The popular media's negative coverage of the insanity defense in contested cases when a defendant claims not to have the rational capacity to commit a crime or has a diminished capacity to conceptualize a criminal intent has caused the public to dismiss forensic psychiatry as providing rationalizations or excuses for bad behavior, rather than possessing a real scientific method. The use of the insanity defense is clearly subject to sociological and societal factors, such as the statistically greater willingness to believe a man who kills his child is competent vs. A woman. However, the authors contend that this ignores the many cases where the defense and the prosecution both agree that the criminal in question was not competent and was operating upon a different schema of 'reality' that affected his or her ability to judge circumstances in the same fashion as a sane person. (It might be argued, in the specific example cited by the article, that men cannot suffer from hormonally caused imbalances like postpartum depression)
However, the problem remains that forensic psychiatry is not an exact science, although the public may wish "expert testimony" to be so. Diminished capacity in particular, where a person may have a mental illness but still knows right from wrong, can be difficult for laypersons to understand. Illnesses such as Post-Traumatic Stress Syndrome may affect person's abilities to make judgments, and be the result of a variety of past stressors in the person's environment.
All theories of criminal guilt or lack of guilt are based in the presupposition that sane persons have the free will to act as they do, and that there are different gradations of sanity. A person with PTSD might hold a job, and be able to drive a car, but is more sensitive to trauma as a result of their past, and may react differently than someone who does not have PTSD. A person like Andrea Yates might commit a horrific action, but may be judged legally insane because of psychotic postpartum depression, as she believed he is 'saving' his/her child from Satan by drowning them in the bathtub. Even a person taking a sleeping pill on his doctor's order who and sleepwalks and drives a car into an embankment, and unintentionally causes harm to a fellow motorist is not operating at full mental capacity to form an intent. In both instances, a person does not will the consequences of his or her action and thus is legally insane or not responsible because they cannot form a criminal intent. Crime is about means and opportunity -- but also motivation, according to this assumption, and punishment is not simply about the act itself, but about reforming the person. Without a coherent ability to form an intent, there is no way to punish the person so he or she will not commit another crime, and as the evil act was not willed by the conscience of the person, no greater moral good will be accomplished by enforcing a penalty. To protect society, a judge or jury might take away a person's driving license, or ensure that a mentally ill person be incarcerated, but this is not mean to punish the person, merely protect society and perhaps the offender him or herself from the illness.
This reasoning becomes problematic, however, when one asks the question -- did someone ask to be born in a gang-ridden environment? Perhaps he or she would not commit a crime if he or she had access to other vocational opportunities, or if his or her background did not normalize violence? Or, what if a drunk driving incident was the 'result' in part of a family predisposition to alcoholism, or a violent act the result of an extra chromosome? Why is drunk driving a crime, if the person thought he or she could drive safely after one drink but has a low tolerance -- but driving while impaired unintentionally by a prescription is excused by the legal system? Responsibility and articulating a negative intent thus becomes more difficult to 'blame' if one examines the causal chain of crime. Also, even sane persons can use police entrapment as a potential defense, namely that they would not have committed a crime unless lured to do so by an agent of the state. Juveniles are assumed to not possess the same capacity as adults -- but can be tried as adults if their crimes are especially heinous, as if premeditated murder gives them added causal reasoning power.
These examples highlight what seems like an inescapable truth of the legal system. The insanity defense is the result of the practical desire of the prison system not to blend the mentally ill and incompetent with hardened criminals. But forensic psychology cannot not iron out the philosophical and legal contradictions of the current system of punishment, or provide concrete answers as to when a criminal thought begins in the mind of a defendant.