The effect of additional Y chromosomes is less clear. Although cases have been reported in which ultrasound and postmortem data linked additional Y chromosomes with brain malformations, several of the available case studies did not detect any differences. There is some controversy regarding whether head size is significantly larger in 47,XYY. This appears particularly interesting given the increased risk for pervasive developmental disorders such as Autism, which are also associated with increased brain size [Nicolson et al., 1998; Courchesne et al., 2007].
