf of the Islamic State of
Iraq and Syria (ISIS), a designated Foreign Terrorist Organization.
Masharipov killed Nawras Alassaf and 38 others. Alassaf’s family
then brought this suit under 18 U. S. C. §2333, an Antiterrorism Act
(ATA) provision that permits U. S. nationals who have been “injured
. . . by reason of an act of international terrorism” to file a civil suit for
damages. Instead of suing ISIS directly under §2333(a), the plaintiffs
(respondents here) invoked §2333(d)(2) to sue three of the largest so-
cial-media companies in the world—Facebook, Twitter (petitioner
here), and Google (which owns YouTube)—for aiding and abetting
ISIS.
The parties today agree on the basic aspects of these platforms: Bil-
lions of people from around the world have signed up for them and
upload massive amounts of content each day. Defendants profit from
that content by placing advertisements on or near it and use “recom-
mendation” algorithms that match content, advertisements, and users
based on information about the use, advertisement, and content being
viewed. As the parties represent things, the algorithms here match
any content with any user who is more likely to view that content, and
the platforms perform little to no front-end screening on any content
before it is uploaded.
Plaintiffs, however, allege that for several years the companies have
knowingly allowed ISIS and its supporters to use their platforms and
“recommendation” algorithms as tools for recruiting, fundraising, and
spreading propaganda; plaintiffs further allege that these companies
have, in the process, profited from the advertisements placed on ISIS’
tweets, posts, and videos. The District Court dismissed the complaint
2 TWITTER, INC. v. TAAMNEH
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for failure to state a claim, but the Ninth Circuit reversed.
Held: Plaintiffs’ allegations that these social-media companies aided and
abetted ISIS in its terrorist attack on the Reina nightclub fail to state
a claim under 18 U. S. C. §2333(d)(2). Pp. 6–31.
(a) In 2016, Congress enacted the Justice Against Sponsors of Ter-
rorism Act (JASTA) to impose secondary civil liability on anyone “who
aids and abets, by knowingly providing substantial assistance, or who
conspires with the person who committed such an act of international
terrorism.” §2333(d)(2). The question here is whether the conduct of
the social-media company defendants gives rise to aiding-and-abetting
liability for the Reina nightclub attack. Pp. 6–8.
(b) The text of JASTA begs two questions: What does it mean to “aid
and abet”? And, what precisely must the defendant have “aided and
abetted”? Pp. 8–21.
(1) Nothing in the statute defines any of the critical terms in the
phrase “aids and abets, by knowingly providing substantial assis-
tance.” The term “aids and abets,” however, is a familiar common-law
term and thus presumably “brings the old soil” with it. Sekhar v.
United States, 570 U. S. 729, 733. Congress also provided additional
context in JASTA by pointing to Halberstam v. Welch, 705 F. 2d 472,
as “provid[ing] the proper legal framework” for “civil aiding and abet-
ting and conspiracy liability.” 130 Stat. 852. Halberstam’s legal
framework, viewed in context of the common-law tradition from which
it arose, confirms that “aids and abets” in §2333(d)(2) refers to a con-
scious, voluntary, and culpable participation in another’s wrongdoing.
Pp. 9–17.
(i) In Halberstam, the D. C. Circuit undertook an extensive sur-
vey of the common law with respect to aiding and abetting and synthe-
sized the surveyed cases as resting on three main elements: (1) there
must be a wrongful act causing an injury performed by the person
whom the defendant aided; (2) at the time assistance was provided,
the defendant must have been “generally aware of his role as part of
an overall illegal or tortious activity;” and (3) the defendant must have
“knowingly and substantially assist[ed] the principal violation.” 705
F. 2d, at 477. The court then articulated six factors to help determine
whether a defendant’s assistance was “substantial.” They are (1) “the
nature of the act assisted,” (2) the “amount of assistance” provided, (3)
whether the defendant was “present at the time” of the principal tort,
(4) the defendant’s “relation to the tortious actor,” (5) the “defendant’s
state of mind,” and (6) the “duration of the assistance” given. Id., at
488 (emphasis deleted). Halberstam also clarified that those who aid
and abet “a tortious act may be liable” not only for the act itself but
also “for other reasonably foreseeable acts done in connection with it.”
Id., at 484. Finally, the court warned that its formulations should “not
Cite as: 598 U. S. ____ (2023) 3
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be accepted as immutable components” but should be “adapted as new
cases test their usefulness in evaluating vicarious liability.” Id., at
489. Pp. 9–11.
(ii) Because the allegations here—involving international ter-
rorist networks and world-spanning internet platforms—are a far cry
from the facts of Halberstam, the Court must ascertain the basic thrust
of Halberstam’s elements to determine how to adapt them to the facts
of this case. To do so, the Court turns to the common law of aiding and
abetting upon which Halberstam rested, and to which JASTA’s com-
mon-law terminology points. At common law, the basic “view of culpa-
bility” animating aiding and abetting liability is that “a person may be
responsible for a crime he has not personally carried out if he helps
another to complete its commission.” Rosemond v. United States, 572
U. S. 65, 70. However, the concept of “helping” in the commission of a
crime or a tort has never been boundless and ordinarily requires some
level of blameworthy conduct; those limits ensure that aiding and abet-
ting does not sweep in mere passive bystanders or those who, for ex-
ample, simply deliver mail that happens to aid criminals. In tort law,
many cases have thus required a voluntary, conscious, and culpable
participation in the wrongful conduct to establish aiding and abetting.
In doing so, they further articulated Halberstam’s framework to cap-
ture those limits. As above, that framework requires that the defend-
ant give knowing and substantial assistance to the primary tortfeasor;
notably, courts often viewed those twin requirements as working in
tandem, with a lesser showing of one demanding a greater showing of
the other to establish a conscious, culpable participation in the tort.
Pp. 11–16.
(iii) Halberstam differentiated types of aid along the same cul-
pability axis that grounded the common law. And its six factors for
“substantial assistance” call for the same balancing that courts had
undertaken previously between the nature and amount of assistance
and the defendant’s scienter. Pp. 16–17.
(2) The parties then vigorously dispute what precisely a defendant
must aid and abet under §2333(d)(2). Plaintiffs assert that it is “the
person,” while defendants insist that it is the “act of international ter-
rorism.” That syntactic dispute makes little difference here, because
aiding and abetting is inherently a rule of secondary liability for spe-
cific wrongful acts. In the tort context, liability is imposed only when
someone commits (not merely agrees to commit) an actual tort. And
in this case, the ATA limits that liability to injuries caused by an “act
of international terrorism,” §2333(a). It thus is not enough for a de-
fendant to have given substantial assistance to a transcendent enter-
prise. A defendant must have aided and abetted (by knowingly provid-
ing substantial assistance) another person in the commission of the
4 TWITTER, INC. v. TAAMNEH
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actionable wrong—here, an act of international terrorism. However,
that does not require a strict nexus between the assistance and the
wrongful act; defendants are liable for other torts that are the foresee-
able risk of an intended tort, and an aider and abettor can assist some-
one without knowing all the details of his plan. Plus, in appropriate
circumstances, a defendant’s role in an illicit enterprise can be so sys-
temic and intentional that the defendant aids and abets each act of the
enterprise—as in Halberstam itself.
To summarize the requirements of §2333(d)(2), the phrase “aids and
abets, by knowingly providing substantial assistance” points to the el-
ements and factors articulated by Halberstam. Those elements and
factors should not be taken as inflexible codes but should be under-
stood in light of the common law and applied as a framework designed
to hold defendants liable when they consciously and culpably “partici-
pate[d] in” a tortious act in such a way as to help “make it succeed.”
Nye & Nissen v. United States, 336 U. S. 613, 619. Pp. 17–21.
(c) Plaintiffs have satisfied Halberstam’s first two elements by al-
leging both that ISIS committed a wrong and that defendants knew
they were playing some sort of role in ISIS’ enterprise. But plaintiffs’
allegations do not show that defendants gave such knowing and sub-
stantial assistance to ISIS that they culpably participated in the Reina
attack. Pp. 21–30.
(1) Plaintiffs allege that defendants aided and abetted ISIS in the
following ways: First, they provided social-media platforms, which are
generally available to the internet-using public; ISIS was able to up-
load content to those platforms and connect with third parties on them.
Second, defendants’ recommendation algorithms matched ISIS-re-
lated content to users most likely to be interested in that content. And,
third, defendants knew that ISIS was uploading this content but took
insufficient steps to ensure that its content was removed. Plaintiffs do
not allege that ISIS or Masharipov used defendants’ platforms to plan
or coordinate the Reina attack. Nor do plaintiffs allege that defend-
ants gave ISIS any special treatment or words of encouragement. Nor
is there reason to think that defendants carefully screened any content
before allowing users to upload it onto their platforms.
None of plaintiffs’ allegations suggest that defendants culpably “as-
sociate[d themselves] with” the Reina attack, “participate[d] in it as
something that [they] wishe[d] to bring about,” or sought “by [their]
action to make it succeed.” Nye & Nissen, 336 U. S., at 619 (internal
quotation marks omitted). Defendants’ mere creation of their media
platforms is no more culpable than the creation of email, cell phones,
or the internet generally. And defendants’ recommendation algo-
rithms are merely part of the infrastructure through which all the con-
tent on their platforms is filtered. Moreover, the algorithms have been
Cite as: 598 U. S. ____ (2023) 5
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presented as agnostic as to the nature of the content. At bottom, the
allegations here rest less on affirmative misconduct and more on pas-
sive nonfeasance. To impose aiding-and-abetting liability for passive
nonfeasance, plaintiffs must make a strong showing of assistance and
scienter. Plaintiffs fail to do so.
First, the relationship between defendants and the Reina attack is
highly attenuated. Plaintiffs make no allegations that defendants’ re-
lationship with ISIS was significantly different from their arm’s
length, passive, and largely indifferent relationship with most users.
And their relationship with the Reina attack is even further removed,
given the lack of allegations connecting the Reina attack with ISIS’ use
of these platforms. Second, plaintiffs provide no reason to think that
defendants were consciously trying to help or otherwise participate in
the Reina attack, and they point to no actions that would normally
support an aiding-and-abetting claim.
Plaintiffs’ complaint rests heavily on defendants’ failure to act; yet
plaintiffs identify no duty that would require defendants or other com-
munication-providing services to terminate customers after discover-
ing that the customers were using the service for illicit ends. Even if
such a duty existed in this case, it would not transform defendants’
distant inaction into knowing and substantial assistance that could
establish aiding and abetting the Reina attack. And the expansive
scope of plaintiffs’ claims would necessarily hold defendants liable as
having aided and abetted each and every ISIS terrorist act committed
anywhere in the world. The allegations plaintiffs make here are not
the type of pervasive, systemic, and culpable assistance to a series of
terrorist activities that could be described as aiding and abetting each
terrorist act by ISIS.
In this case, the failure to allege that the platforms here do more
than transmit information by billions of people—most of whom use the
platforms for interactions that once took place via mail, on the phone,
or in public areas—is insufficient to state a claim that defendants
knowingly gave substantial assistance and thereby aided and abetted
ISIS’ acts. A contrary conclusion would effectively hold any sort of
communications provider liable for any sort of wrongdoing merely for
knowing that the wrongdoers were using its services and failing to stop
them. That would run roughshod over the typical limits on tort liabil-
ity and unmoor aiding and abetting from culpability. Pp. 21–27.
(2) The Ninth Circuit’s analysis obscured the essence of aiding-
and-abetting liability. First, the Ninth Circuit framed the issue of sub-
stantial assistance as turning on defendants’ assistance to ISIS’ activ-
ities in general, rather than with respect to the Reina attack. Next,
the Ninth Circuit misapplied the “knowing” half of “knowing and sub-
stantial assistance,” which is designed to capture the defendants’ state
6 TWITTER, INC. v. TAAMNEH
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of mind with respect to their actions and the tortious conduct (even if
not always the particular terrorist act). Finally, the Ninth Circuit ap-
pears to have regarded Halberstam’s six substantiality factors as a se-
quence of disparate, unrelated considerations without a common con-
ceptual core. In doing so, the Ninth Circuit focused primarily on the
value of defendants’ platforms to ISIS, rather than whether defend-
ants culpably associated themselves with ISIS’ actions. Pp. 27–29.
(3) There is also one set of allegations specific to Google: that
Google reviewed and approved ISIS videos on YouTube as part of a
revenue-sharing system and thereby shared advertising revenue with
ISIS. But the complaint here alleges nothing about the amount of
money that Google supposedly shared with ISIS, the number of ac-
counts approved for revenue sharing, or the content of the videos that
were approved. Nor does it give any other reason to view Google’s rev-
enue sharing as substantial assistance. Without more, plaintiffs thus
have not plausibly alleged that Google knowingly provided substantial
assistance to the Reina attack, let alone (as their theory of liability
would require) every single terrorist act committed by ISIS. Pp. 29–
30.
(d) The concepts of aiding and abetting and substantial assistance
do not lend themselves to crisp, bright-line distinctions. Applying the
guideposts provided by the common law and Halberstam, the nexus
between defendants and the Reina attack is far removed. As alleged
by plaintiffs, defendants designed virtual platforms and knowingly
failed to do “enough” to remove ISIS-affiliated users and ISIS-related
content from their platforms. Yet, plaintiffs have failed to allege that
defendants intentionally provided any substantial aid to the Reina at-
tack or otherwise consciously participated in it—much less that de-
fendants so pervasively and systemically assisted ISIS as to render
them liable for every ISIS attack. Plaintiffs accordingly have failed to
state a claim under §2333(d)(2). Pp. 30–31.
2 F. 4th 871, reversed.
THOMAS, J., delivered the opinion for a unanimous Court. JACKSON, J.,
filed a concurring opinion.
Cite as: 598 U. S. ____ (2023) 1
Opinion of the Court
NOTICE: This opinion is subject to formal revision before publication in the
United States Reports. Readers are requested to notify the Reporter of
Decisions, Supreme Court of the United States, Washington, D. C. 20543,
pio@supremecourt.gov, of any typographical or other formal errors.
SUPREME COURT OF THE UNITED STATES
_________________
No. 21–1496
_________________
TWITTER, INC., PETITIONER v. MEHIER
TAAMNEH, ET AL.
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE NINTH CIRCUIT
[May 18, 2023]
JUSTICE THOMAS delivered the opinion of the Court.
Under 18 U. S. C. §2333, United States nationals who
have been “injured . . . by reason of an act of international
terrorism” may sue for damages. §2333(a). They are not
limited to suing the individual terrorists or organizations
that directly carried out the attack, however. That is be-
cause §2333(d)(2) also imposes civil liability on “any person
who aids and abets, by knowingly providing substantial as-
sistance, or who conspires with the person who committed
such an act of international terrorism.” Victims of terrorist
acts therefore may seek to recover from those who aided
and abetted the terrorist act that injured them.
The plaintiffs (who are respondents) contend that they
have stated a claim for relief under §2333(d)(2). They were
allegedly injured by a terrorist attack carried out by ISIS.
But plaintiffs are not suing ISIS. Instead, they have
brought suit against three of the largest social-media com-
panies in the world—Facebook, Twitter (who is petitioner),
and Google (which owns YouTube)—for allegedly aiding
and abetting ISIS. As plaintiffs allege, ISIS has used de-
fendants’ social-media platforms to recruit new terrorists
2 TWITTER, INC. v. TAAMNEH
Opinion of the Court
and to raise funds for terrorism. Defendants allegedly
knew that ISIS was using their platforms but failed to stop
it from doing so. Plaintiffs accordingly seek to hold Face-
book, Twitter, and Google liable for the terrorist attack that
allegedly injured them. We conclude, however, that plain-
tiffs’ allegations are insufficient to establish that these de-
fendants aided and abetted ISIS in carrying out the rele-
vant attack.
I
Plaintiffs’ case arises from a 2017 terrorist attack on the
Reina nightclub in Istanbul, Turkey. The attack was car-
ried out by Abdulkadir Masharipov on behalf of the Islamic
State of Iraq and Syria (ISIS).1 Born in Uzbekistan,
Masharipov had received military training with al Qaeda
in Afghanistan in 2011 and eventually became affiliated
with ISIS. In 2016, he was ordered by ISIS to travel to Tur-
key and launch an attack in Istanbul on New Year’s Eve.
After planning and coordinating the attack with ISIS emir
Abu Shuhada, Masharipov entered the Reina nightclub in
the early hours of January 1, 2017, and fired over 120
rounds into a crowd of more than 700 people. In total,
Masharipov killed 39 people and injured 69 others. The
next day, ISIS released a statement claiming responsibility
for the attack. Two weeks later, Masharipov was arrested
in Istanbul after hiding out in ISIS safe houses.
One of Masharipov’s victims was Nawras Alassaf, who
was killed in the attack. Several members of Alassaf ’s fam-
ily then brought the present lawsuit under §2333, alleging
that they had been injured by the attack.2 Invoking
——————
1 In one form or another, ISIS has been designated a Foreign Terrorist
Organization since 2004; it has also been known as the Islamic State of
Iraq and the Levant, al Qaeda in Iraq, and the al-Zarqawi Network.
2 Plaintiffs appear to offer several theories as to the nature of their in-
jury; because that question is not before us, we take no position as to any
of those theories.
Cite as: 598 U. S. ____ (2023) 3
Opinion of the Court
§2333(d)(2), plaintiffs sued three major social-media com-
panies—Facebook, Inc., Google, Inc., and Twitter, Inc.—
claiming that they aided and abetted ISIS and thus were
liable for the Reina nightclub attack.3
As is common knowledge, these three companies control
three of the largest and most ubiquitous platforms on the
internet: Facebook, YouTube, and Twitter. At the time of
the Reina attack, Facebook had over 2 billion active users
each month, YouTube had over 1 billion, and Twitter had
around 330 million. See Facebook, Inc., Form 10–K for Fis-
cal Year Ended Dec. 31, 2017, p. 34; Twitter, Inc., Form 10–
K for Fiscal Year Ended Dec. 31, 2017, p. 47; YouTube,
YouTube Hits a Billion Monthly Users (Mar. 21, 2013),
https://blog.youtube/news-and-events/onebillionstrong. At
least for Facebook and YouTube, those numbers are even
higher today. See Meta Platforms, Inc., Form 10–K for Fis-
cal Year Ended Dec. 31, 2022, p. 56 (nearly 3 billion);
L. Ceci, Statista, YouTube–Statistics & Facts (Mar.
22, 2023), https://www.statista.com/topics/2019/youtube/
#topicOverview (2.56 billion).
Everyone before us today agrees on the basic aspects of
these platforms’ business models. People from around the
world can sign up for the platforms and start posting con-
tent on them, free of charge and without much (if any) ad-
vance screening by defendants. Once on the platforms, us-
ers can upload messages, videos, and other types of content,
which others on the platform can then view, respond to, and
share. As noted above, billions of people have done just
that. As a result, the amount of content on defendants’ plat-
forms is staggering. It appears that for every minute of the
day, approximately 500 hours of video are uploaded to
——————
3 Although Twitter, Inc., is the named petitioner and defendant, Twit-
ter, Inc., has since been merged into X Corp., a subsidiary of X Holdings
Corp. Similarly, although Facebook, Inc., and Google, Inc., are the
named defendants, Facebook, Inc., is now known as Meta Platforms, Inc.,
and Google, Inc., is now Google LLC, a subsidiary of Alphabet, Inc.
4 TWITTER, INC. v. TAAMNEH
Opinion of the Court
YouTube, 510,000 comments are posted on Facebook, and
347,000 tweets are sent on Twitter. See Statista, Media
Usage in an Internet Minute as of April 2022 (2023),
https://www.statista.com/statistics/195140/new-user-
generated-content-uploaded-by-users-per-minute; Statista,
YouTube–Statistics & Facts; B. Marr, How Much Data Do
We Create Every Day? Forbes, May 21, 2018. On YouTube
alone, users collectively watch more than 1 billion hours of
video every day. See YouTube Advertising, Reach Your
Customers—and Discover New Ones, https://youtube.com/
intl/en_us/ads/how-it-works/set-up-a-campaign/audience.
Defendants profit from this content largely by charging
third parties to advertise on their platforms. Those adver-
tisements are placed on or near the billions of videos, posts,
comments, and tweets uploaded by the platforms’ users. To
organize and present all those advertisements and pieces of
content, defendants have developed “recommendation” al-
gorithms that automatically match advertisements and
content with each user; the algorithms generate those out-
puts based on a wide range of information about the user,
the advertisement, and the content being viewed. So, for
example, a person who watches cooking shows on YouTube
is more likely to see cooking-based videos and advertise-
ments for cookbooks, whereas someone who likes to watch
professorial lectures might see collegiate debates and ad-
vertisements for TED Talks.
But not all of the content on defendants’ platforms is so
benign. As plaintiffs allege, ISIS and its adherents have
used these platforms for years as tools for recruiting, fund-
raising, and spreading their propaganda. Like many others
around the world, ISIS and its supporters opened accounts
on Facebook, YouTube, and Twitter and uploaded videos
and messages for others to see. Like most other content on
those platforms, ISIS’ videos and messages were then
matched with other users based on those users’ information
Cite as: 598 U. S. ____ (2023) 5
Opinion of the Court
and use history. And, like most other content, advertise-
ments were displayed with ISIS’ messages, posts, and vid-
eos based on information about the viewer and the content
being viewed. Unlike most other content, however, ISIS’
videos and messages celebrated terrorism and recruited
new terrorists. For example, ISIS uploaded videos that
fundraised for weapons of terror and that showed brutal ex-
ecutions of soldiers and civilians alike. And plaintiffs allege
that these platforms have been crucial to ISIS’ growth, al-
lowing it to reach new audiences, gain new members, and
spread its message of terror.
Plaintiffs also allege that defendants have known that
ISIS has used their platforms for years. Yet, plaintiffs
claim that defendants have failed to detect and remove a
substantial number of ISIS-related accounts, posts, and
videos. (For example, plaintiffs aver that defendants “have
failed to implement . . . a basic account detection methodol-
ogy” to prevent ISIS supporters from generating multiple
accounts on their platforms. App. 150.) Accordingly, plain-
tiffs assert that defendants aided and abetted ISIS by
knowingly allowing ISIS and its supporters to use their
platforms and benefit from their “recommendation” algo-
rithms, enabling ISIS to connect with the broader public,
fundraise, and radicalize new recruits. And, in the process,
defendants allegedly have profited from the advertisements
placed on ISIS’ tweets, posts, and videos.
Plaintiffs also provide a set of allegations specific to
Google. According to plaintiffs, Google has established a
system that shares revenue gained from certain advertise-
ments on YouTube with users who posted the videos
watched with the advertisement. As part of that system,
Google allegedly reviews and approves certain videos before
Google permits ads to accompany that video. Plaintiffs al-
lege that Google has reviewed and approved at least some
ISIS videos under that system, thereby sharing some
amount of revenue with ISIS.
6 TWITTER, INC. v. TAAMNEH
Opinion of the Court
The District Court dismissed plaintiffs’ complaint for fail-
ure to state a claim.4 But the Ninth Circuit reversed, find-
ing that plaintiffs had plausibly alleged that defendants
aided and abetted ISIS within the meaning of §2333(d)(2)
and thus could be held secondarily liable for the Reina
nightclub attack. Gonzalez v. Google, 2 F. 4th 871 (2021).
We granted certiorari to resolve whether plaintiffs have ad-
equately stated such a claim under §2333(d)(2). See 598
U. S. ___ (2022).
II
Section 2333 was originally enacted as part of the
Antiterrorism Act (ATA) in 1990. 104 Stat. 2250. At that
time, Congress authorized United States nationals or their
“estate, survivors, or heirs” to bring civil lawsuits when “in-
jured in [their] person, property, or business by reason of
an act of international terrorism.” Id., at 2251.5 In such a
——————
4 Plaintiffs also raised other claims, including that defendants were di-
rectly liable for having provided material support to ISIS. See, e.g., 18
U. S. C. §§2333(a), 2339A, 2339B, 2339C. The District Court dismissed
those claims as well, and plaintiffs did not appeal them.
5 The ATA defines “international terrorism” to mean
“activities that—
“(A) involve violent acts or acts dangerous to human life that are a
violation of the criminal laws of the United States or of any State, or that
would be a criminal violation if committed within the jurisdiction of the
United States or of any State;
“(B) appear to be intended—
“(i) to intimidate or coerce a civilian population;
“(ii) to influence the policy of a government by intimidation or coercion;
or
“(iii) to affect the conduct of a government by mass destruction, assas-
sination, or kidnapping; and
“(C) occur primarily outside the territorial jurisdiction of the United
States, or transcend national boundaries in terms of the means by which
they are accomplished, the persons they appear intended to intimidate
or coerce, or the locale in which their perpetrators operate or seek asy-
lum.” §2331(1).
Cite as: 598 U. S. ____ (2023) 7
Opinion of the Court
lawsuit, the plaintiff could recover treble damages and the
cost of the suit, including attorney’s fees. See 18 U. S. C.
§2333(a). But the ATA did not explicitly impose liability on
anyone who only helped the terrorists carry out the attack
or conspired with them. Prior to 2016, some courts accord-
ingly determined that the ATA did not authorize that sort
of secondary civil liability. See, e.g., Rothstein v. UBS AG,
708 F. 3d 82, 97–98 (CA2 2013).
Then, in 2016, Congress enacted the Justice Against
Sponsors of Terrorism Act (JASTA) to provide for a form of
secondary civil liability. 130 Stat. 852. Thus, as the law
now stands, those injured by an act of international terror-
ism can sue the relevant terrorists directly under
§2333(a)—or they can sue anyone “who aids and abets, by
knowingly providing substantial assistance, or who
conspires with the person who committed such an act of in-
ternational terrorism” under §2333(d)(2). For such a
secondary-liability claim, there is an additional condition:
The “act of international terrorism” must have been “com-
mitted, planned, or authorized by an organization that had
been designated as a foreign terrorist organization under [8
U. S. C. §1189] as of the date on which such act of interna-
tional terrorism was committed, planned, or authorized.”
Ibid. Plaintiffs seeking secondary liability can likewise re-
cover treble damages and the cost of the suit, including at-
torney’s fees. See §§2333(a), (d)(2).
The parties here do not dispute that the first three com-
ponents of §2333(d)(2) have been adequately alleged: The
Reina nightclub attack was an “act of international terror-
ism”; the attack was “committed, planned, or authorized”
by ISIS; and ISIS was “designated as a foreign terrorist or-
ganization” as of the date of the Reina nightclub attack.
——————
In short, the act generally must be violent, criminal, intended to in-
timidate or coerce civilians or a government, and occur either primarily
outside the United States or transcending national boundaries. See ibid.
8 TWITTER, INC. v. TAAMNEH
Opinion of the Court
§2333(d)(2). The central question is thus whether defend-
ants’ conduct constitutes “aid[ing] and abett[ing], by know-
ingly providing substantial assistance,” such that they can
be held liable for the Reina nightclub attack. Ibid.
III
As always, we start with the text of §2333. See Barten-
werfer v. Buckley, 598 U. S. 69, 74 (2023). Here, that text
immediately begs two questions: First, what exactly does it
mean to “aid and abet”? Second, what precisely must the
defendant have “aided and abetted”?
A
We turn first to the meaning of the phrase “aids and
abets, by knowingly providing substantial assistance.”
Nothing in the statute defines any of those critical terms.
Yet terms like “aids and abets” are familiar to the common
law, which has long held aiders-and-abettors secondarily li-
able for the wrongful acts of others. See Central Bank of
Denver, N. A. v. First Interstate Bank of Denver, N. A., 511
U. S. 164, 181 (1994). We generally presume that such
common-law terms “brin[g] the old soil” with them. Sekhar
v. United States, 570 U. S. 729, 733 (2013) (internal quota-
tion marks omitted). In enacting JASTA, Congress pro-
vided additional context by pointing to Halberstam v.
Welch, 705 F. 2d 472 (CADC 1983), as “provid[ing] the
proper legal framework” for “civil aiding and abetting and
conspiracy liability.” §2(a)(5), 130 Stat. 852.6 We thus
begin with Halberstam’s “legal framework,” viewed in con-
text of the common-law tradition from which it arose.
——————
6 The provision reads in full: “The decision of the United States Court
of Appeals for the District of Columbia in Halberstam v. Welch, 705 F.2d
472 (D.C. Cir. 1983), which has been widely recognized as the leading
case regarding Federal civil aiding and abetting and conspiracy liability,
including by the Supreme Court of the United States, provides the proper
legal framework for how such liability should function in the context of
chapter 113B of title 18, United States Code.” §2(a)(5), 130 Stat. 852.
Cite as: 598 U. S. ____ (2023) 9
Opinion of the Court
1
Long regarded as a leading case on civil aiding-and-
abetting and conspiracy liability, see 130 Stat. 852, Hal-
berstam arose from a distinctive fact pattern. Bernard
Welch was a serial burglar who had killed Michael Hal-
berstam during a break-in. 705 F. 2d, at 474. Halberstam’s
estate then sued Welch’s live-in partner, Linda Hamilton,
for aiding and abetting and conspiring with Welch. Id., at
474, 476.7 Hamilton was not present for Halberstam’s mur-
der, or even allegedly aware of the murder. See id., at 474–
476. But the facts made clear that “[s]he was a willing part-
ner in [Welch’s] criminal activities.” Id., at 474 (internal
quotation marks omitted). Hamilton had lived with Welch
for five years, during which time the couple had risen from
modest circumstances to possess a substantial fortune.
Ibid. This rapid ascent was remarkable because Welch had
no outside employment. Id., at 475. Rather, he left the
house most evenings and returned with antiques, jewelry,
and precious metals—some of which he melted down into
gold and silver ingots by using a smelting furnace that he
had installed in their garage. Meanwhile, Hamilton did
bookkeeping work for Welch’s “business,” facilitating the
sale of those stolen goods. Ibid. She had Welch’s customers
make checks payable to her, falsified her tax returns at
Welch’s direction, and kept records of incoming payments
from Welch’s customers—with no records of outgoing funds
to his “suppliers.” Ibid. Their arrangement continued until
Welch was arrested after he killed Halberstam while bur-
glarizing Halberstam’s home. Ibid.To determine Hamilton’s liability, the D. C. Circuit un-
dertook an extensive survey of the common law, examining
——————
7 Halberstam’s estate also sued Welch himself; a default judgment was
entered against Welch, who did not appeal. See Halberstam, 705 F. 2d,
at 474.
10 TWITTER, INC. v. TAAMNEH
Opinion of the Court
a series of state and federal cases, the Restatement (Sec-
ond) of Torts, and prominent treatises that discussed sec-
ondary liability in tort. Id., at 476–478, 481–486. With re-
spect to aiding and abetting, the court synthesized the cases
as resting on three main elements: First, “the party whom
the defendant aids must perform a wrongful act that causes
an injury.” Id., at 477. Second, “the defendant must be
generally aware of his role as part of an overall illegal or
tortious activity at the time that he provides the assis-
tance.” Ibid. And, third, “the defendant must knowingly
and substantially assist the principal violation.” Ibid. (cit-
ing, e.g., Landy v. Federal Deposit Ins. Corp., 486 F. 2d 139,
162–163 (CA3 1973); Woodward v. Metro Bank of Dallas,
522 F. 2d 84, 94–95 (CA5 1975)). Halberstam then articu-
lated six factors to help determine whether a defendant’s
assistance was “substantial.” 705 F. 2d, at 486–488. Those
factors are (1) “the nature of the act assisted,” (2) the
“amount of assistance” provided, (3) whether the defendant
was “present at the time” of the principal tort, (4) the de-
fendant’s “relation to the tortious actor,” (5) the “defend-
ant’s state of mind,” and (6) the “duration of the assistance”
given. Id., at 488 (emphasis deleted). Last, Halberstam
clarified that those who aid and abet “a tortious act may be
liable” not only for the act itself but also “for other reasona-
bly foreseeable acts done in connection with it.” Id., at 484.
Applying that framework, the D. C. Circuit held that
Hamilton was liable for aiding and abetting Halberstam’s
murder. Id., at 489.8 The court first determined that Welch
had committed a wrong (in killing Halberstam during the
burglary) and that Hamilton was generally aware of her
role in Welch’s criminal enterprise. Id., at 488. It then ex-
plained that Hamilton had given knowing and substantial
——————
8 The D. C. Circuit also held that Hamilton was liable as a
co-conspirator. Id., at 489.
Cite as: 598 U. S. ____ (2023) 11
Opinion of the Court
assistance to Welch’s activities by helping him turn his “sto-
len goods into ‘legitimate’ wealth,” thereby intending to
help Welch succeed by performing a function crucial to any
thief. Ibid. And it clarified that Hamilton knew Welch was
committing some sort of “personal property crime,” the
“foreseeable risk” of which was “violence and killing.” Ibid.The court therefore concluded that Hamilton substantially
helped Welch commit personal property crimes and was li-
able for Halberstam’s death, which was a foreseeable result
of such crimes. Ibid.That articulation of the common law thus resolved Hal-
berstam. But Halberstam recognized that the elements and
factors it provided could “be merged or articulated some-
what differently without affecting their basic thrust.” Id.,
at 478, n. 8. It thus cautioned—in a typical common-law
fashion—that its formulations should “not be accepted as
immutable components.” Id., at 489. Rather, Halberstam
suggested that its framework should be “adapted as new
cases test their usefulness in evaluating vicarious liability.”
Ibid.2
The allegations before us today are a far cry from the
facts of Halberstam. Rather than dealing with a serial bur-
glar and his live-in partner-in-crime, we are faced with in-
ternational terrorist networks and world-spanning internet
platforms. By Halberstam’s own lights, its precise three-
element and six-factor test thus may not be entirely ade-
quate to resolve these new facts. Ibid. And JASTA itself
points only to Halberstam’s “framework,” not its facts or its
exact phrasings and formulations, as the benchmark for
aiding and abetting. §2(a)(5), 130 Stat. 852. We therefore
must ascertain the “basic thrust” of Halberstam’s elements
and determine how to “adap[t]” its framework to the facts
before us today. See 705 F. 2d, at 478, 489, and n. 8. To do
so, we turn to the common law of aiding and abetting upon
12 TWITTER, INC. v. TAAMNEH
Opinion of the Court
which Halberstam rested and to which JASTA’s common-
law terminology points.
As we have recognized, “[a]iding and abetting is an an-
cient criminal law doctrine” that has substantially influ-
enced its analog in tort. Central Bank of Denver, 511 U. S.,
at 181. In one early statement of the criminal-law doctrine,
William Blackstone explained that those who were “pre-
sent, aiding and abetting the fact to be done,” or “procure[d],
counsel[ed], or command[ed] another to commit a crime,”
were guilty and punishable. 4 Commentaries on the Laws
of England 34, 36 (1795). Over the years, many statutes
and courts have offered variations on that basic rule. See
United States v. Peoni, 100 F. 2d 401, 402 (CA2 1938) (L.
Hand, J., for the court) (collecting authorities). Yet, to this
day, the basic “view of culpability” that animates the doc-
trine is straightforward: “[A] person may be responsible for
a crime he has not personally carried out if he helps another
to complete its commission.” Rosemond v. United States,
572 U. S. 65, 70 (2014).
Importantly, the concept of “helping” in the commission
of a crime—or a tort—has never been boundless. That is
because, if it were, aiding-and-abetting liability could
sweep in innocent bystanders as well as those who gave
only tangential assistance. For example, assume that any
assistance of any kind were sufficient to create liability. If
that were the case, then anyone who passively watched a
robbery could be said to commit aiding and abetting by fail-
ing to call the police. Yet, our legal system generally does
not impose liability for mere omissions, inactions, or non-
feasance; although inaction can be culpable in the face of
some independent duty to act, the law does not impose a
generalized duty to rescue. See 1 W. LaFave, Substantive
Criminal Law §6.1 (3d ed. 2018) (LaFave); W. Keeton, D.
Dobbs, R. Keeton, & D. Owen, Prosser and Keeton on Law
of Torts 373–375 (5th ed. 1984) (Prosser & Keeton). More-
over, both criminal and tort law typically sanction only
Cite as: 598 U. S. ____ (2023) 13
Opinion of the Court
“wrongful conduct,” bad acts, and misfeasance. J. Goldberg,
A. Sebok, & B. Zipursky, Tort Law: Responsibilities and Re-
dress 31 (2004). Some level of blameworthiness is therefore
ordinarily required. But, again, if aiding-and-abetting lia-
bility were taken too far, then ordinary merchants could be-
come liable for any misuse of their goods and services, no
matter how attenuated their relationship with the wrong-
doer. And those who merely deliver mail or transmit emails
could be liable for the tortious messages contained therein.
See Restatement (Second) of Torts §876, Comment d, Illus.
9, p. 318 (1979) (cautioning against this result).
For these reasons, courts have long recognized the need
to cabin aiding-and-abetting liability to cases of truly cul-
pable conduct. They have cautioned, for example, that not
“all those present at the commission of a trespass are liable
as principals” merely because they “make no opposition or
manifest no disapprobation of the wrongful” acts of another.
Brown v. Perkins, 83 Mass. 89, 98 (1861); see also Hilmes v.
Stroebel, 59 Wis. 74, 17 N. W. 539 (1883); Duke v. Feldman,
245 Md. 454, 457–458, 226 A. 2d 345, 347 (1967). Put an-
other way, overly broad liability would allow for “one person
[to] be made a trespasser and even a felon against his or her
consent, and by the mere rashness or precipitancy or over-
heated zeal of another.” Bird v. Lynn, 49 Ky. 422, 423
(1850). Moreover, unlike its close cousin conspiracy, aiding
and abetting does not require any agreement with the pri-
mary wrongdoer to commit wrongful acts, thus eliminating
a significant limiting principle. See Nye & Nissen v. United
States, 336 U. S. 613, 620 (1949).
To keep aiding-and-abetting liability grounded in culpa-
ble misconduct, criminal law thus requires “that a defend-
ant ‘in some sort associate himself with the venture, that
he participate in it as in something that he wishes to bring
about, that he seek by his action to make it succeed’ ” before
he could be held liable. Id., at 619 (quoting Peoni, 100 F. 2d,
at 402). In other words, the defendant has to take some
14 TWITTER, INC. v. TAAMNEH
Opinion of the Court
“affirmative act” “with the intent of facilitating the offense’s
commission.” Rosemond, 572 U. S., at 71. Such intentional
participation can come in many forms, including abetting,
inducing, encouraging, soliciting, or advising the commis-
sion of the offense, such as through words of encouragement
or driving the getaway car. 2 LaFave §13.2(a), at 457–460;
see also J. Hawley & M. McGregor, The Criminal Law 81
(3d ed. 1899). Regardless of the particulars, however, it is
clear that some culpable conduct is needed. See Rosemond,
572 U. S., at 73, 77.9
Similar principles and concerns have shaped aiding-and-
abetting doctrine in tort law, with numerous cases directly
employing them to help articulate the standard for tortious
aiding and abetting. See, e.g., Zoelsch v. Arthur Andersen
& Co., 824 F. 2d 27, 35–36 (CADC 1987); Woodward, 522
F. 2d, at 95, n. 23; Landy, 486 F. 2d, at 163–164 (all relying
on Nye & Nissen, 336 U. S., at 619); see also Camp v. Dema,
948 F. 2d 455, 459 (CA8 1991) (relying on People v. Terman,
4 Cal. App. 2d 345, 346–347 (1935), another criminal case).
Similar to the criminal-law rule, some cases have required
that the defendant’s assistance “must have had a direct re-
lation to the trespass, and have been calculated and in-
tended to produce it” to warrant liability for the resulting
tort. Bird, 49 Ky., at 423; see also Smith v. Thompson, 103Idaho 909, 911, 655 P. 2d 116, 118 (App. 1982); Brown, 83
Mass., at 98. Other cases have emphasized the need for
some “culpable conduct” and “some degree of knowledge
that [a defendant’s] actions are aiding the primary violator”
before holding the defendant secondarily liable. Camp, 948
F. 2d, at 460. Still others have explained that “[c]ulpability
——————
9 Conversely, conspiracy liability could be premised on a “more attenu-
ated relation with the principal violation” because the defendant and the
principal wrongdoer had agreed to a wrongful enterprise. Halberstam,
705 F. 2d, at 485; see also Restatement (Second) of Torts §876, Comment
a, p. 316 (1979) (“The theory of the early common law was that there was
a mutual agency of each to act for the others”).
Cite as: 598 U. S. ____ (2023) 15
Opinion of the Court
of some sort is necessary to justify punishment of a second-
ary actor,” lest mostly passive actors like banks become lia-
ble for all of their customers’ crimes by virtue of carrying
out routine transactions. Monsen v. Consolidated Dressed
Beef Co., 579 F. 2d 793, 799 (CA3 1978). And others have
suggested that “inaction cannot create liability as an aider
and abettor” absent a duty to act. Zoelsch, 824 F. 2d, at 36;
see also Woodward, 522 F. 2d, at 96.
In articulating those limits, courts simultaneously began
to crystalize the framework for aiding and abetting that
Halberstam identified and applied. See, e.g., Monsen, 579
F. 2d, at 799 (stating a similar three-part test).10 As in Hal-
berstam, that framework generally required what the text
of §2333(d)(2) demands: that the defendant have given
knowing and substantial assistance to the primary tortfea-
sor. See, e.g., Monsen, 579 F. 2d, at 799; Landy, 486 F. 2d,
at 162–163. Notably, courts often viewed those twin re-
quirements as working in tandem, with a lesser showing of
one demanding a greater showing of the other. E.g., Wood-
ward, 522 F. 2d, at 97; Woods v. Barnette Bank of Ft.
Lauderdale, 765 F. 2d 1004, 1010 (CA11 1985). In other
words, less substantial assistance required more scienter
before a court could infer conscious and culpable assistance.
See Woodward, 522 F. 2d, at 97. And, vice versa, if the as-
sistance were direct and extraordinary, then a court might
more readily infer conscious participation in the underlying
tort. See ibid. In moving back and forth between all these
guideposts, the courts thus largely tracked the same dis-
tinctions drawn above to ensure that liability fell only on
——————
10 Others, however, have wondered whether any of these “elaborate dis-
cussions of the aiding and abetting standard . . . ‘have added anything
except unnecessary detail’ ” to the formulation set forth by Judge
Learned Hand in United States v. Peoni, 100 F. 2d 401 (CA2 1938), and
adopted by this Court in Nye & Nissen v. United States, 336 U. S. 613(1949). SEC v. Apuzzo, 689 F. 3d 204, 212, n. 9 (CA2 2012) (quoting IIT,
an Int’l Inv. Trust v. Cornfeld, 619 F. 2d 909, 922 (CA2 1980)).
16 TWITTER, INC. v. TAAMNEH
Opinion of the Court
those who had abetted the underlying tort through con-
scious, “culpable conduct.” Camp, 948 F. 2d, at 460.
3
Halberstam’s framework reflected and distilled those
common-law principles. Indeed, Halberstam started with a
survey of many earlier common-law cases, including many
of the same cases cited above. 705 F. 2d, at 477, 483 (citing,
e.g., Woodward, 522 F. 2d, at 94–95; Landy, 486 F. 2d, at
162–163; Duke, 245 Md., at 457–458, 226 A. 2d, at 347). As
part of that survey, Halberstam explicitly distinguished dif-
ferent types of aid along the same culpability axis that
grounded the common law. 705 F. 2d, at 481–483. For ex-
ample, Halberstam recognized that giving verbal encour-
agement (such as yelling “ ‘Kill him!’ ”) could be substantial
assistance, id., at 481, but that passively watching an as-
sault after hearing an assailant threaten the victim likely
would not be, id., at 483. Those same lines have long been
drawn for aiding-and-abetting liability under the common
law. See Rosemond, 572 U. S., at 70; see also, e.g., 2 LaFave
§13.2(a), at 457–460; Restatement (Second) of Torts §876,
Comment d, Illus. 9–10, at 318; Brown, 83 Mass., at 98.
And Halberstam’s six factors for “substantial assistance”
call for the same balancing that courts had undertaken pre-
viously between the nature and amount of assistance on the
one hand and the defendant’s scienter on the other. 705
F. 2d, at 484–485, 487–488.
Despite that deep-rooted common-law basis, the Ninth
Circuit appears to have understood JASTA’s approval of
Halberstam’s “legal framework” as requiring it to hew
tightly to the precise formulations that Halberstam used.
The parties before us similarly make a conscious effort to
draw analogies to the facts of that case. But any approach
that too rigidly focuses on Halberstam’s facts or its exact
phraseology risks missing the mark. Halberstam is by its
own terms a common-law case and provided its elements
Cite as: 598 U. S. ____ (2023) 17
Opinion of the Court
and factors as a way to synthesize the common-law ap-
proach to aiding and abetting. And JASTA employs the
common-law terms “aids and abets,” pointing to Hal-
berstam’s common-law “framework” as the primary guide-
post for understanding the scope of §2333(d)(2).
At bottom, both JASTA and Halberstam’s elements and
factors rest on the same conceptual core that has animated
aiding-and-abetting liability for centuries: that the defend-
ant consciously and culpably “participate[d]” in a wrongful
act so as to help “make it succeed.” Nye & Nissen, 336 U. S.,
at 619. To be sure, nuances may establish daylight between
the rules for aiding and abetting in criminal and tort law;
we have described the doctrines as “rough[ly] simila[r],” not
identical. Central Bank of Denver, 511 U. S., at 181. But
we need not resolve the extent of those differences today; it
is enough for our purposes to recognize the framework that
Halberstam set forth and the basis on which it rests. The
phrase “aids and abets” in §2333(d)(2), as elsewhere, refers
to a conscious, voluntary, and culpable participation in an-
other’s wrongdoing.
B
The next question, however, is what precisely a defend-
ant must aid and abet. As noted above, JASTA imposes li-
ability on anyone “who aids and abets, by knowingly provid-
ing substantial assistance, or who conspires with the person
who committed such an act of international terrorism.”
§2333(d)(2). The parties vigorously dispute the textual ob-
ject of the term “aids and abets”: Plaintiffs assert that it is
“the person,” and defendants insist that it is the “act of in-
ternational terrorism.” So, plaintiffs contend, defendants
can be liable if they aided and abetted ISIS generally—
there is no need for defendants to have aided and abetted
the specific Reina nightclub attack. Conversely, defendants
posit that they are liable only if they directly aided and
18 TWITTER, INC. v. TAAMNEH
Opinion of the Court
abetted the Reina nightclub attack, with a strict nexus be-
tween their assistance and that attack. Neither side is
quite right.
To start, we find it unnecessary to parse whether the tex-
tual object of “aids and abets” is “the person” or the “act of
international terrorism.” That syntactic dispute makes lit-
tle difference here, because aiding and abetting is inher-
ently a rule of secondary liability for specific wrongful acts.
See Prosser & Keeton 323. As stated above, the rule im-
poses liability for a wrong on those who “hel[p] another to
complete its commission.” Rosemond, 572 U. S., at 70 (em-
phasis added). Or, as Halberstam put it, the defendant
must aid and abet “a tortious act.” 705 F. 2d, at 484.
Nor would a contrary rule make sense for torts. That is
because tort law imposes liability only when someone com-
mits an actual tort; merely agreeing to commit a tort or sug-
gesting a tortious act is not, without more, tortious. See
Prosser & Keeton 324; Halberstam, 705 F. 2d, at 479.11
“Enterprises” or “conspiracies” alone are therefore not tor-
tious—the focus must remain on the tort itself. The same
is true here: The ATA opens the courthouse doors only if the
plaintiff is “injured . . . by reason of an act of international
terrorism.” §2333(a). JASTA further restricts secondary
liability by requiring that the “act of international terror-
ism” be “committed, planned, or authorized by” a foreign
terrorist organization designated as such “as of the date on
which such act of international terrorism was committed,
planned, or authorized.” §2333(d). Thus, it is not enough,
——————
11 In this respect, tort law is different from criminal law, which does
punish mere agreements to commit crimes. See, e.g., 2 LaFave §12.2(b),
at 372 (“At common law a conspiracy was punishable even though no act
was done beyond the mere making of the agreement”); see also 1 J. Ohlin,
Wharton’s Criminal Law §8.7, p. 242 (16th ed. 2021) (noting that statu-
tory requirements of an “overt act” generally do not require that the overt
act be criminal); Iannelli v. United States, 420 U. S. 770, 785, n. 17 (1975)
(similar).
Cite as: 598 U. S. ____ (2023) 19
Opinion of the Court
as plaintiffs contend, that a defendant have given substan-
tial assistance to a transcendent “enterprise” separate from
and floating above all the actionable wrongs that constitute
it. Rather, a defendant must have aided and abetted (by
knowingly providing substantial assistance) another per-
son in the commission of the actionable wrong—here, an act
of international terrorism. See, e.g., Restatement (Second)
of Torts §876(b), and Comment d; Halberstam, 705 F. 2d, at
488.
Plaintiffs insist that Halberstam proves the contrary, but
their argument misses the gist of that case. To be sure,
Linda Hamilton was not on the scene for the burglary of
Halberstam’s house and did not lend any specific support to
Halberstam’s murder. Ibid. But Hamilton’s assistance to
Welch was so intentional and systematic that she assisted
each and every burglary committed by Welch; any time that
Welch left the house to burglarize, he would have relied on
Hamilton’s assistance in laundering the stolen goods and
transforming them into usable wealth. See ibid. Thus,
Hamilton did aid and abet Welch in burglarizing Hal-
berstam’s home—and, as noted above, killing Halberstam
was a foreseeable consequence of that burglary. See ibid.
On the other hand, defendants overstate the nexus that
§2333(d)(2) requires between the alleged assistance and the
wrongful act. To start, aiding and abetting does not require
the defendant to have known “all particulars of the primary
actor’s plan.” Restatement (Third) of Torts: Intentional
Torts to Persons §10, Comment c, p. 104 (Tent. Draft No. 3,
Apr. 6, 2018). For example, a defendant might be held lia-
ble for aiding and abetting the burning of a building if he
intentionally helped others break into the building at night
and then, unknown to him, the others lit torches to guide
them through the dark and accidentally started a fire. See
American Family Mutual Ins. Co. v. Grim, 201 Kan. 340,
345–347, 440 P. 2d 621, 625–626 (1968); Restatement (Sec-
ond) of Torts §876, Comment d, Illus. 10, at 318. That leads
20 TWITTER, INC. v. TAAMNEH
Opinion of the Court
to the next problem: As Halberstam makes clear, people
who aid and abet a tort can be held liable for other torts
that were “a foreseeable risk” of the intended tort. 705
F. 2d, at 488. Accordingly, a close nexus between the assis-
tance and the tort might help establish that the defendant
aided and abetted the tort, but even more remote support
can still constitute aiding and abetting in the right case.
Moreover, in appropriate circumstances, a secondary de-
fendant’s role in an illicit enterprise can be so systemic that
the secondary defendant is aiding and abetting every
wrongful act committed by that enterprise—as in Hal-
berstam itself. Id., at 487–488. At this point, aiding-and-
abetting liability begins to blur with conspiracy liability,
which typically holds co-conspirators liable for all reasona-
bly foreseeable acts taken to further the conspiracy. See
Pinkerton v. United States, 328 U. S. 640, 647–648 (1946);
see also Halberstam, 705 F. 2d, at 482–483 (noting the oc-
casional overlap). Yet, as noted above, aiding and abetting
lacks the requisite agreement that justifies such extensive
conspiracy liability. See Restatement (Second) of Torts
§876, Comment a, at 316 (“The theory of the early common
law was that there was mutual agency of each [conspirator]
to act for the others”); Pinkerton, 328 U. S., at 646. Thus,
while the facts of Halberstam are not totemic (lest courts
strain to compare Linda Hamilton with international crim-
inal syndicates), its facts are useful when determining
whether a defendant has so consciously “participate[d] in”
a series of tortious acts in order to “make [each one] suc-
ceed.” Nye & Nissen, 336 U. S., at 619 (internal quotation
marks omitted).
* * *
To summarize the requirements of §2333(d)(2), the
phrase “aids and abets, by knowingly providing substantial
assistance,” points to the elements and factors articulated
by Halberstam. But, those elements and factors should not
Cite as: 598 U. S. ____ (2023) 21
Opinion of the Court
be taken as inflexible codes; rather, they should be under-
stood in light of the common law and applied as a frame-
work designed to hold defendants liable when they con-
sciously and culpably “participate[d] in” a tortious act in
such a way as to help “make it succeed.” Nye & Nissen, 336
U. S., at 619 (internal quotation marks omitted). And the
text requires that defendants have aided and abetted the
act of international terrorism that injured the plaintiffs—
though that requirement does not always demand a strict
nexus between the alleged assistance and the terrorist act.
IV
Under the appropriate framework, some aspects of to-
day’s case become immediately clear: First, because they
are trying to hold defendants liable for the Reina attack,
plaintiffs must plausibly allege that defendants aided and
abetted ISIS12 in carrying out that attack. Next, plaintiffs
have satisfied Halberstam’s first two elements by alleging
both that ISIS committed a wrong and that defendants
knew they were playing some sort of role in ISIS’ enter-
prise. The key question, therefore, is whether defendants
gave such knowing and substantial assistance to ISIS that
they culpably participated in the Reina attack. The allega-
tions here fall short of that showing under Halberstam’s
framework as properly understood by reference to the com-
mon-law principles it applied.
A
To start, recall the basic ways that defendants as a group
allegedly helped ISIS. First, ISIS was active on defendants’
——————
12 For purposes of this case, we need not resolve whether defendants
must have aided and abetted ISIS, Masharipov, or some subgroup of ISIS
operatives in committing the Reina attack. In other words, we need not
resolve whether “the person” referred to in §2333(d)(2) encompasses in-
ternational terrorist syndicates or is somehow otherwise limited; either
way, defendants need to have aided and abetted that “person” in carrying
out the Reina attack.
22 TWITTER, INC. v. TAAMNEH
Opinion of the Court
social-media platforms, which are generally available to the
internet-using public with little to no front-end screening
by defendants. In other words, ISIS was able to upload con-
tent to the platforms and connect with third parties, just
like everyone else. Second, defendants’ recommendation al-
gorithms matched ISIS-related content to users most likely
to be interested in that content—again, just like any other
content. And, third, defendants allegedly knew that ISIS
was uploading this content to such effect, but took insuffi-
cient steps to ensure that ISIS supporters and ISIS-related
content were removed from their platforms. Notably, plain-
tiffs never allege that ISIS used defendants’ platforms to
plan or coordinate the Reina attack; in fact, they do not al-
lege that Masharipov himself ever used Facebook,
YouTube, or Twitter.
None of those allegations suggest that defendants culpa-
bly “associate[d themselves] with” the Reina attack, “par-
ticipate[d] in it as something that [they] wishe[d] to bring
about,” or sought “by [their] action to make it succeed.” Nye
& Nissen, 336 U. S., at 619 (internal quotation marks om-
mitted). In part, that is because the only affirmative “con-
duct” defendants allegedly undertook was creating their
platforms and setting up their algorithms to display content
relevant to user inputs and user history. Plaintiffs never
allege that, after defendants established their platforms,
they gave ISIS any special treatment or words of encour-
agement. Nor is there reason to think that defendants se-
lected or took any action at all with respect to ISIS’ content
(except, perhaps, blocking some of it).13 Indeed, there is not
even reason to think that defendants carefully screened any
content before allowing users to upload it onto their plat-
forms. If anything, the opposite is true: By plaintiffs’ own
——————
13 Plaintiffs concede that defendants attempted to remove at least some
ISIS-sponsored accounts and content after they were brought to their at-
tention.
Cite as: 598 U. S. ____ (2023) 23
Opinion of the Court
allegations, these platforms appear to transmit most con-
tent without inspecting it.
The mere creation of those platforms, however, is not cul-
pable. To be sure, it might be that bad actors like ISIS are
able to use platforms like defendants’ for illegal—and some-
times terrible—ends. But the same could be said of cell
phones, email, or the internet generally. Yet, we generally
do not think that internet or cell service providers incur cul-
pability merely for providing their services to the public
writ large. Nor do we think that such providers would nor-
mally be described as aiding and abetting, for example, il-
legal drug deals brokered over cell phones—even if the pro-
vider’s conference-call or video-call features made the sale
easier. See Doe v. GTE Corp., 347 F. 3d 655, 659 (CA7
2003).
To be sure, plaintiffs assert that defendants’ “recommen-
dation” algorithms go beyond passive aid and constitute ac-
tive, substantial assistance. We disagree. By plaintiffs’
own telling, their claim is based on defendants’ “provision
of the infrastructure which provides material support to
ISIS.” App. 53. Viewed properly, defendants’ “recommen-
dation” algorithms are merely part of that infrastructure.
All the content on their platforms is filtered through these
algorithms, which allegedly sort the content by information
and inputs provided by users and found in the content itself.
As presented here, the algorithms appear agnostic as to the
nature of the content, matching any content (including
ISIS’ content) with any user who is more likely to view that
content. The fact that these algorithms matched some ISIS
content with some users thus does not convert defendants’
passive assistance into active abetting. Once the platform
and sorting-tool algorithms were up and running, defend-
ants at most allegedly stood back and watched; they are not
alleged to have taken any further action with respect to
ISIS.
At bottom, then, the claim here rests less on affirmative
24 TWITTER, INC. v. TAAMNEH
Opinion of the Court
misconduct and more on an alleged failure to stop ISIS from
using these platforms. But, as noted above, both tort and
criminal law have long been leery of imposing aiding-and-
abetting liability for mere passive nonfeasance. To show
that defendants’ failure to stop ISIS from using these plat-
forms is somehow culpable with respect to the Reina attack,
a strong showing of assistance and scienter would thus be
required. Plaintiffs have not made that showing.
First, the relationship between defendants and the Reina
attack is highly attenuated. As noted above, defendants’
platforms are global in scale and allow hundreds of millions
(or billions) of people to upload vast quantities of infor-
mation on a daily basis. Yet, there are no allegations that
defendants treated ISIS any differently from anyone else.
Rather, defendants’ relationship with ISIS and its support-
ers appears to have been the same as their relationship
with their billion-plus other users: arm’s length, passive,
and largely indifferent. Cf. Halberstam, 705 F. 2d, at 488.
And their relationship with the Reina attack is even further
removed, given the lack of allegations connecting the Reina
attack with ISIS’ use of these platforms.
Second, because of the distance between defendants’ acts
(or failures to act) and the Reina attack, plaintiffs would
need some other very good reason to think that defendants
were consciously trying to help or otherwise “participate in”
the Reina attack. Nye & Nissen, 336 U. S., at 619 (internal
quotation marks omitted). But they have offered no such
reason, let alone a good one. Again, plaintiffs point to no
act of encouraging, soliciting, or advising the commission of
the Reina attack that would normally support an aiding-
and-abetting claim. See 2 LaFave §13.2(a), at 457. Rather,
they essentially portray defendants as bystanders, watch-
ing passively as ISIS carried out its nefarious schemes.
Such allegations do not state a claim for culpable assistance
or participation in the Reina attack. See Halberstam, 705
F. 2d, at 481, 483.
Cite as: 598 U. S. ____ (2023) 25
Opinion of the Court
Because plaintiffs’ complaint rests so heavily on defend-
ants’ failure to act, their claims might have more purchase
if they could identify some independent duty in tort that
would have required defendants to remove ISIS’ content.
See Woodward, 522 F. 2d, at 97, 100. But plaintiffs identify
no duty that would require defendants or other
communication-providing services to terminate customers
after discovering that the customers were using the service
for illicit ends. See Doe, 347 F. 3d, at 659; People v. Brophy,
49 Cal. App. 2d 15, 33–34 (1942).14 To be sure, there may
be situations where some such duty exists, and we need not
resolve the issue today. Even if there were such a duty
here, it would not transform defendants’ distant inaction
into knowing and substantial assistance that could estab-
lish aiding and abetting the Reina attack.
If there were any doubt, the expansive scope of plaintiffs’
claims would put it to rest. Given the lack of any concrete
nexus between defendants’ services and the Reina attack,
plaintiffs’ claims would necessarily hold defendants liable
as having aided and abetted each and every ISIS terrorist
act committed anywhere in the world. Under plaintiffs’ the-
ory, any U. S. national victimized by an ISIS attack could
bring the same claim based on the same services allegedly
provided to ISIS. Plaintiffs thus must allege that defend-
ants so systemically and pervasively assisted ISIS that de-
fendants could be said to aid and abet every single ISIS at-
tack. Viewed in that light, the allegations here certainly
——————
14 Plaintiffs have not presented any case holding such a company liable
for merely failing to block such criminals despite knowing that they used
the company’s services. Rather, when legislatures have wanted to im-
pose a duty to remove content on these types of entities, they have ap-
parently done so by statute. See, e.g., Telecommunications Act of 1996,
§502, 110 Stat. 133–134 (codified, as amended, at 47 U. S. C. §223); but
see Reno v. American Civil Liberties Union, 521 U. S. 844, 857, 874
(1997) (holding parts of §223 unconstitutional under the First Amend-
ment).
26 TWITTER, INC. v. TAAMNEH
Opinion of the Court
fall short. Plaintiffs do not claim that defendants intention-
ally associated themselves with ISIS’ operations or affirm-
atively gave aid that would assist each of ISIS’ terrorist
acts. Nor have they alleged that defendants and ISIS
formed a near-common enterprise of the kind that could es-
tablish such broad liability. These allegations are thus a
far cry from the type of pervasive, systemic, and culpable
assistance to a series of terrorist activities that could be de-
scribed as aiding and abetting each terrorist act.
To be sure, we cannot rule out the possibility that some
set of allegations involving aid to a known terrorist group
would justify holding a secondary defendant liable for all of
the group’s actions or perhaps some definable subset of ter-
rorist acts. There may be, for example, situations where
the provider of routine services does so in an unusual way
or provides such dangerous wares that selling those goods
to a terrorist group could constitute aiding and abetting a
foreseeable terror attack. Cf. Direct Sales Co. v. United
States, 319 U. S. 703, 707, 711–712, 714–715 (1943)
(registered morphine distributor could be liable as a co-
conspirator of an illicit operation to which it mailed mor-
phine far in excess of normal amounts). Or, if a platform
consciously and selectively chose to promote content pro-
vided by a particular terrorist group, perhaps it could be
said to have culpably assisted the terrorist group. Cf. Pas-
saic Daily News v. Blair, 63 N. J. 474, 487–488, 308 A. 2d
649, 656 (1973) (publishing employment advertisements
that discriminate on the basis of sex could aid and abet the
discrimination).
In those cases, the defendants would arguably have of-
fered aid that is more direct, active, and substantial than
what we review here; in such cases, plaintiffs might be able
to establish liability with a lesser showing of scienter. But
we need not consider every iteration on this theme. In this
case, it is enough that there is no allegation that the plat-
forms here do more than transmit information by billions of
Cite as: 598 U. S. ____ (2023) 27
Opinion of the Court
people, most of whom use the platforms for interactions
that once took place via mail, on the phone, or in public ar-
eas. The fact that some bad actors took advantage of these
platforms is insufficient to state a claim that defendants
knowingly gave substantial assistance and thereby aided
and abetted those wrongdoers’ acts. And that is particu-
larly true because a contrary holding would effectively hold
any sort of communication provider liable for any sort of
wrongdoing merely for knowing that the wrongdoers were
using its services and failing to stop them. That conclusion
would run roughshod over the typical limits on tort liability
and take aiding and abetting far beyond its essential culpa-
bility moorings.
B
In holding that plaintiffs had stated a claim, the Ninth
Circuit went astray through a series of missteps that, to-
gether, obscured the essence of aiding-and-abetting liabil-
ity. To correct those errors, we proceed through the Ninth
Circuit’s application of Halberstam’s framework.
The Ninth Circuit framed the issue of substantial assis-
tance as turning on defendants’ assistance to ISIS’ activi-
ties in general. See 2 F. 4th, at 909. But, as we explained
above, the question is whether defendants gave substantial
assistance to ISIS with respect to the Reina attack. The
focus thus must remain on the Reina attack; plaintiffs’ fail-
ure to allege any definable nexus between the defendants’
assistance and that attack therefore—at minimum—dras-
tically increases their burden to show that defendants
somehow consciously and culpably assisted the attack.
Next, the Ninth Circuit misapplied the “knowing” half of
“knowing and substantial assistance.” It first separated the
“knowing” and “substantial” subelements; it then analyzed
the “knowing” subelement as a carbon copy of the anteced-
ent element of whether the defendants were “generally
aware” of their role in ISIS’ overall scheme. Ibid.; see also
28 TWITTER, INC. v. TAAMNEH
Opinion of the Court
id., at 908. But, as discussed above, “the knowledge and
substantial assistance” components “should be considered
relative to one another” as part of a single inquiry designed
to capture conscious and culpable conduct. Camp, 948
F. 2d, at 459 (internal quotation marks omitted). The
“knowing” part of that inquiry is therefore designed to cap-
ture the defendants’ state of mind with respect to their ac-
tions and the tortious conduct (even if not always the par-
ticular terrorist act), not the same general awareness that
defines Halberstam’s second element.
Finally, when applying Halberstam’s six substantiality
factors, the Ninth Circuit appears to have regarded the fac-
tors as a sequence of disparate, unrelated considerations
without a common conceptual core. See 2 F. 4th, at 909–
910. That is incorrect. The point of those factors is to help
courts capture the essence of aiding and abetting: partici-
pation in another’s wrongdoing that is both significant and
culpable enough to justify attributing the principal wrong-
doing to the aider and abettor. The Ninth Circuit thus
erred in focusing (as it did) primarily on the value of defend-
ants’ platforms to ISIS, rather than whether defendants
culpably associated themselves with ISIS’ actions. For ex-
ample, when applying the second factor (the amount and
kind of assistance), the Ninth Circuit should have consid-
ered that defendants’ platforms and content-sorting algo-
rithms were generally available to the internet-using pub-
lic. That focus reveals that ISIS’ ability to benefit from
these platforms was merely incidental to defendants’ ser-
vices and general business models; it was not attributable
to any culpable conduct of defendants directed toward ISIS.
And, when considering the fourth and fifth factors (the de-
fendants’ relationship to ISIS and the defendants’ state of
mind), the Ninth Circuit should have given much greater
weight to defendants’ arm’s-length relationship with ISIS—
which was essentially no different from their relationship
Cite as: 598 U. S. ____ (2023) 29
Opinion of the Court
with their millions or billions of other users—and their un-
disputed lack of intent to support ISIS. See id., at 910.
Taken as a whole, the Ninth Circuit’s analytic approach
thus elided the fundamental question of aiding-and-
abetting liability: Did defendants consciously, voluntarily,
and culpably participate in or support the relevant wrong-
doing? As we have explained above, the answer in this case
is no. Plaintiffs allege only that defendants supplied gen-
erally available virtual platforms that ISIS made use of,
and that defendants failed to stop ISIS despite knowing it
was using those platforms. Given the lack of nexus between
that assistance and the Reina attack, the lack of any de-
fendant intending to assist ISIS, and the lack of any sort of
affirmative and culpable misconduct that would aid ISIS,
plaintiffs’ claims fall far short of plausibly alleging that de-
fendants aided and abetted the Reina attack.
C
That leaves the set of allegations specific to Google. As
explained above, plaintiffs allege that Google reviewed and
approved ISIS videos on YouTube as part of its revenue-
sharing system and thereby shared advertising revenue
with ISIS. The Ninth Circuit briefly mentioned those alle-
gations when analyzing plaintiffs’ complaint here. How-
ever, in addressing another, materially identical complaint,
the Ninth Circuit held that the same allegations “failed to
state a claim for aiding-and-abetting liability” because they
were “devoid of any allegations about how much assistance
Google provided” and therefore did not plausibly allege
“that Google’s assistance was substantial.” Id., at 907.
We think that the Ninth Circuit was correct in that hold-
ing. The complaint here alleges nothing about the amount
of money that Google supposedly shared with ISIS, the
number of accounts approved for revenue sharing, or the
content of the videos that were approved. It thus could be
the case that Google approved only one ISIS-related video
30 TWITTER, INC. v. TAAMNEH
Opinion of the Court
and shared only $50 with someone affiliated with ISIS; the
complaint simply does not say, nor does it give any other
reason to view Google’s revenue sharing as substantial as-
sistance. Without more, plaintiffs thus have not plausibly
alleged that Google knowingly provided substantial assis-
tance to the Reina attack, let alone (as their theory of lia-
bility requires) every single terrorist act committed by ISIS.
V
By their very nature, the concepts of aiding and abetting
and substantial assistance do not lend themselves to crisp,
bright-line distinctions. However, both the common law
and Halberstam provide some clear guideposts: The point
of aiding and abetting is to impose liability on those who
consciously and culpably participated in the tort at issue.
The focus must remain on assistance to the tort for which
plaintiffs seek to impose liability. When there is a direct
nexus between the defendant’s acts and the tort, courts may
more easily infer such culpable assistance. But, the more
attenuated the nexus, the more courts should demand that
plaintiffs show culpable participation through intentional
aid that substantially furthered the tort. And, if a plain-
tiff ’s theory would hold a defendant liable for all the torts
of an enterprise, then a showing of pervasive and systemic
aid is required to ensure that defendants actually aided and
abetted each tort of that enterprise.
Here, however, the nexus between defendants and the
Reina attack is far removed. As alleged by plaintiffs, de-
fendants designed virtual platforms and knowingly failed
to do “enough” to remove ISIS-affiliated users and ISIS-
related content—out of hundreds of millions of users world-
wide and an immense ocean of content—from their plat-
forms. Yet, plaintiffs have failed to allege that defendants
intentionally provided any substantial aid to the Reina at-
tack or otherwise consciously participated in the Reina at-
Cite as: 598 U. S. ____ (2023) 31
Opinion of the Court
tack—much less that defendants so pervasively and sys-
temically assisted ISIS as to render them liable for every
ISIS attack. Plaintiffs accordingly have failed to state a
claim under §2333(d)(2).
We therefore reverse the judgment of the Ninth Circuit.
It is so ordered.
Cite as: 598 U. S. ____ (2023) 1
JACKSON, J., concurring
SUPREME COURT OF THE UNITED STATES
_________________
No. 21–1496
_________________
TWITTER, INC., PETITIONER v. MEHIER
TAAMNEH, ET AL.
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE NINTH CIRCUIT
[May 18, 2023]
JUSTICE JACKSON, concurring.
I join the opinion of the Court with the understanding
that today’s decisions are narrow in important respects. In
this case and its companion, Gonzalez v. Google, 598 U. S.
___ (2023) (per curiam), the Court has applied 18 U. S. C.
§2333(d)(2) to two closely related complaints, filed by the
same counsel. Both cases came to this Court at the motion-
to-dismiss stage, with no factual record. And the Court’s
view of the facts—including its characterizations of the
social-media platforms and algorithms at issue—properly
rests on the particular allegations in those complaints.
Other cases presenting different allegations and different
records may lead to different conclusions.
The Court also draws on general principles of tort and
criminal law to inform its understanding of §2333(d)(2).
General principles are not, however, universal. The
common-law propositions this Court identifies in interpret-
ing §2333(d)(2) do not necessarily translate to other con-
texts.