One of yesterday's comments asked me whether I believed in free will. I think of free will as being epiphenomenal. When we engage in deliberation, we are examining the pros and cons of alternative courses of action. When we complete our deliberation, either the pros or the cons will be weightier, and we go with the weightier side of the balance.
This is not to deny that people are morally and legally responsible for their deliberate actions. But I take the function of the concept of "responsibility" to be to add a thumb to the balance described in the previous paragraph. The question for the law is not whether a defendant's crime was the product of an exercise of free will, but whether attaching a penalty to the kind of conduct in which he engaged is likely to reduce the incidence of that conduct by making it more costly. If so, we say that his decision to engage in the conduct was culpable, was "his fault." We say he "could have chosen" not to engage in the conduct. But probably, if we knew everything about his psychology, we would realize that his choice was foreordained. What we mean when we say that he "had a choice" is that the penalty would have deterred most people from engaging in such behavior.
As Willard Quine put it, a choice is "free" if the individual's "motives and drives" are part of the causal chain that produces the "chosen" act, even if those motives and drives are themselves rigidly determined, perhaps as a result of a heavy threat of punishment or a powerful financial incentive. If the individual because of youth, insanity, or retardation is incapable of deliberation, his behavior is excused or his responsibility mitigated.
The upshot is that ascriptions of responsibility are based on social need (for example to deter crime) rather than on metaphysics, philosophy of mind, theology, or moral philosophy.
For a fuller discussion, see the index references to "free will" in my book The Problems of Jurisprudence (1990).