The likelihood that an autistic person has another condition correlates strongly with the age at which they received their autism diagnosis. Also, autistic girls are more likely than autistic boys to have another condition, according to a new study.

The study assessed whether an autistic person’s age, age of diagnosis or birth sex changed their chance of having any of 11 commonly co-occurring conditions, including epilepsy, anxiety and attention deficit hyperactivity disorder (ADHD). It drew on data from roughly 16,000 people with autism and more than 650,000 people without autism up to 16 years old in the Danish National Patient Registry, a large dataset that records the birth date and sex of people in the Danish hospital system and all diagnoses they receive.

Among people diagnosed late, at 11 to 15 years old, 26 percent of girls and 13 percent of boys were also diagnosed with an affective disorder, the study found. An opposite trend occurred for intellectual disability; 40 percent of people with an early autism diagnosis had an intellectual disability, compared with just 10 percent of people with a late autism diagnosis.

The findings may suggest that young men and women are not diagnosed with autism unless they also have another condition, the researchers say. Still, they urge caution when interpreting the results.

“We consider several possible explanations for this finding, but with our current data we cannot say with certainty which ones are true,” says study investigator Eya-Mist Rødgaard, a research assistant at the University of Copenhagen, who collaborated with Laurent Mottron, professor of psychiatry at the University of Montreal in Canada.

Perhaps the diverse nature of autism is partly explained by age, such that different ‘types’ of autism appear at different stages in life, Rødgaard says. Or “symptoms of psychiatric conditions, such as anxiety or ADHD, make it more likely that a child is also assessed for autism, potentially causing autism to commonly be diagnosed during the age period where a given comorbid condition would be more likely to develop.”

The data may also reflect uncertainty in how autism is diagnosed. Because autism and co-occurring conditions, such as anxiety, are often diagnosed close in time, it may suggest that there's no clear boundary delineating where autism ends and those other conditions begin, Mottron says. "Is autism a natural category or not? That's not solved."

For all of the 11 co-occurring conditions considered in the paper, the age of autism diagnosis was the single biggest predictor for whether a participant had that condition. But sex was another major factor for many of the conditions.

“In psychiatry, you have some conditions that appear much more frequently in boys than in girls, such as conduct disorders,” Mottron says. “But amazingly, when these conditions are comorbid with autism, the odds ratios reverse. It means that conditions that are more frequent in boys than in girls in the general population, in autism it goes completely the other way around.”

Among autistic individuals, girls were 2.2 times more likely to have anxiety than boys, the study found. By contrast, anxiety is about 1.4 times higher in non-autistic girls than in non-autistic boys. And whereas non-autistic boys are 2.6 times more likely to have ADHD than non-autistic girls, that ratio dropped within the autism population. Autistic boys are just 1.6 times more likely than autistic girls to have ADHD. The findings appeared in Acta Psychiatrica Scandinavica in July.

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