Even if we had certain answers to these big, complicated questions of free will vs. determinism, or understood the precise neurological cause of mental problems, that wouldn't necessarily dictate how the legal system should deal with those diagnosed as mentally ill. Some physical determinists have concluded that neuroscience cannot tell us whether we should hold people legally responsible for their actions…Juries and judges need to know what happened and why. As the Yates and Clark cases illustrate, psychiatry and neuroscience are not much help in answering those questions, despite the constant promise that a deeper and more expansive understanding of the relationship between brain and mind is just around the corner.
Criminal responsibility does not require purely "uncaused" action but, instead, requires that action be immediately caused in a certain way. If a harmful act is connected to the actor's desires or goals, and if the actor held certain beliefs about the harm that could result from the act, then he or she is criminally responsible. Thus, to be guilty of murder under the U.S. Model Penal Code, for example, one must have a mens rea (or a mental state) that includes: (1) the desire to perform an act that results in the death of another; and (2) performance of the act with the desire to kill, knowledge one will kill, or with reckless indifference to the chance that one may kill. It is possible—even likely—that future science may provide a complete physical causal description of the mental states required for criminal responsibility—the desire to kill and the understanding that the act will result in a death. But this need not affect the categories of culpability (such as murder, rape, and theft) themselves. The breadth of category is a policy decision determined by legislatures and judges. New scientific knowledge should only act to help us, as a society, better categorize defendants as "guilty" or "not guilty" based upon the existing categories of culpability and defense.