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Based on experts who studied genetic data from more than 45,000 autistic individuals in Europe and the US, autism must not be viewed as a unified disorder with a common root origin.

The international research revealed that those identified with autism early in childhood, usually before six years old, tend to display a different biological makeup than those identified later.

Key Findings from the Research

• Younger-diagnosed individuals tended to show social difficulties from early childhood, such as difficulties in social interaction, but stay consistent over time.
• Individuals identified with autism at an older age, often after age 10, tended to develop growing behavioral and social difficulties during teenage years and additionally showed an higher risk of psychological conditions such as depression.

"The term ‘autism’ likely refers to several disorders," stated the lead researcher, principal investigator of the study. "Initially, we have discovered that early and late identified autism have distinct underlying and growth-related characteristics."

Not Two Categories, But a Spectrum

The experts are not recommending a shift to two identification groups, explaining that such a step could be counterproductive for the numerous who are in between.

"It represents a gradient," Warrier said. "There are also many additional elements that influence age of diagnosis, so the moment you go from general trends to anything that is relevant to an person, it's misleading to compare."

Rising Autism Spectrum Identification Rates

These results come at a time when autism identification has risen steeply, with a almost 800% rise in cases in the UK between 1998 and 2018.

Experts explain this is mainly because of a broadening of the diagnostic criteria and increased awareness of the disorder.

Variability and Categories in Autism

Furthermore, while autism is characterized by having challenges with interacting with others, responding to senses, and restrictive actions, there is significant diversity in how these difficulties appear between individuals.

Researchers have been investigating whether the group forms subgroups, with shared characteristics or paths, that could make studying autism more tractable.

Study Approach and Biological Findings

This latest study analyzed observational data from several birth cohorts and genetic data from two extensive investigations, with more than 45,000 participants.

Previously, it was commonly assumed that those identified earlier tended to be those with more marked autistic traits, supported by people carrying a higher amount of autism-linked gene variants.

However, the latest study uncovered a different trend.

Distinct Genetic Patterns and Developmental Trajectories

This analysis found that the underlying genetic patterns differed between those identified with autism earlier and later in life, with only a small similarity.

Their average biological pattern of later-diagnosed autism is more similar to that of ADHD, as well as to mental health conditions like anxiety and PTSD, than it is to autism identified in early childhood.

• Those identified prior to the age of six years were more likely to be delayed to walk and have trouble understanding hand gestures and often experience social and communication difficulties that appeared early but stayed stable.
• Individuals identified after the age of 10 years were more likely to experience an increase in challenges during adolescence and, by late adolescence, presented with more significant challenges.

"This gives me hope that additional subgroups will be identified, and every one will find an appropriate diagnostic label," commented Prof Uta Frith, who was not involved in the research. "It is time to realize that ‘autism’ has become a ragbag of various disorders."