Autism and ADHD diagnoses have climbed sharply over the past two decades, stoking fears of a hidden epidemic. A new analysis from Sweden offers a different story: the traits that underlie these conditions appear far more stable than the headlines suggest. The disconnect points to changes in how we identify, label, and support neurodivergent people—not a sudden surge in underlying symptoms.
Drawing on data from nearly 10,000 twins, researchers tracked parent-reported autism and ADHD symptoms across nine consecutive birth cohorts and found little evidence that symptom levels in late adolescence rose in tandem with clinical diagnoses. The findings, published in Psychiatry Research, challenge simple narratives and raise practical questions for schools, clinicians, and policymakers.
What the Swedish twin study set out to test
Recorded rates of autism spectrum disorder and attention-deficit/hyperactivity disorder have increased in many countries, Sweden included. But are more young people actually experiencing more symptoms, or has the system changed in ways that bring more people into diagnostic pathways?
To probe that question, the team turned to the Child and Adolescent Twin Study in Sweden, an ongoing project that follows twins to better understand mental and physical health. For this investigation, researchers examined parent-reported symptoms when participants reached age 18, focusing on twins born across nine consecutive years from 1993 to 2001. Using the same questionnaires, at the same age, across adjacent cohorts allowed them to look for population-wide shifts in traits over time—without the confounding effects of changing tools or different life stages.
What they found: diagnoses up, traits largely steady
Autism-related traits did not increase. Average levels of parent-reported autism symptoms among 18-year-olds were stable across all nine birth years. The share of young people with the highest scores—those clustered at the top end of the trait distribution—also showed no statistically significant change. In other words, while clinical diagnoses of autism have risen, the underlying distribution of autism traits in the population appeared consistent.
ADHD traits told a more nuanced story. Among boys, both the average levels of parent-reported ADHD symptoms and the proportion of boys in the highest-scoring group were essentially unchanged across cohorts. Among girls, however, there was a small but statistically detectable increase in ADHD symptoms over time, visible in both average scores and in the upper tail of the distribution.
Crucially, the size of that increase was modest. Birth year explained only a very small fraction of the variation in girls’ ADHD scores. The effect was not nearly large enough to account for the dramatic rise in documented ADHD diagnoses among girls and young women reported in many clinical systems. The authors’ bottom line: shifting diagnoses are not being mirrored by a wholesale shift in symptoms.
Why diagnoses can rise even when traits don’t
If the population hasn’t changed much, why do diagnosis rates keep climbing? The study can’t assign causes, but it adds weight to several plausible drivers that have reshaped the diagnostic landscape over the past 20 years.
Broader criteria and better screening
Clinical definitions have evolved. In 2013, the DSM-5 integrated several autism subtypes into a single spectrum and updated criteria for ADHD, including raising the age by which some symptoms must appear and refining how impairment is assessed across settings. Alongside those changes, routine screening in pediatric and school settings expanded. More screens mean more referrals, especially for people whose difficulties were previously overlooked.
Reduced stigma and greater health-seeking
Public understanding of neurodevelopmental conditions has grown, and stigma has eased—significantly in some communities. Families who once hesitated to pursue an assessment are more likely to do so now, and young adults themselves are more willing to seek answers for lifelong challenges. As acceptance rises, the gap between lived experience and formal recognition narrows.
Recognition of diverse presentations
Autism and ADHD do not present the same way for everyone. Girls and women, for example, are more likely to show inattentive ADHD symptoms and to “mask” social or sensory differences, which can delay recognition. As clinicians, educators, and families learn to spot less stereotyped patterns, more people—particularly girls and young women—come into view.
Systems and incentives
In many countries, diagnoses unlock access to accommodations, therapies, and educational supports. As these supports become better known and more systematically available, obtaining a formal diagnosis becomes a practical step toward help. In parallel, the growth of private assessment pathways has shortened waits for some and broadened access—though it has also prompted concerns about variability in evaluation quality. The net effect is more diagnoses, even if the underlying traits are relatively stable.
Strengths, caveats, and what comes next
The study’s design is a major strength. By using the same parent-report measures at age 18 across consecutive birth cohorts, the researchers reduced the risk that shifting tools or ages would muddy the trends. The large sample of twins drawn from the same national context adds statistical power and consistency.
There are limits worth noting. Parent reports capture behavior as seen at home and may miss context-dependent challenges; they are not clinical diagnoses. The cohorts come from Sweden, a country with particular healthcare structures and social supports, so patterns may differ elsewhere. And while the slight rise in girls’ ADHD traits is intriguing, the study doesn’t explain why it’s happening.
Even so, the data clarify one key point: it is difficult to argue that a genuine population-level surge in symptoms is driving the surge in diagnoses. The more likely story is that systems of detection and recognition have changed faster than the traits themselves.
Practical implications for schools, clinics, and policy
For educators, the message is to keep calibrating supports to individual needs rather than labels alone. If more students are being recognized—not necessarily newly impaired—then classrooms should continue to emphasize flexible instruction, predictable routines, and sensory- and attention-friendly environments that benefit many learners.
For clinicians, the findings reinforce the value of robust, multi-informant assessments that look beyond checklists. That includes developmental history, observation across settings, and consideration of conditions that can overlap with or mimic autism and ADHD—such as anxiety, language disorders, sleep problems, or trauma-related difficulties. Better recognition of diverse presentations, especially in girls and gender-diverse youth, can reduce both underdiagnosis and misdiagnosis.
For policymakers and planners, the disconnect between symptoms and diagnoses is a budgeting signal. Demand for services is rising, regardless of whether population traits have changed, and systems must keep pace. That can mean investing in school-based supports, expanding evidence-based behavioral and occupational therapies, and ensuring equitable access to high-quality assessments—while also monitoring evaluation standards to guard against inconsistent practices.
A reframed narrative
It is tempting to describe rising diagnosis curves as proof of an unfolding epidemic. This study suggests a different narrative: we have widened our lens, reduced stigma, and gotten better at matching names and supports to longstanding human differences. The traits were there all along; our systems are finally finding more of the people who have them.
That reframing matters. It replaces alarm with pragmatism—focus on meeting needs, improving assessment quality, and making supports accessible across the lifespan. And it invites curiosity where fear once stood: if traits are steady but recognition is rising, how can we ensure that recognition translates into meaningful help?
The answer will not come from a single dataset. But Sweden’s twin cohorts offer a clear signal through the noise: diagnoses may be rising, yet the population’s underlying profile is remarkably constant. As health and education systems continue to evolve, keeping that distinction in mind can help steer resources—and public conversation—toward what works.