Environmental Factors Key to Autism

A new study published earlier this week will likely dump buckets of fuel on the fire that rages in the debate over autism. For the past decade, there has been ongoing vitriol spilled over the (potential) relationship between early childhood vaccination and the development of autism. The truth of the matter, however, is that there has been no definitive answer to why some children become autistic and others do not. A recent study may help guide researchers to resolve this open medical mystery. 

According to this new study, conducted jointly by UCSF and Stanford, and published Monday in the Archives of General Psychiatry, environmental factors play a much larger role in causing autism than previously thought—apparently, an even larger role than genetics.

The study, the largest of its kind to date, looked at 192 pairs of twins in California—all of which had at least one autistic sibling; some of which had two. The results, after being wrung through complicated mathematical modeling and analysis, are that genetics account for 38 percent of the risk of autism, and environmental factors are to blame for the remaining 62 percent.

These findings are sure to provide firepower to those who believe that vaccination can cause autism, people like Jenny McCarthy, the former pinup model and self-proclaimed champion of children everywhere, who leads the global anti-vaccine campaign via both personal grandstanding and through her non-profit Generation Rescue. McCarthy and others like her hold the belief that the MMR (Mumps, Measles, and Rubella) vaccine leads to cancer. The MMR vaccine, introduced in 1971, is generally considered standard preventative care for kids. But it is no stranger to controversy.

McCarthy and others who believe autism is caused by the MMR vaccine base their beliefs in a controversial and immensely influential 1998 study performed by Dr. Andrew Wakefield. Dr. Wakefield’s study has been largely discredited, pronounced an “elaborate fraud” by the British Medical Journal (BMJ) and retracted in 2010 by The Lancet, the same publication that had originally published the study a decade earlier.

On the other side of the debate are those who have believed autism is a highly inheritable disease—a group comprising the majority of the medical community. Previous studies have suggested that autism is 90 percent genetic, with only a slight environmental factor at play.

This study, however, blew these beliefs wide open. 

Here’s why: Of the 192 twin sets, 54 were identical; 138 were fraternal. As a refresher, identical twins are nearly 100 percent genetically identical; fraternal twins share the same genetic makeup only to the extent any siblings do, so in the range of 50 percent.

If autism is, in fact, a primarily genetic disease, scientists would expect that if one identical twin had the disorder, the other twin would too, nearly 100 percent of the time. Among fraternal twins, if one twin was autistic, the other should have a slightly higher-than-normal risk of being autistic as well, but not huge.

However, the study found that autism occurred in both siblings in 77 percent of male identical twins and in 50 percent of female identical twins. Perhaps even more surprising were the rates among fraternal twins: 31 percent of males and 36 percent of females.

When plugged into a mathematical equation taking into account expected rates if autism were to be found entirely genetic- or environmentally-caused, these results suggested that only 38 percent of the autism cases could be attributed to genetic factors.

As for the 62 percent of cases attributable to environmental factors, the authors of the study stressed that it remains very unclear what those factors are. There is no proof that vaccines cause autism, but there are also not that many other ideas floating around the medical community.