Purpose: The current study addresses a gap in the literature regarding syntactic development of adolescent boys with fragile X syndrome (FXS) and Down syndrome (DS). Specifically, we ask whether syntactic skills plateau or continue to change during adolescence for these groups and whether the profile of syntactic change differs between boys with…

Background: Children with Down syndrome (DS) are at risk for autism spectrum disorder (ASD). They are often diagnosed later than other children in part due to difficulty differentiating ASD-related impairment from that associated with DS. The psychometric properties of the Pervasive Developmental Disorder in Mental Retardation Scale (PDD-MRS) were…

Background: Fragile X syndrome (FXS) and Down syndrome (DS) are the two leading genetic causes of intellectual disability, and FXS is the most common known genetic condition associated with autism. Both FXS and DS are associated with significant language impairment, but little is known about expressive language across domains over time or the role…

Background: A population sample of people with Down Syndrome, repeatedly studied since infancy, has now been followed up at the age of 45 years. The paper is intended to give an overview of their abilities, as represented by the results of psychological tests, over their life span to date. Methods: As at all previous occasions from age 30 onwards,…

Purpose: To compare the phonological accuracy and speech intelligibility of boys with fragile X syndrome with autism spectrum disorder (FXS-ASD), fragile X syndrome only (FXS-O), Down syndrome (DS), and typically developing (TD) boys. Method: Participants were 32 boys with FXS-O (3-14 years), 31 with FXS-ASD (5-15 years), 34 with DS (4-16 years),…

Background: Longitudinal studies show that in the general population IQ declines with age: early and rapidly in the case of performance IQ, later and more slowly in the case of verbal IQ. These populations have not apparently included people with intellectual disabilities (ID). Method: A literature search identified 11 studies, some…