A man Richard Speck, shown in the image on the left, killed 8 nurses in July 1966 in Chicago. Speck systematically raped and strangled each women individually. One nurse, Corazon Amurao, hid under a bed and after Speck miscounted the number of victims he had, he left. He was eventually caught and identified by Corazon. It was reported that Speck was an XYY. He was 6 foot 1, intellectually challenged (he miscounted his victims, repeated the eighth grade, and dropped out of school before he was 16) and his face was covered with acne scarring.
It turned out that Speck wasn't XYY at all, but now people were itching to prove, one way or another, if XYY had a link to violence. Sarnoff Mednick took on the challenge when he gathered a group of 28,884 men from Copenhagen. He then ran chromosome screenings on all the men that were over 6 feet tall. Out of the 4,139 men that were screened, only 12 had the XYY gene. After comparing the criminal backgrounds of those with the XYY gene to those with a normal XY gene, it was found that XYY gene is affiliated with crime. The XYY men had a crime rate of 41.7% and the average XY males had a crime rate of 9.3%. The violent crimes in the XYY men were 8.4%, while only 1.8% in XY males. Although this is a significatn difference, it needs to be kept in mind that this was a small sampling, so the violence statistics are insignificant. This study, although not producing any statistically sound evidence on violence, did provide insight on the genetic basis to crime.
First, men with the XYY chromosome did not commit more violent crimes, they did commit more pretty property crimes. Second, the XYY chromosome is a malfunction in the body, and is not something that is heritable. Therefore, the XYY chromosome can not be used to determine whether crime or violence is heritable. The fact that the XYY chromosome does not prove crime to be heritable, it does not invalidate any other findings through twin and adoption studies. Finally, in more larger scale studies on younger males, they do seem to display more aggressive behavior than the controls, it may be more than an extra Y chromosome that can produce criminal behavior.