There have been many theories as to the cause of autism over the years, including an infamous claim by British researcher Andrew Wakefield in 1998 that there was a link between MMR vaccines and autism. His paper on the matter has since been retracted.
But autism is almost entirely genetic in origin, according to new research which suggests the condition is more heritable than previously thought.
In 74% to 98% of cases, Autism Spectrum Disorder (ASD) is caused by genetic make-up, the study of 516 twins found.
Genetic risk factors for ASD were also found to overlap with genes that influence less extreme autistic traits seen in the general population.
The study was carried out by researchers at the Institute of Psychiatry, Psychology & Neuroscience (IoPPN), King's College London, and appears in the journal JAMA Psychiatry.
Lead author Beata Tick said: "Our main finding was that the heritability of ASD was high. 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.
"They also confirm that genetic factors lead to a variety of autistic skills and behaviours across the general population."
The researchers analysed data from the population-based Twins Early Development Study (Teds), funded by the Medical Research Council (MRC).
Co-author Professor Patrick Bolton, also from King's College, said: "The comparison of identical and non-identical twins is a well established way of clarifying the extent of genetic and environmental influences in autism.
"The novel aspect of this study was the inclusion of twins regardless of whether they had a clinical diagnosis.
"This enabled us to get 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.
"Our findings add weight to the view that ASD represents the extreme manifestation of autistic skills and behaviours seen in the general population."