The same genes that cause higher intellectual achievement may trigger autism, a study suggests.
A study of more than 5,000 people found that genes linked with exceptional brain power were also associated with the disorder.
The Yale University authors said that the findings may explain why autism has not been eliminated by natural selection.
Genes that have a negative effect on reproductive success normally die out.
The researchers said that a variety of genes known to have effects including boosting brain growth were beneficial in most cases, and that is why they continue to be passed on.
But the downside is that the same genes increase the chances of autism.
Autism – more properly known as autism spectrum disorders (ASD) – include Asperger's syndrome.
It has long been known that some people with autism have superior abilities in areas such as mathematics and science. It affects four to five times as many males as females.
People with the condition can also have difficulties with understanding language and the rules of social interaction as well as understanding emotion.
The researchers used a computer model that simulated the effects of natural selection on different sets of genes.
They concluded that the genetics variants found in people with autistic disorder were present at a much higher rate than would be expected by chance.
This showed that the genes have a 'signature for positive selection'.
The authors, writing in PLOS Genetics said: 'This strongly suggests that these variants have undergone positive selection during the course of human evolutionary history.'
Co-author Joel Gelernter, professor of genetics and of neuroscience at Yale University said: 'It might be difficult to imagine why the large number of gene variants that together give rise to traits like ASD 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 authors said that previous research had found many overlaps between autism and having a high IQ.
These included having a larger brain, faster brain growth, increased visual, sensory and spatial abilities, increased attentional focus, and coming from a higher socio-economic background.
Other studies have also shown that the risk of autism disorders is higher in people who have spent longer in studies and highly intelligent as children.
But this was the first time that it had been shown that the genes linked to higher IQ and autism are being positively selected in evolution.
The authors said that the gene variants boosting brain function did so in subtle and small ways.
This is because genes having a 'large effect' on the development of the nervous system would in the vast majority of cases being extremely damaging – leading to very damaging disabilities or death and rapidly be eliminated from the human gene pool.
But an accumulation of lots of small beneficial modifications to the brain 'would be to the benefit of most but to the detriment of some'.
Renato Polimanti of Yale said of the genes in the study: 'In this case, we found a strong positive signal that, along with autism spectrum disorder, these variants are also associated with intellectual achievement.'
Link to original article: http://www.dailymail.co.uk/health/article-4264798/Is-child-destined-intelligent-autistic.html