A UK twin study estimates that between 56 and 95 percent of autism spectrum disorder (ASD) cases are attributable to genetics.
The findings, published in JAMA Psychiatry, suggest that the risk of ASD and related traits is more strongly influenced by genetics than previously thought.
'These results further demonstrate the importance of genetic effects on ASD, despite the dramatic increase in prevalence of the disorder over the last 20 years,' explained study author Beata Tick from the Kings College London team.
The researchers used data on over 6000 twin pairs from a UK population-based study who had been followed from birth. They carried out in-depth home-based evaluations of 207 twin pairs, who were identified from the initial sample as being likely that at least one twin had ASD.
The likelihood of both twins in a pair having ASD was higher in identical twins, who share the same DNA, than among non-identical twins, confirming the high heritability of the disorder.
Depending on which type of autism assessment was used, the researchers' estimates of the heritability of ASD ranged from 56 percent to 95 percent.
In comparison to previously published studies which have focused on children with a clinical diagnosis of autism, the research analysed a wider sample of twins, in the hope of achieving a better understanding of ASD within the general population.
Professor Patrick Bolton, a co-author of the study, explained that this approach enabled the team to achieve a 'more accurate picture of how influential a child's environmental experiences and their genetic make-up is on ASD, as well as on subtler expressions of autistic skills and behaviours'.
The number of autism diagnoses has increased over the past 20 years, which some have suggested is due to environmental factors such as pre-natal exposure to toxins.
However, speaking to the BBC, study author Professor Francesca Happé explained: 'Our findings suggest environmental factors are smaller, which is important because some parents are concerned whether things like high pollution might be causing autism.'
She added: 'The main consensus now is that the rise in diagnosis has more to do with increased awareness of the condition.'
Estimates suggest that one in 100 people in the UK have ASD, which can impair the behaviour, communication, and social interactions of affected individuals. Autism is a spectral disorder, encompassing a range of conditions of varying severities. Recent research suggests the genetic basis of the disorder involves the complex interactions of hundreds of genes.