The effect of age on language in adults with intellectual disabilities: A comparison of passives in Down syndrome and Williams syndrome

Individuals with intellectual disabilities often show limitations in language functioning, commonly linked to their overall poor cognitive skills. However, despite the intellectual impairments and language delays, it is well established that language is more vulnerable in some populations, e.g. Down syndrome (DS), and relatively preserved in others, e.g. Williams syndrome (WS). Individuals with DS are also known to be at increased risk of cognitive decline due to the earlier onset of Alzheimer’s dementia, although little is known about how aging affects language skills in this population. Individuals with WS, though with relatively developed language, are reported to never acquire some grammatical structures that appear late in typically developing (TD) individuals, such as passives of psychological verbs. In an attempt to better understand how linguistic deficits in individuals with intellectual disabilities can be teased apart from the effects of general language delays, chronological age, and overall intellectual impairment, we compare the comprehension of passives in adults with DS (mean age: 38) and WS (mean age: 30). Passives are known to develop late in typical development and present difficulties for individuals with developmental disorders. This has been observed especially in their generally poorer performance on passives of psychological verbs than on passives of actional verbs. Our results reveal divergent patterns of performance in our participants. Adults with WS performed no different from younger TD controls on actives and passives of both actional and psychological verbs. In contrast, adults with DS showed exceptionally poor performance on all sentence types, even on actives of actional verbs, considerably poorer than observed in the TD and WS groups. While the good performance of adults with WS might be due to individual variation, rather than continuous language development, we argue that the poor performance of participants with DS is due to an age-related decline of cognitive and language abilities, possibly linked to Alzheimer’s-type dementia.