Written by Pam MS, NCSP | Fact checked by Psychology Dictionary staff 

a chromosomal anomaly found in 1961 and correlated to males who demonstrated aggressive or violent behavior in criminal facilities. It was initially presumed that the extra Y chromosome set males up for these actions, but the theory was changed as XYY anomalies were later discovered in normal males. Commonly referred to as double-Y condition.

XYY SYNDROME: "XYY syndrome is when a male has two Y chromosomes. "

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