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Rh Long regarded as a leading case on civil aiding-and-abetting and conspiracy liability, see 130 Stat. 852, Halberstam arose from a distinctive fact pattern. Bernard Welch was a serial burglar who had killed Michael Halberstam 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. Hamilton was not present for Halberstam’s murder, or even allegedly aware of the murder. See id., at 474–476. But the facts made clear that “[s]he was a willing partner 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 burglarizing Halberstam’s home. Ibid.

To determine Hamilton’s liability, the D. C. Circuit undertook an extensive survey of the common law, examining