Two lines of evidence have emerged from a study of XYY males in the literature that tend to refute the argument that aggressive behavior in these individuals is a myth promoted by the mass media. First, there is a higher incidence (1.8% to 12%) of XYY males in institutions than in the general population (0.14% to 0.38%) as determined from newborn surveys. Secondly, a psychological evaluation of a group of XYY men characterized them as passive, dependent on others, and incapable of controlling their impulses. The extra Y chromosome does have an apparent effect on behavior, and should be considered a factor in assessing a defendant's responsibility in a court of law.
Gardner LI, Neu RL. Evidence Linking an Extra Y Chromosome to Sociopathic Behavior. Arch Gen Psychiatry. 1972;26(3):220-222. doi:10.1001/archpsyc.1972.01750210028005