Some empirical evidence that the belief in free will affects behavior:
Belief in free will can be manipulated with methods developed by Vohs and Schooler (2007). These researchers had some participants read an essay by a well known scientist (Francis Crick, a Nobel laureate) rejecting and indeed mocking the notion of free will. Others read a neutral essay. Another manipulation involved a procedure in which participants read aloud a series of statements emphasizing either freedom of action or lack of freedom and determinism. In those studies, participants who had been induced to disbelieve in free will were later more willing than controls to cheat on a test.
Likewise, manipulations of belief in free will have been shown by Baumeister, Masicampo, and DeWall (2006) to affect other social behaviors. Participants who had been led to disbelieve in free will were significantly more aggressive and less helpful toward others.
We are not suggesting that these studies be taken as proof of the existence of free will. Nevertheless, taken together, these findings indicate that not only is the belief in free will normative, but it is also socially beneficial. Undermining that belief leads to an increase in antisocial actions (cheating and aggression) and a reduction in socially desirable behavior (helping).
Also, in these studies, manipulations aimed at promoting belief in free will typically yielded results identical to neutral controls, which suggests that encouraging people to believe in free will simply reaffirms their ordinary state. That is, people normally believe in free will, and getting them to disbelieve in it is the departure from normal. This pattern indicates that belief in free will is woven into the fabric of everyday social life and the assumptions according to which people perceive and interact with each other.
Some necessary caveats: the notion of free will that was manipulated was almost certainly some kind of contracausal notion (it’s hard to be sure; both the studies cited are unpublished). The authors of this paper note that many philosophers are compatibilists, but having noted it they seem immediately to forget it. Second, we need to distinguish the following two situations: shaking belief in free will affects behavior (for the worse) and shaking belief in a cherished notion affects behavior (for the worse). Still, some very interesting results for optimistic disbelievers in free will to mull over.