The genetic and environmental factors influencing cognitive outcomes in 120 children (80 boys and 40 girls) with the fragile X full mutation and their unaffected siblings were determined by in-home evaluations and reported from the Departments of Psychiatry and Pediatrics, Stanford University, CA. The MPIQ (mean full-scale IQ of biological parents) was the primary determinant of the IQ of comparison siblings, and the quality of the home environment had an additional but small effect on verbal development. Using multiple regression analyses, cognitive outcome of girls with fragile X was correlated with the mean IQ. of their parents, and to a lesser degree with the quality of home environment. FMR 1 protein % (FMRP) was correlated with girls’ levels of distractibility. Girls with fragile X had higher IQs than boys, with relatively stronger Verbal Scales. The affected boys’ Performance IQ was related to the mean parental IQ, while the boys’ Full Scale IQ was correlated with FMRP %. The quality of boys’ home environments affected their cognitive outcomes more so than in affected girls. 
COMMENT. Both genetic/parental and environmental factors are significant in the prediction of cognitive outcome of children with fragile X syndrome. The specific factors influencing IQ are different in girls and boys.
Brain anatomy in fragile X syndrome. MRI scans and cognitive testing were performed in 37 children and adolescents with fragile X syndrome at Stanford University. Decreases in grey matter and increases in white matter were age- and gender-related. Caudate and ventricular CSF volumes were significantly enlarged, and caudate volumes decreased with age. IQ scores and volumes of cortical and subcortical grey matter were not significantly correlated, but were different from the correlations observed in normal children.