XYY syndrome is a disorder in which a male infant is born with an extra Y chromosome.

About 1 in every 1,000 boys is born with XYY syndrome. Boys with XYY syndrome tend to be tall and have difficulties with language. The intelligence quotient (IQ) tends to be slightly lower than that of other family members. Learning disabilities, hyperactivity, attention-deficit disorder, and minor behavioral disorders can develop. XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved.

Last full review/revision April 2013 by Nina N. Powell-Hamilton, MD