The Council for Exceptional Children 1110 N. Glebe Rd.
Boys are typically more severely affected than girls.
Emotional and behavioral problems are common in both sexes. Fragile X syndrome is the single most common inherited cause of mental impairment. Current estimates of its prevalence vary, but some experts believe that fragile X affects at least 1 in 1000 males and females of all races and ethnic groups. The discrepancy in these numbers is due to the fact that large-scale population studies of the incidence of fragile X have not yet been done. In either case, fragile X is one of the most common genetic diseases in humans.