Autism genes may have been conserved during human evolution because they make us smarter, say scientists.
More inherited genetic variants linked to autism have been naturally selected than would be expected by chance, a study has shown.
The same variants were associated with traits linked to brain performance, such as molecular functions involved in the creation of new neurons.
Lead researcher Dr Renato Polimanti, from Yale School of Medicine in the US, said: "We found a strong positive signal that, along with autism spectrum disorder, these variants are also associated with intellectual achievement."
Under the laws of natural selection outlined by Charles Darwin, evolutionary variants that have a negative impact on reproductive success are quickly eliminated from a population.
But those providing a better chance of survival tend to remain for generation after generation, if their advantages outweigh their adverse effects.
Study co-author Professor Joel Gelernter, from Yale University, said: "It might be difficult to imagine why the large number of gene variants that together give rise to traits like ASD (autism spectrum disorder) are retained in human populations. Why aren't they just eliminated by evolution?.
"The idea is that during evolution these variants that have positive effects on cognitive function were selected, but at a cost - in this case an increased risk of autism spectrum disorders."
The scientists, whose findings are published in the journal Public Library of Science Genetics, studied more than 5,000 cases of ASD and conducted an analysis of evolutionary gene selection.