By contrast, XYY shows a distinct pattern of behavioral features. Given the role of the Y chromosome in creating male-specific sexual differentiation, it is interesting that the behavioral phenotypes associated with Y chromosome polynomial also tend to be among those generally more common in males, such as hyperactivity with impulsivity, aggression, and pervasive developmental disorders. The reason for the increased risk of disorders such as autism in males is not known, although it has been hypothesized to be an exaggeration of male-typical characteristics, perhaps as a result of high levels of prenatal testosterone exposure [Auyeung et al., 2009]. As additional Y chromosomes do not appear to be associated with abnormalities in testosterone production [Aksglaede et al., 2007b], the findings suggest that in addition to hormonal factors, another route to increased risk in males for these disorders is through direct effects of Y chromosome genes.
