In Richard Wright's novel, Native Son, Bigger Thomas could possibly be classified as guilty but mentally ill by today's standards. Stress, anxiety, and aggression in Thomas" social and romantic relationships, living conditions, family life, and childhood and adolescent experiences are all factors.
The older basis for pleading insanity requires the person to be "so severely disturbed at the time of his act that he did not know what he was doing, or if he did know what he was doing, did not know that it was wrong". Some states have also added "irresistible impulse", which states that "some mentally ill individuals may respond correctly when asked if a particular act is morally right or wrong but may be unable to control their behavior" (Atkinson et al. 632-633).
Although standards have changed a bit, the idea behind the insanity plea remains the same. In the 1970's, not all, but most state and federal courts adopted a new legal definition proposed by the American Law Institute (ALI) stating, "A person is not responsible for criminal conduct if at the time of such conduct, as a result of mental disease or defect, he lacks substantial capacity either to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of the law." The ALI uses words like "substantial" to suggest that "any incapacity is not enough to avoid criminal responsibility but that total incapacity is not required either," and the word "appreciate" rather than "know" suggests that "intellectual awareness of right or wrong is not enough; individuals must have some understanding of the moral or legal consequences of their behavior before they can be held criminally responsible" (Atkinson et al. 632-633).
Guilty but mentally ill is one more option. A verdict of guilty but mentally ill is found "when a defendant is found to have a substantial disorder of thought of mood that afflicted him at the time of the crime and that significantly impaired his judgement, behavior, capacity to recognize reality, or ability to cope with the ordinary demands of life.