An international research team led by the University of Cambridge has found that autism diagnosed in early childhood differs genetically and developmentally from autism diagnosed later in life. The study, published in Nature, analyzed behavioral data from children and adolescents in the UK and Australia, as well as genetic data from more than 45,000 autistic individuals across Europe and the US.
The researchers discovered that children diagnosed with autism before age six often displayed behavioral difficulties such as problems with social interaction from an early age. In contrast, those diagnosed later were more likely to develop social and behavioral challenges during adolescence and had higher rates of mental health conditions including depression.
By linking genetic data to age at diagnosis, the team found significant differences between the genetic profiles of those diagnosed earlier versus later. There was only a modest overlap between these groups. The average genetic profile for later-diagnosed autism was closer to that seen in attention deficit hyperactivity disorder (ADHD) and other mental health conditions like depression and post-traumatic stress disorder (PTSD) than to early-diagnosed autism.
Researchers noted that lack of support in early childhood could contribute to increased vulnerability to mental health issues among those diagnosed later, potentially due to factors such as bullying prior to diagnosis. However, they emphasized that genetics also play a role: "Nevertheless, scientists say that the stronger genetic overlap between later-diagnosed autism and certain psychiatric disorders suggests there may be some genetic factors that partly increase the risk of mental health conditions among those diagnosed with autism later in life."
Lead author Xinhe Zhang from the University of Cambridge stated: "We found that, on average, individuals diagnosed with autism earlier and later in life follow different developmental pathways, and surprisingly have different underlying genetic profiles." Zhang added: "Our findings suggest that the timing of autism diagnosis reflects more than just differences in access to healthcare or awareness, important as these are. However, it is important to note that these are average differences on a gradient, so earlier and later diagnosed autism are not valid diagnostic terms."
The study focused on polygenic factors—sets of thousands of common genetic variants—which explained about 11% of variation in age at diagnosis.
Senior author Dr Varun Warrier said: "The term ‘autism’ likely describes multiple conditions. For the first time, we have found that earlier and later diagnosed autism have different underlying biological and developmental profiles." Warrier continued: "An important next step will be to understand the complex interaction between genetics and social factors that lead to poorer mental health outcomes among later-diagnosed autistic individuals."
The findings suggest implications for how autism is understood both clinically and socially. Genetic variation appears to influence when autistic traits emerge and how they are recognized. Warrier explained: "Some of the genetic influences predispose people to show autism traits from a very young age that may be more easily identified, leading to an earlier diagnosis. For others, genetic influences may alter which autism features emerge and when. Some of these children may have features that are not picked up by parents or caregivers until they cause significant distress in late childhood or adolescence."
He concluded: "Understanding how the features of autism emerge not just in early childhood but later in childhood and adolescence could help us recognise, diagnose, and support autistic people of all ages."