Part 1 of the article discusses selected theories and research on the genetic, biological, environmental, and sociological influences on criminal behavior and examines law cases and defenses utilizing this research. Part 2 examines the Biosocial Study and other longitudinal studies of juvenile and adult crime and assesses the rationale of a biological deficiency defense. Part 3 examines models that would predict criminal behavior, finding them unable to demonstrate where responsibility ends and excuse based on biological deficiency begins. Part 3 also examines a probability theory of behavior and presents several arguments against the biological deficiency defense. The article concludes that until social science research shows considerable improvement, it should not be invoked when making judgments about a defendant's criminal responsibility. On the other hand, social science research may have a legitimate role to play in sentencing policy. 310 footnotes.