A child's genes are the cause of 98 per cent of autism cases, say scientists.
Researchers from, King's College London, studied identical twins and concluded that genetics, not lifestyle and environmental factors, are responsible for the condition.
Dr Francesca Happe told the BBC: "Our findings suggest environmental factors are smaller, which is important because some parents are concerned whether things like high pollution might be causing autism.
"The main consensus now is that the rise in diagnosis has more to do with increased awareness of the condition."
Estimates suggest that as many as one in 100 people - 700,000 - in the UK now has autism and there have been concerns that the condition is fuelled by environmental factors such as pollution.
But the team found these causes were a distant second to genetics.
The study looked at 128 pairs of twins in which at least one sibling had an autism diagnosis, between the ages of 12 and 14.
Comparisons were drawn between the behaviour of pairs of identical twins, who share all their DNA, and non-identical twins, who have only half their genes in common.
Scientists found the condition was far more common in identical twins than in non-identical ones.
The study, in the journal JAMA Psychiatry, concluded that DNA was behind between 74 to 98 per cent of autism cases.
Dr Judith Brown, of the National Autistic Society, said: "Autism is a highly complex story of genes not only interacting with other genes, but with non-genetic factors too.
"This large population-based twin sample is significant because it helps us to understand much more about the role genetics play in autism and opens up the possibility of whole families gaining a better understanding of a condition they may share."
But she added: "We are still a long way from knowing what leads to autism.
"What people with the condition, their families and carers need most of all is access now to the right kind of support to be able to lead full lives."