relatively common human sex chromosome anomaly in which a male has two Y chromosomes rather than one. It occurs in 1 in 500–1,000 live male births, and individuals with the anomaly are often characterized by tallness and severe acne and sometimes by skeletal malformations and mental deficiency. It has been suggested that the presence of an extra Y chromosome in an individual may cause him to be more aggressive and prone to criminal behaviour, a condition called the “supermale” syndrome. Studies of prison populations have tended to confirm this hypothesis; but subsequent studies of the general population, especially those in which affected individuals were observed from early childhood over a long period of time, have cast serious doubt on the validity of linking the chromosomal anomaly directly to behavioral abnormalities.