A boomerang effect involves the movement of a message recipient's belief, attitude, or behavior in a direction contrary to the advocated position in the message. There has been limited scholarly attention to the mechanism of belief boomerang. To better understand boomerang effects, we tested the effect of message discrepancy on belief boomerang and the mechanism underlying such an effect with a between-subjects factorial study (N = 458). Our results revealed that a counterattitudinal message resulted in belief boomerang, whereas a proattitudinal message resulted in persuasion. The relationship between discrepancy and belief boomerang was accounted for by a motivational (i.e., reactance) process. The results suggest the significance of examining the boomerang effect as an outcome distinct from a failure to persuade.