Our findings support the notion that those with co-morbid FXS and autism represent a distinct subtype of FXS, with more impairment in receptive language and theory of mind even when controlling for their lower non-verbal IQ relative to those with only FXS. The greater cognitive impairments observed in those with co-morbid FXS and autism continues into adolescence and young adulthood; and the autism seen in those with FXS appears to be the same as idiopathic autism.

We investigated the receptive language of adolescents and young adults with Down syndrome (n = 25) or fragile X syndrome (n = 19). We were interested in syndrome differences and gender differences within fragile X. Comparison of the syndromes and MA-matched typically developing children (n = 24) revealed that individuals with the syndromes differed in relative achievements across the domains of receptive vocabulary, receptive syntax, and nonverbal cognition as well as in the organization of their linguistic knowledge. Comparison of males and females with fragile X revealed that each displayed synchronous development across the three domains, despite the fact that the receptive language levels of females surpassed that of males.

Fragile X syndrome (FXS) is the leading inherited cause of intellectual disability. The syndrome is caused by a single gene mutation on the X chromosome. Although individual differences are large, most individuals with FXS display weaknesses across all language and literacy domains compared to peers of the same chronological age with typical cognitive and language development. Expressive, receptive, and pragmatic language abilities as well as literacy skills are similar to those of younger, typically developing peers at similar cognitive and language developmental levels, although there are areas in which impairments exceed developmental-level expectations. One area of special impairment is the higher occurrence of repetition in the language of individuals with FXS compared to developmentally matched peers. In this paper, we review the behavioral, language, and literacy characteristics of individuals with FXS and discuss potential clinical implications.

We consider possible causes of these maternal differences, the implications for clinical practice, needs for future research, and the importance of understanding child and contextual factors as well as the dynamics leading to these differences.