Autism, ADHD and Schizophrenia: Why Has Natural Selection Not Eliminated These Genetic Risks?
Neurodevelopmental and psychiatric conditions raise a longstanding question for researchers: if certain genetic predispositions increase the likelihood of significant difficulties throughout life, why do they persist across generations? Autism, attention-deficit/hyperactivity disorder (ADHD), schizophrenia and bipolar disorder all have important genetic components. Yet these conditions continue to occur in human populations.
At first glance, this seems to contradict the principle of natural selection. If certain genetic variants are associated with difficulties that may reduce reproductive success, their frequency might be expected to decline over time. Recent research using ancient DNA provides new insights into this evolutionary puzzle, although it does not offer a definitive explanation for every condition.
Complex conditions shaped by many genes
Genetics plays an important role in many neurodevelopmental and psychiatric conditions, but it does not determine their development on its own. In most cases, risk reflects the combined influence of numerous genetic variants, each often having a relatively small effect, alongside biological, developmental, and environmental factors.
This polygenic architecture explains why there is generally no single “autism gene” or one gene solely responsible for ADHD or schizophrenia. Some genetic variants may contribute to the risk of several conditions, while others are more closely associated with particular characteristics.
Large-scale genetic studies have identified overlaps between several psychiatric disorders. These shared influences do not mean that the conditions are identical. Each retains distinct clinical features, and individuals can experience very different developmental and clinical trajectories.
It is also important to distinguish heritability from individual risk. Heritability estimates the proportion of variation within a population that is statistically associated with genetic differences. High heritability does not mean that a condition is entirely genetic or inevitable for someone carrying certain variants.
Ancient DNA sheds light on recent human evolution
A study published in Nature in April 2026 analyzed genetic data from 15,836 individuals from West Eurasia, including 10,016 newly analyzed samples. The researchers developed a method to track changes in genetic variant frequencies over time and better distinguish natural selection from changes caused by migration or population structure.
Their findings indicate that natural selection continued to shape the human genome over the past 10,000 years. The researchers identified changes involving hundreds of genetic variants and observed shifts in genetic profiles associated today with several complex traits. These included a decrease in genetic predisposition to higher body fat and schizophrenia, alongside increases in certain genetic indicators associated with cognitive performance.
These findings are particularly valuable because they draw on ancient genetic data rather than relying solely on comparisons between contemporary populations. They demonstrate that human genetic evolution did not stop with the development of agriculture, culture or complex social organization.
An important limitation must nevertheless be emphasized: the researchers studied genetic profiles associated with particular traits in present-day populations. They did not retrospectively diagnose schizophrenia or other psychiatric conditions in the individuals whose ancient DNA was analyzed. The findings concern statistical genetic predispositions, not direct evidence of changes in the number of people who had a particular disorder.
Why do some genetic predispositions persist?
One possible explanation is the continuous emergence of new mutations. Even when a genetic variant becomes less common because of natural selection, new mutations can introduce further genetic variation associated with certain risks.
This possibility is particularly relevant to complex brain-related traits. Brain development and function involve numerous genes, creating many opportunities for genetic variation to influence cognition, behavior, and mental health. The effects of individual variants are often small, but their combined influence may contribute to changes in risk.
Natural selection therefore does not operate as a mechanism that can permanently eliminate every variant associated with a disadvantage. Its effects depend on the strength of that disadvantage, mutation rates, reproductive patterns, environmental conditions and interactions between genes.
Another important factor is that a genetic variant can influence several characteristics at once, a phenomenon known as pleiotropy. The consequences of a predisposition may therefore vary according to age, environment and a person’s wider genetic background.
Could some traits once have offered evolutionary advantages?
Some hypotheses suggest that characteristics now associated with certain disorders may have been tolerated or even favored in particular past environments. Traits linked to creativity, attention to detail or highly specific interests have sometimes been proposed as possible explanations for the persistence of certain genetic predispositions.
These ideas, however, require considerable caution. Reconstructing the psychological experiences of ancient populations is difficult, and it cannot be assumed that people who would receive a particular diagnosis today necessarily enjoyed an evolutionary advantage in the past.
The 2026 study adds an important qualification. The researchers identified negative selection acting on genetic predispositions associated with schizophrenia and bipolar disorder. This suggests that these predispositions may have been associated with reduced reproductive success over the period studied. However, it does not mean that every variant linked to these conditions was eliminated, or that all neurodevelopmental conditions followed the same evolutionary trajectory.
Findings concerning autism also require careful interpretation. The absence of a significant genetic shift for certain predispositions does not prove that autism, across all its forms, provided an adaptive advantage. It means only that the selection signal measured in this study was not equivalent to that observed for some other traits.
Natural selection does not follow modern definitions of health
Natural selection does not necessarily favor traits that improve well-being, adaptability or quality of life according to modern standards. It is influenced, among other things, by how individuals contribute to subsequent generations within a particular environment.
Factors that influenced reproductive success thousands of years ago may differ substantially from those affecting health and everyday functioning today. Social environments, lifestyles, educational systems and family structures have changed considerably. Findings about past genetic evolution therefore cannot be directly translated into conclusions about the lives of people today.
Furthermore, a genetic predisposition is not a destiny. An individual’s characteristics emerge from a complex combination of biological and environmental influences. Experiences vary widely, and people with the same diagnosis may have very different strengths, challenges and support needs.
What these findings mean for our understanding of mental health
Ancient DNA research offers new ways to investigate how natural selection has influenced human genetic characteristics. It shows that some predispositions have changed over millennia and that the brain, like other biological systems, remains subject to evolutionary processes.
However, these findings do not reduce autism, ADHD, schizophrenia or bipolar disorder to a simple story of natural selection. Nor do they justify ranking people according to their genetic characteristics or viewing these conditions as defects that evolution should eliminate.
The central question remains scientific: how do numerous genetic variants, biological mechanisms and environmental influences interact to produce such different outcomes? By combining present-day genetic data, ancient DNA and clinical research, scientists may gradually gain a clearer understanding of these processes.
The paradox, then, is not necessarily that natural selection has failed to eliminate these conditions. Rather, it reflects the complexity of human genetics, the continuous emergence of mutations and the fact that evolution does not operate as a process designed to remove every vulnerability. Recognizing this complexity can help foster a more scientifically accurate and less simplistic understanding of neurodevelopmental and psychiatric conditions.
