XYY's theory- humans have 23 pairs of chromosomes, males have XY and women have XX.
An abnormality in the male chromosome creates an extra Y chromosome making XYY.
Criminals are believed to have the extra Y chromosome
Males with XYY syndrome have an increased risk of behavioural, social and emotional difficulties. This can include ADHD, more aggression and slower than average learning.
John Wayne Gacy is said to have XYY, he sexually assaulted, tortured and raped at least 33 men in America.
PatriciaJacobs took blood samples from 197 men in a maximum security prison in London. All participants had aggressive behaviour, 12 having chromosome abnormalities. The rate is prison (3.5%) was much higher than the general population (0.2%).