The proportion of intellectual disability in autism spectrum disorder over two decades

The registered prevalence of autism has increased substantially over the past two decades, potentially impacting the co-occurrence of diagnosed intellectual disability (ID) in autism. Thus, the aim of this study was to investigate the proportion of ID in individuals with autism over two decades (2001-2020).

Data for this study was retrieved for individuals born between 1988-2016 from Swedish population based national registers. A total of 3,045,819 individuals were eligible for inclusion in the current study, out of whom 81,286 had autism assigned 2001-2020. The annual proportion of ID in autism was calculated across year of autism diagnosis and the Cochrane Armitage trend tests were conducted.

From 2001-2020 the prevalence of autism increased by 800%. Among those with autism, 13,403 also had an ID diagnosis, resulting in an overall proportion of ID in autism of 16.5%. The proportion of ID in autism decreased from 55.8% in 2001 to 6.7% in 2020, with a comparable decrease in females and males. A similar trend was discerned when limiting the sample to those diagnosed with autism before age 13 and 7 years.

The results of this study indicate less congruity between the autism phenotype observed today and earlier conceptualizations of autism, which often included intellectual disability (ID). The findings suggest that changes in diagnostic criteria during this period contributed to a broadening of the ways in which individuals could meet criteria for autism, encompassing a wider range of cognitive abilities.