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. The 
Wechsler Intelligence Scales assessed intellectual functioning; the ANT program 
evaluated cognitive function; and the CBCL assessed behavioral functioning. 
ANOVA was used for group comparisons. Hierarchical regressions assessed variance 
explained by each independent variable: parental education, timing of diagnosis, 
testosterone, age, and nationality. Parental education, timing of diagnosis, and 
hormonal treatment all played an important role in neurocognitive performance. 
The observed higher IQ and better attention regulation in the US group as 
compared to the NL group was observed with decreased levels of behavioral 
problems in the US group. Cognitive measures that were different between the NL 
and US groups, i.e., attention regulation and IQ scores, were also significantly 
influenced by external factors including timing of diagnosis, testosterone 
treatment, and parental education. On the ANT, a cognitive phenotype of 47,XXY 
was observed, with similar scores on 9 out of the 10 ANT subtests for the NL and 
US groups. This study lays additional features to the foundation for an 
algorithm linking external variables to outcome on various neurodevelopmental 
measures.