For decades, autism has been understood as a condition that reveals itself in the first few years of life, with clinicians looking for tell-tale difficulties in toddlers. Yet a growing number of people today are diagnosed only in adolescence or adulthood, often after years of struggling in school or with relationships. A new Nature study asks whether these cases were simply missed earlier or reflect something biologically distinct.

To investigate this question, an international team of researchers led by Varun Warrier, associate professor at University of Cambridge, combined two streams of evidence. From four long-term studies that tracked groups of children from birth to adolescence, they examined how children's behaviour unfolded using structured behavioural questionnaires. In parallel, they analysed genetic information from nearly 50,000 autistic individuals -- one of the largest such datasets assembled to date.

The researchers tested two possibilities in particular. The first, which they called the "unitary model", proposed that autism has the same genetic roots regardless of when it is diagnosed, with later cases reflecting subtler traits that were simply overlooked earlier. The second, called the "developmental model", suggested that earlier- and later-diagnosed autism may follow partly distinct genetic and developmental pathways. In other words -- there could be more than one kind of trajectory into autism.

Two pathways

The behavioural data from the long-term studies supported the latter model. One group showed difficulties in social interactions, communication, and behaviour that were evident earlier in life and persisted into adulthood. These children were more often diagnosed in preschool or primary school. Another group showed far fewer difficulties at first, although these became more pronounced in adolescence, particularly as schoolwork and friendships became more demanding. These children tended to be diagnosed later in life.

The group diagnosed later also showed genetic links to higher educational attainment, which the researchers suggested could help explain why early difficulties are less apparent in some children.

These findings challenge the idea that autism diagnosed later is simply missed earlier. Instead, they suggest there may be a second pathway in which difficulties emerge more strongly only in adolescence -- a distinction with both biological and social roots.

Fingerprints of timing

The genetic analysis revealed two partly distinct patterns of genetic variants across many people. One was linked more closely to earlier diagnoses and was associated with pronounced social and communication difficulties in early life but showed only weak genetic ties to conditions such as attention-deficit/hyperactivity disorder (ADHD) or depression. The other, linked to later diagnoses, carried stronger associations with ADHD, post-traumatic stress disorder, depression, and self-harm.

The two profiles were only partly overlapping, suggesting that while they shared some roots, they weren't identical.

"It could be a missed diagnosis -- there are many factors associated with later diagnosis, and genetics explains only about 10% of the variation in age at autism diagnosis," said Varun Warrier, assistant professor at Cambridge University and the study's corresponding author. "But what it shows is that there are two different underlying genetic causes."

He added that because genetics accounted for only a small share of when autism is recognised, social and environmental factors play a larger role in shaping who gets diagnosed and at what stage.

When challenges arise

Sally J. Rogers, a professor at the University of California, Davis, said the field has been aware of differences in the age of onset.

"The field has known for a long time that the times of onset or recognition vary in autism, so the paper is helpful in understanding one biological aspect to age of onset differences, along with other variables at play," she said. "At any time children, or adults for that matter, are having difficulty coping with the challenges of everyday life, the next steps involve understanding the nature of those challenges and providing interventions appropriate to their difficulties and strengths."

In practice, this means conducting an assessment for autism followed by tailored support, regardless of whether one's difficulties have surfaced at a young or later ages.

This said, adolescents who receive a diagnosis later often struggle with mental health problems that can compound their challenges. "We need urgent support for co-occurring mental health conditions as this can have a huge impact on quality of life," Dr. Warrier said. The challenge for health systems is to recognise these vulnerabilities early and provide integrated care that addresses both autism and its common companions in mental health.

Life beyond diagnosis

Patricia Howlin, a professor of clinical child psychology at the Institute of Psychiatry in London, noted that findings from long-term studies resonate with what clinicians see in practice: that autism can unfold in different ways across development. "Later diagnosis is much more common among individuals whose early symptoms are subtle or atypical," she said.

In the past, many autistic girls have gone unrecognised because diagnostic criteria were largely derived from early-onset autism in boys.

These clinical realities can blur the boundary between autism that actually emerges later and autism that's recognised late. And when the individuals' difficulties are overlooked, they may miss out on vital educational and social support. By adolescence, their problems can be compounded by anxiety, depression, or social isolation. Ultimately, in adulthood, the individual with undiagnosed autistic may be misdiagnosed with other psychiatric disorders and deprived of appropriate help. "Social, educational, and health systems need to be much more aware of the different trajectories of autism," Dr. Howlin added. "Lack of typical early symptoms does not exclude a later diagnosis."

Like much of large-scale genetic research, this study is also based mainly on people of European descent, which limits how directly its findings can be applied to other regions, including India.

"Who receives a diagnosis and when they receive it differs substantially across cultures," Dr. Warrier said. "I'd be very surprised if we see more people being diagnosed as autistic in adolescence than in childhood in India, possibly because of different social and cultural norms." He added that priorities, presumably for researchers and policymakers, must include raising awareness, reducing stigma, and developing tests and checklists that work well in local languages and cultural settings.

Taken together, the new study underscores that autism is not a single story but many. Some pathways emerge in early childhood and others in adolescence, and each one carries partly distinct genetic patterns. As Dr. Warrier put it, autism is best seen as "multiple spectra with possibly different biological and social pathways." For families, clinicians, and policymakers, acknowledging this diversity may be the first step towards better support across their loved ones' lives.

Anirban Mukhopadhyay is a geneticist by training and science communicator from New Delhi.

New study finds different roots for early and late autism diagnoses