An international team of researchers led by the University of Cambridge has found that autism diagnosed early in life looks different from autism diagnosed later in childhood or adolescence—both in development and in genetics.
This challenges the idea that autism is a single condition with one underlying cause.
The study, published in Nature, analyzed behavioral data from children and teenagers in the UK and Australia, as well as genetic data from more than 45,000 autistic individuals across Europe and the U.S.
The researchers discovered two broad patterns. Children diagnosed with autism before the age of six were more likely to show clear difficulties in social interaction and communication from infancy.
In contrast, those diagnosed later in childhood or adolescence tended to develop social and behavioral difficulties as they grew older. This later-diagnosed group also had higher rates of conditions like ADHD, depression, and PTSD.
When the scientists compared the genetic data, they found that the underlying genetic profiles of early- and late-diagnosed autism were different, with only a modest overlap. In fact, the genetic profile of later-diagnosed autism was closer to that of ADHD and some mental health conditions than to autism diagnosed early in life.
The study also noted that a lack of support in early childhood may increase the risk of mental health issues for people diagnosed later, such as by making them more vulnerable to bullying. Still, the genetic overlap suggests that some of the risk for these mental health conditions is inherited.
Lead author Xinhe Zhang explained that the timing of an autism diagnosis reflects more than just access to healthcare or awareness. It may also show differences in biology and development. However, she emphasized that early- and late-diagnosed autism are not official diagnostic categories but rather patterns seen on a spectrum.
The researchers studied “polygenic” factors—thousands of small genetic differences that together influence traits. They found that these genetic factors explained about 11% of the differences in the age at autism diagnosis.
Senior author Dr. Varun Warrier said that “autism” likely describes multiple conditions. This study is the first to show that earlier- and later-diagnosed autism have different biological and developmental profiles. The next step is to study how genetics and social factors interact to influence mental health outcomes for later-diagnosed individuals.
The findings suggest that autism traits can emerge at different times in life depending on a person’s genetic makeup. For some children, traits are obvious in infancy and lead to early diagnosis. For others, traits emerge later and may not be noticed until they cause significant distress in adolescence.
Understanding how autism features appear not just in early childhood but also in later stages could improve how we identify, diagnose, and support autistic people of all ages.
The study is published in Autism.
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