My understanding is that the burden of proof in a tort case is preponderance of evidence. that is, more likely than not. Consider the following:
Defendant hits the plaintiff. The plaintiff suffers a broken arm, and did not have one before he was hit. The act was witnessed by several people. I would guess that plaintiff would likely be able to prove his case by preponderance of evidence. Is that likely to be the case?
The plaintiff was also hit in the arm a few seconds later by a third party, X. This confuses the picture. A medical examination of the arm shows a broken bone by one blow, and internal bleeding by a lesser blow. So consider the following cases:
Case 1: The defendant is 6'0", 180 pounds, and X is 5'4", 120 pounds. (He may be even weaker, basically, "weak" to the point where he could not have administered the harder blow without an adrenaline rush.) I would guess the preponderance of evidence would work against the defendant. Is that the case?
Case 2, a: The defendant is the "weak man," and X is the stronger. I would guess that the preponderance of evidence would work in favor of the defendant. Is this true?
Case 2, b: The defendant is the weaker, but yelled at the plaintiff prior to hitting him, possibly creating the required adrenalin rush. Does this impact the result in 2a?
Case 3, Defendant and X are of equal size, leading to a 50-50 chance that each cast the harder blow that probably broke the arm. In addition, there is a non-zero chance that the softer blow may also have broken the arm, through leverage, etc. The defendant clearly cast one of the two blows. Does the preponderance of evidence work against him?
One more thing. Under (Arizona) State vs. McNair, Assault and battery is a crime, and even a "bystander" can become an accessory or accomplice that is equally guilty with the principal, if there was a relationship between them, or if they acted in concert (confronted the plaintiff together). Could defendant and X share criminal liability at least in Arizona? Does this extend to civil liability?