Early development, temperament, and functional impairment in autism and fragile X syndrome.

We compared the developmental status, functional abilities, and temperament of 31 young boys with fragile X syndrome (FXS) who did not have autism, matched on chronological age, gender, and race, with 31 boys with autism but no FXS. Children with autism exhibited a more variable profile of development in comparison with a relatively flat profile for children with FXS. Children with autism were significantly more delayed in social skills and were rated by observers as exhibiting a greater degree of impairment in cognitive, communication, and social skills. On temperament ratings, both groups were slower to adapt, less persistent, and more withdrawing than the reference group. Boys with FXS were rated as more active than the referent group, whereas boys with autism were rated as less intense, more distractible, having a higher threshold for response, and less rhythmic than the reference group. A smaller three-group analysis compared boys with FXS, boys with autism, and boys with both FXS and autism. Children with both autism and FXS were substantially more delayed than children with autism or FXS alone.