British Columbia, Canada
The following excerpt is from Javier v. Hooper, 2014 BCSC 1253 (CanLII):
Where, as here, there are multiple tortfeasors, the defendants are not excused from liability because other causal factors for which they are not responsible also contributed to produce the harm. As long as a defendant is part of the cause of an injury, the defendant is liable, even though his or her act alone was not enough to create the injury: Athey v. Leonati, 1996 CanLII 183 (SCC),  3 S.C.R. 458 at paras. 17-20.
The defendants are not required to compensate the plaintiff for expenses or loss she would have suffered in her life in any event, even without the accident: Blackwater v. Plint, 2005 SCC 58 at para. 78. The defendants must compensate for the damages they caused but not for the effects of another other wrongful act that would have occurred anyway. This is known as the “crumbling skull rule”: Blackwater, at para. 80.