In new research published in the Personality and Social Psychology Bulletin, Baumeister, Masicampo and DeWall (2009) theorise that a belief in free will may be partly what oils the wheels of society, what encourages us to treat each other respectfully. They explore this theory with three studies, two on helping behaviours and one on aggression.This reminds me of a study that found economists are more likely to defect in prisoners dilemmas.
These experiments aren't the first to examine how a belief in free will (or otherwise) affects our behaviour. In a recent study Vohs and Schooler (2008) also found that a belief in free will seems to have a positive effect on people's behaviour. In that experiment (covered by Cognitive Daily) participants whose disbelief in free will was encouraged were more likely to cheat on a test.
What's going on here?
My instinctual response is that people are responding to cognitive dissonance-- as determinists/economists, it's more difficult to rationalize altruism. (Selection bias is another possibility, though the determinist study appears to have ruled that out, so it could only apply to economists.)
If you were curious, I'm a compatibilist.