The XYY syndrome is a disorder in which a male infant is born with an extra Y chromosome.
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. The XYY syndrome was once thought to cause aggressive or violent criminal behavior, but this theory has been disproved.
Resource: Merck Manual Home Edition