Can a defendant be found guilty as a principal even though he personally did not commit the crime?

We have held that under 18 U.S.C. 2(a) a person charged as an aider and abettor "cannot be found guilty . . . unless a principal whom he has aided and abetted committed the criminal act." United States v. Bernstein, 533 F.2d 775, 799 (2d Cir. 1976). See also United States v. Erb, 543 F.2d 438, 446 (2d Cir.), cert. denied, 429 U.S. 981, 97 S.Ct. 493, 50 L.Ed.2d 590 (1976). Under 18 U.S.C. 2(b) a person who causes an innocent party to commit an act which, if done with the requisite intent, would constitute an offense may be found guilty as a principal even though he personally did not commit the criminal act. ¹¹