XYY syndrome is a congenital sex chromosome abnormality of males. Usually males have an X and Y chromosome (XY), and females two X chromosomes (XX). In one in 500 males, an extra Y chromosome is found to cause the XYY syndrome. Rarely even more Y chromosomes may be added to create XYYY and XYYYY. It is diagnosed by examining the chromosomes in almost any bodily cell, and may be made before birth by amniocentesis or chorionic villus sampling.
These males are tall, heavily built, aggressive and violent. Behavioural therapy is the only treatment as there is no cure. The population in jails has a far higher proportion of men with this syndrome than is found in the general population.