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

Syllabus

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 Halb