Answer to Question 1
The legal system starts with the assumption that individuals are in possession of free will, that they have control over their behavior, and that they make choices and freely act on them. Thus, the law can hold people accountable for their actions. Mental health professionals tend to start from the assumption of determinism, that is, all human behavior is the result of complex interactions between genetics, biological factors, past experience, local conditions, social pressures, etc., and they recognize that sometimes, as the result of factors, such as brain tumors or social coercion, individuals actually can be made to behave against their will. The law does make one exception with regard to the assumption of free will in the insanity defense that recognizes that an individual can be suffering from a mental disorder that clouds his reason, perception, or judgment so much that it no longer makes any logical or legal sense to hold him fully responsible for his actions. At this point, the two definitions overlap, and a judge and/or jury needs to sort out the issue of responsibility.
Answer to Question 2
There are three ways in which the law assumes that mental disorders may affect an individual's ability to exercise his or her rights and responsibilities: (1) Defendants who are not guilty by reason of insanity are not criminally responsible for their actions; (2) Defendants who are incompetent to stand trial are unable to exercise their right to participate in their own defense; and (3) mental illness may be a mitigating factor that can lead to a less harsh sentence, or a harsher one. Since neither judges nor juries have expert psychological knowledge that allows them to make these decisions, mental health experts testify about these matters. One conflict between mental health and the law concerns the role of expert witnesses, specialists allowed to testify about matters of opinion (not just fact) that lie within their area of expertise. In Daubert v.Merrell Dow Pharmaceuticals (1993), the U.S. Supreme Court ruled that expert opinion must be based on an ... inference or assertion . . . derived by the scientific method, and courts must determine whether the reasoning or methodology underlying the testimony is scientifically valid and ... whether that reasoning or methodology can be applied to the facts in issue. Since experts can and do interpret the same information in different ways, and lawyers shop for friendly experts who have a history of interpreting evidence in a way that will help their case. Lawyers expect challenges to their expert witnesses' testimony, and they anticipate that experts for the opposing side will present conflicting testimony. One way to limit conflict, and hopefully improve expert testimony, is for courts to appoint neutral experts rather than having each side employ its own hired gun.