Introductory Note: This chapter considers several defenses which the defendant may raise regarding his lack of mental responsibility for the alleged offense. These include: (1) the insanity defense (including the “XYY chromosome” defense); (2) the defense of diminished responsibility (which can negate the existence of the required mens rea); (3) automatism (the doing of acts while in an unconscious state); (4) the defense of intoxication; and (5) infancy.