Although we currently do not know the precise cause of autism, the prevailing idea is that both genetics and the environment play a role. However, the relative contributions of both have been difficult to establish.

Now, having investigated a large number of twins in the U.K., scientists have once again highlighted the role of genetics in autism, but this time their results place less weight on environmental influences than some previous studies. According to the investigation, autism can be largely explained by genetic factors, with 74-98% of autism cases attributable to genes. Precisely which genes are involved is unknown at this stage, although the authors note that it is likely that tens, if not hundreds, could be playing a role.

Of course, the study does not rule out the possibility that environmental factors also contribute to the development of autism. Indeed, a study just last year concluded that the environment and genetics play equal roles. However, what these environmental factors are still remains controversial.

Autism Spectrum Disorder (ASD) is an umbrella term used to describe several neurodevelopmental disorders, including autism and Asperger’s syndrome, that affect social interaction, communication and behavior. Although we do not yet fully understand the origins of ASD, the possible influence of environmental factors has been highlighted by the fact that diagnoses have increased dramatically in recent years. For example, in the U.S., diagnosis of ASDs increased 10-fold over a period of ten years, from 4-5 children per 10,000 in the ‘80s to 30-60 in the ‘90s.

While there have been a few different ideas put out to explain this trend, such as the now discredited study linking autism to the MMR, many believe it could simply be due to an increase in our understanding of the condition and better diagnostic criteria. However, it is clear that there is still considerable confusion over the etiology of ASD, which is why scientists from King’s College London decided to conduct a study to find out more.

As described in JAMA Psychiatry, they compiled data from all twins born in England and Wales between 1994 and 1996, and then conducted home-based evaluations on 258 pairs of twins from the initial group of more than 6,000. Twins are useful in genetics because identical twins share 100% of their DNA, which means differences observed can be attributed to the environment.

After conducting diagnostic interviews and observing them playing, 181 twins were diagnosed with ASD. Next, they compared identical and non-identical twins, which revealed that the rate of ASD was significantly higher in the former, suggesting it is highly heritable. Further analysis led them to the conclusion that in 74-98% of cases, ASD was caused by genetics.

“Our findings suggest environmental factors are smaller, which is important because some parents are concerned whether things like high pollution might be causing autism,” study author Dr. Francesca Happé told The BBC. “The main consensus now is that the rise in diagnosis has more to do with increased awareness of the condition.”