What is the test for giving jurors an alibi instruction in a criminal case?

It is well established that, under proper circumstances, the jury must be given an alibi instruction when the defense so requests. United States v. Megna, 450 F.2d 511 (5th Cir. 1971); United States v. Marcus, 166 F.2d 497, 503--04 (3d Cir. 1948). The reasoning behind this rule is not difficult to appreciate. Jurors are, by definition, untrained in the specifics of the law and, accordingly, must be instructed as to the legal standards they are bound to apply. In those cases where an alibi defense is presented, there exists the danger that the failure to prove that defense will be taken by the jury as a sign of the defendant's guilt.