International investigation of neurocognitive and behavioral phenotype in 47,XXY (Klinefelter syndrome): Predicting individual differences.

47,XXY (KS) occurs in 1:650 male births, though less than 25% are ever identified. We assessed stability of neurocognitive features across diverse populations and quantified factors mediating outcome. Forty-four boys from the Netherlands (NL) and 54 boys from the United States (US) participated.