Why Humans Are More Prone to Autism Than Other Species
Human brain evolution may explain why autism is more common in our species than in other primates.
Humans have a higher rate of autism than any other species, but what makes us uniquely susceptible?
A new study, analyzing gene activity in neurons from humans and other primates, found that rapid evolution in specific cortical neurons coincided with changes in autism-associated genes.
“Our results suggest that some of the same genetic changes that make the human brain unique also made humans more neurodiverse,” said lead author Alexander L. Starr, a doctoral candidate at Stanford University.
Autism and related traits are uniquely human
In the US, ~1 in 31 children (3.2%) were identified with autism in a 2022 sample. Globally, the World Health Organization estimates ~1 in 100 children have autism. Autism affects language, social cognition and complex behavior – traits also involved in schizophrenia.
Many researchers have suspected that both autism and schizophrenia are either unique to humans or much more common in our species than in other primates. Most cortical neuronal types are conserved across mammals; however, subtle differences in gene expression or connectivity may underpin uniquely human cognition.
Comparative genomics has linked human-accelerated regions (HARs), parts of the genome that changed rapidly in our lineage, and synaptic gene expression shifts to autism and other neurodevelopmental problems.
Why some neuron types evolve faster and whether this relates to neurodevelopmental disorders remains unclear.
The new study asks whether a general principle explains why some neuronal types evolve faster, and whether human evolution produced unusual shifts in neurons carrying autism-linked genes.
Autism-related genes show signs of positive selection
The study looked at gene activity in over a million neurons drawn from six species: humans, chimpanzees, gorillas, macaques, marmosets and mice. The researchers used single-nucleus RNA sequencing data from three brain areas – the medial temporal gyrus, dorsolateral prefrontal cortex and motor cortex. These regions are tied to memory, decision-making and movement, and are believed to play roles in higher cognition.
The more common a type of neuron was, the slower it tended to evolve in terms of gene expression; this correlation was consistent across brain regions and species.
Humans, however, stood out. The most common cortical neurons – layer 2/3 intratelencephalic neurons – showed rapid divergence in our lineage compared to other primates. These cells connect distant parts of the cortex and are linked to abilities such as abstract thought and communication. Instead of being highly conserved, they had shifted dramatically.
The team found strong evidence of downregulation of genes tied to autism. This trend appeared in multiple datasets and was confirmed using human–chimpanzee hybrid brain organoids, which allow direct comparisons under identical cellular conditions. The pattern pointed to polygenic positive selection, rather than drift or relaxed constraint.
Polygenic positive selection
Polygenic positive selection occurs when natural selection favors small changes across many genes at once, rather than a single gene, leading to evolutionary adaptation in complex traits.
Some schizophrenia-related genes followed the same path.
Human-specific brain evolution may explain autism prevalence
The results suggest that the same evolutionary changes that gave humans a larger cognitive range may have also increased the risk of autism.
“The exceptionally high prevalence of autism in humans may be a direct result of natural selection,” said the authors.
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The team argues that selection may have favored traits such as slower brain development after birth or greater capacity for language, with the side effect of higher autism prevalence. This fits a long-standing idea: that human evolution for advanced cognition came with trade-offs in vulnerability to neurodevelopmental conditions.
The study provides some of the strongest evidence yet that positive selection on human-specific traits can also raise disease risk.
At the same time, the authors are cautious: “Our results come with important caveats. As with most correlations, causality is not implied,” they note. The sample sizes for non-human primates were limited, and organoid models only approximate real brain development.
Looking ahead, the team points to larger cross-species datasets, deeper sequencing and extending the analysis to other brain regions.
“Although our results strongly suggest natural selection for down-regulation of ASD-linked genes, the reason why this conferred fitness benefits to our ancestors remains an open question.”
Reference: Starr AL, Fraser HB. A general principle of neuronal evolution reveals a human-accelerated neuron type potentially underlying the high prevalence of autism in humans. Mol Biol Evol. 2025;42(9):msaf189. doi: 10.1093/molbev/msaf189
This article is a rework of a press release issued by Oxford University Press USA. Material has been edited for length and content.
