The findings may explain why ASD still remains in the human population. The prevalence of ASD is estimated at 1.1 percent of the UK population, higher than expected given its evolutionary disadvantage.
'Why aren't [these variants] just eliminated by evolution?' asked Joel Gelernter, professor of genetics and neuroscience at Yale School of Medicine and co-author of the study.
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.'
Researchers used a computer model to simulate the effects of natural selection on genomic data from 5000 individuals. They found that common variants associated with ASD risk showed signatures of positive selection, meaning they are retained in the ongoing evolution of the human genome and therefore must confer some beneficial effects.
These genes tend to be expressed in brain and pituitary tissue and are implicated in the development of the nervous system and the creation of neurons.
The researchers also showed these ASD variants correlated strongly with several measures of intelligence, including years of schooling, college completion and childhood intelligence.
Lead researcher Dr Renato Polimanti, from Yale School of Medicine, said: 'We found a strong positive signal that, along with autism spectrum disorder, these variants are also associated with intellectual achievement.'
Intelligence is a highly complex trait, in which many small genetic effects can add up. As the evolution of the human brain has been pivotal in the success of the species, genes that have a large negative effect on neural activity would be quickly deleted from the gene pool. Signatures of this 'purifying selection' have previously been observed for rare genetic variants linked to ASD.
The authors speculate that small-effect common variants are 'accumulated across the genome… to the benefit of most but to the detriment of some'.
The findings also explain the well-documented coincidence of certain traits in ASD and those with high IQ, such as larger brain size, increased sensory and visuospatial abilities, and high socioeconomic status.
Previous studies have also shown ASD correlates strongly with measures of intelligence, i.e. years of schooling, college completion and childhood intelligence.
The study was published in PLOS Genetics.