Almost 20% of young people with autism spectrum disorder (ASD) also were diagnosed with depression in a study that tracked participants into their 20s.

The findings also showed that individuals with autism who were higher functioning and free of intellectual disabilities were at greater risk of depression than those with more severe forms of the disorder, according to investigators led by Dheeraj Rai, MBBS, MRC PSYCH, PhD, of the University of Bristol in the United Kingdom.

“This work highlights the extent of the problem,” Rai told MD Magazine®. “Hopefully the results will raise awareness of the need for better services, and also for more research on how we measure depression and mental health problems in this population—and how we treat these problems.”

Although depression is a frequently occurring mental disorder, few longitudinal population-based studies have been conducted to examine its association with autism, Rai and his team wrote.

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To investigate, the team analyzed data from of 223,842 individuals in the Stockholm Youth Cohort, which includes all children and young adults (age range, 0-17 years) who lived in Stockholm County, Sweden, from 2001 to 2011 (n = 735,096). The data were updated in 2011, when the oldest participants were 27. A total of 4073 individuals were diagnosed with an autism spectrum disorder. Of those, 2927 had no intellectual disability and 1146 had an intellectual disability.

The study found that 19.8% of the individuals with autism had a history of depression by their mid-to-late 20s. That was more than triple the 6% of people with depression in the general population.

Non-autistic full siblings and half-siblings of individuals with ASD also had a higher risk of depression than the general public. However, compared with their non-autistic full siblings, individuals with ASD had more than a 2-fold risk of a depression diagnosis in young adulthood.

Rai said the results suggest that the risk of depression in people with autism may not solely be explained by factors siblings share.

“This could mean that depression in this population may be modifiable/preventable if the important environmental factors are identified and addressed,” he said.

For instance, bullying may be a modifiable factor. Researchers must identify such mechanisms and then study whether interventions can be designed to address them and whether such interventions lead to improved mental health in people with autism, Rai said.

Asked why depression was more common in individuals with autism, Rai said biological, psychological and social factors all may play a role.

“Biologically it has been suggested that autism and depression may share common genetic vulnerabilities,” he said.

As for why individuals with ASD who were not intellectually impaired were at greater risk for depression than their lower functioning peers, the authors speculated that higher functioning individuals might have a greater awareness of their circumstances.

“Knowledge that one is different and the problems relating to social exclusion, bullying, isolation, and navigating day-to-day life in environments that are not always receptive to autism-specific needs could be contributing factors,” Rai said.

Overall, the findings highlight the importance of paying more attention to mental health problems in people with a diagnosis of autism, the authors said.

“Co-occurring conditions in autism receive little attention,” Rai said. “The services, at least in European countries, mainly focus on diagnosis of autism rather than management of co-occurring conditions.”

The study, “Association Between Autism Spectrum Disorders With or Without Intellectual Disability and Depression in Young Adulthood,” is published in JAMA Network Open.