The genetic defence: the impact of genetics on the concept of criminal responsibility.

Early attempts to employ a genetic defence in the United States failed. Many of these cases concerned the XYY syndrome — a genetic abnormality in which men have a second Y chromosome. This syndrome was thought to be a likely target because the extra Y chromosome suggested that these men were more “male” and therefore more aggressive. However, “critiques of the methodologically questionable XYY studies raised serious doubts that XYY individuals were significantly predisposed to aggression or violence.”