Xyy Syndrome

Based in Lombrosian theory that criminal behavior is rooted in heredity, some contemporary theorists have focused their research on genetic composition that may influence or even cause criminal conduct. The notion that criminal conduct may be biologically predisposed is certainly not new, nor is it without foundation. However, the research on genetic makeup, specifically the XYY chromosomal syndrome, has been very controversial. In 1965, Jacobs and associates reported that the presence of an extra Y chromosome in human males was correlated with mental retardation, tall stature, and excessive levels of aggressive conduct. This report spawned a number of researchers to hypothesize that the XYY chromosomal anomaly was related to violent crime by certain males.

Chromosomes are chains of genetic material known as DNA, which direct the growth of living cells in all living organisms. This includes eye color, height, temperament, and predisposition for all other human traits, such as depression. Each cell of a normal female has two X chromosomes, whereas normal males have an X and a Y chromosome in each cell. When combined, these cells constitute 23 pairs. In rare instances, two Y chromosomes in the male pair with a single X, leaving the person with 47 chromosomes rather than the normal 46. Such persons can demonstrate unusual height, acne, borderline intelligence, and bouts of extreme aggressiveness.

A few noted cases of murder have involved persons with the XYY syndrome, including Daniel Hugon in Paris in 1968. He was charged with the savage killing of an elderly woman. Another was Sean Farley, known as “Big Bad John,” a giant of a man from New York City who murdered and mutilated a woman in 1969. In neither case did the XYY syndrome acquit the offender by reason of insanity.

Thus far, the research on XYY syndrome has failed to adequately substantiate linkage between possessing the extra chromosome and violent criminal conduct. Indeed, the possibility that in rare cases some persons possessing the XYY anomaly commit violent crimes is certainly feasible. But how does that explain the majority of cases of XYY syndrome that do not result in violent criminal behavior? Jarvik, in a global review of 26 studies that included over 5,000 criminals, found that in the general population, the presence of XYY is between .11% and .14%, in psychiatric patients between .13% and .20%, and in criminal populations significantly higher at 1.9%. The 1.9% is an extremely small part of the total prison population and does little to support the occurrence of violence in society. In fact, most of the crimes committed by persons with XYY syndrome were crimes against property, not people.