Autism without intellectual impairments prevalence

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More than half of autistic people in the United States have an average or above-average intelligence quotient (IQ), an uptick over previous estimates, a new longitudinal study of children in Minnesota suggests.

The rise could reflect a heightened awareness and understanding of the condition, as well as improvements in recognizing and detecting it, says lead investigator Maja Katusic, a developmental pediatrician at the Mayo Clinic in Rochester, Minnesota.

In 2016, the proportion of autistic children in the U.S. with an average or higher IQ was 42 percent, according to the most recent autism prevalence data published by the Centers for Disease and Control and Prevention (CDC) in 2020.

The new study, however, suggests that this number could be as high as 59 percent.

Katusic and her colleagues scoured a trove of medical and school records from more than 30,000 people born between 1976 and 2000 in Olmsted County, Minnesota. They identified people with autism based on behavioral assessments and descriptions in the records, using either an inclusive or narrow definition of the condition in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV). The inclusive definition flagged children who met criteria for autistic disorder, Asperger syndrome or pervasive developmental disorder-not otherwise specified, whereas the narrow definition included only those with autistic disorder. The researchers also tallied clinical autism diagnoses documented in the records.

Of the 890 individuals for whom the records contained IQ data and who met the inclusive definition of autism, 59 percent had an IQ score that was average or higher — defined in the study as 86 or above. The same was true for 51 percent of the 453 people who met the narrow definition. By contrast, only 43 percent of the 187 people with a clinical diagnosis had an average or higher IQ score. The study appeared in November in Pediatrics.

The results suggest that autistic people with a below-average IQ may be more easily recognized by clinicians — and that those with higher IQs are more likely overlooked and may be missing out on services that would benefit them.

A greater proportion of autistic boys had an average or higher IQ than did autistic girls across all three diagnostic groups. Autistic girls tend to be diagnosed later in life and to hide their traits, Katusic said, which could help explain this gender gap.

The findings come with some important limitations. For example, more than 90 percent of the participants are white and highly educated, meaning that the findings are not necessarily generalizable to the wider population.

Also, IQ is important, Katusic says, but it does not capture autistic people’s ‘adaptive skills’: practical, day-to-day abilities such as taking care of oneself, taking part in activities and generally being able to function in a community.