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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 

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8 The  D. C.  Circuit  also  held  that  Hamilton  was  liable  as  a 

co-conspirator.  Id., at 489.