Jacobs suggested that men with the XYY syndrome were more aggressive than normal XY men. Men with XYY syndrome are typically more violent as a result of higher levels of testosterone. They usually have a lower IQ and be immature and unstable. They’re typically very tall and have a higher than average eBMI, they may develop severe acne during adolescence. They may have learning disabilities and behavioural problems.
The extra Y chromosome means testosterone secretion is higher than in the average male. This extra dose of testosterone may lead to a more aggressive personality or hostile attribution bias which leads to increased violence.
