Autism is caused by genetic make-up in 74-98% of cases, a Medical Research Council study of 516 twins indicates. The King’s College team said 181 of the teenagers had autism, but the rate was far higher in the identical twins, who share the same DNA. The researchers told JAMA Psychiatry that many genes were involved, and they do not rule out environmental factors entirely.
Both twins in each pair had been raised by their parents in the same household. Autism can be tricky to diagnose. It is a spectrum of conditions rather than a single disorder, and its severity can vary widely from person to person. Researcher Dr Francesca Happe said, although not perfect, all the evidence pointed to genes playing a bigger role in autism than previously thought.
"Our findings suggest environmental factors are smaller, which is important because some parents are concerned whether things like high pollution might be causing autism. "Some people think there might be a big environmental component because autism has become more common in recent years but that’s happened too fast for genetics to be a probable cause.