Lexicon of Madness -- XYY Syndrome

 

XYY Syndrome: The result of a mutation marked by the presence of an extra male (Y) sex chromosome. These men tend to be taller (generally over six feet), have lower IQs, and have histories of severe acne. In a study of 4,139 Danish men over six feet tall, 12 were found to be XYY (which is twice the 1-in-1,000 rate seen in the general population). Five of those 12 (41.7%) had criminal histories for non-violent offenses, versus only 9.3% in a normal XY control group. In other words, men with the XYY chromosomal mutation were four times more likely than XY men to have convictions for property crimes. Because XYY is the result of a genetic mutation, it is not a heritable condition; therefore, this finding does not demonstrate the genetic heritability of criminal behavior -- twin studies are needed for that. Studies of the concordance rates of criminal behavior among monozygotic (MZ; identical) and dizygotic (DZ; fraternal) twins consistently show significantly greater concordance rates among MZ twins than among DZ twins. If you are a male, and your identical twin is a criminal, the chance that you are also a criminal is about 50%. If your fraternal twin is a criminal, the chance that you are also a criminal is only about 20%. Studies such as these generate a heritability estimate for criminal behavior of about .60, which suggests that more than half of the variability in law-abiding/criminal behavior in a population can be attributed to genetic, not environmental, differences.