Here's a nice summary of current scientific understanding on free will, written by Dennis Overbye in the New York Times:
A bevy of experiments in recent years suggest that the conscious mind is like a monkey riding a tiger of subconscious decisions and actions in progress, frantically making up stories about being in control. [Our] conscious brain [is] only playing catch-up to what the unconscious brain [is] already doing.
In other words, our unconscious (genetically programmed) brain perceives the world for us, motivates us toward some situations over others, colors our experiences with pre-wired dispositions (emotions), and triggers our reactions. The higher brain is left to make up (confabulate) reasons for why we did what we did, based on the illusion of free will.
One concern raised in the article is whether "talking about this in public will fan the culture wars," as if scientific truth should be subjected to a popularity contest. Scientists are supposed to follow the facts, and do the right thing in the face of opposition, not bury their noses in the sand! Fortunately, it seems British publications are willing to be less politically correct than Americans. For example, the Economist magazine recently reported another case against free will:
In the late 1990s a previously blameless American began collecting child pornography and propositioning children. On the day before he was due to be sentenced to prison for his crimes, he had his brain scanned. He had a tumour. When it had been removed, his paedophilic tendencies went away. When it started growing back, they returned. When the regrowth was removed, they vanished again. Who then was the child abuser?
It's easy to see how a brain tumor could remove your capabilities (like ability to speak or walk), but how was it in this case able to add new (albeit evil) complex desires and motivations, unless these already existed in the brain (merely unleased by - not caused by - the tumor)?
The same question arises with gambling. Certain people -- who previously never felt the urge to gamble -- suddenly became addicted to gambling after taking a certain drug. Drugs are very simple molecules... there is no "gambling-ness" in the drug itself. The drug is simply unlocking a complex behavior that we already have inside.