A new study published in the journal JAMA Psychiatry suggests that 74 to 98 percent of cases of autism are caused by genetics.
The Medical Research Council studied 516 twins and discovered that identical twins who share the same DNA have a higher rate of Autism Spectrum Disorder* (ASD).
"Our main finding was that the heritability of ASD was high," lead author Beata Tick from King's College London told The Independent. "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 behaviors across the general population."
However, since the twins studied were raised in the same house by the same parents, the researchers could not entirely rule out environmental factors.
"The comparison of identical and non-identical twins is a well-established way of clarifying the extent of genetic and environmental influences in autism," co-author Patrick Bolton told The Independent.
"The novel aspect of this study was the inclusion of twins regardless of whether they had a clinical diagnosis," Bolton continued. "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 behaviors. Our findings add weight to the view that ASD represents the extreme manifestation of autistic skills and behaviors seen in the general population."
Another of the study's authors, researcher Francesca Happe, thinks that the increased number of cases of autism and other spectrum conditions is due to more accurate diagnosis.
"Our findings suggest environmental factors are smaller, which is important because some parents are concerned whether things like high pollution might be causing autism," Happe told BBC News. "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. Autism is a highly complex story of genes not only interacting with other genes but with non-genetic factors too."
Scientists are working on identifying the gene (or genes) that are involved in autism development.
"There may be perhaps hundreds of genes that contribute to autistic traits," Happe told BBC News.
"Autism is a highly complex story of genes not only interacting with other genes, but with non-genetic factors too," said Judith Brown, from the National Autistic Society. "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. However, we are still a long way from knowing what leads to autism. What people with condition, their families and careers need most of all is access now to the right kind of support to be able to lead full lives."
According to Autism Speaks, an autism advocacy organization, brain development can be affected by - in combination with genetics - environmental factors, such as advanced age of the parents, maternal illness during pregnancy, any lack of oxygen to the baby's brain before or after birth or lack of folic acid intake during pregnancy.
* In May 2013, the DSM 5 diagnostic handbook, redefined autism by combining autistic disorder, childhood disintegrative disorder, pervasive developmental disorder, Asperger syndrome and other variations of the condition under the umbrella diagnosis ASD.
