it’s now clear that there isn’t a single cause of autism—just like there isn’t a single cause of cancer or heart disease. as with many complex real-world disorders both environmental and genetic causes play a role. some recent studies are helping us understand the complex interrelationship between these causes. a study of 192 twin pairs published in november 2011 tried to isolate environmental versus genetic causes by comparing identical to fraternal twins. identical twins have the same genes; fraternal twins have the same parents but are not any more similar genetically than any other pair of siblings. using fancy math these researchers confirmed that there are both genetic and environmental causes—and that the influence of both was about in the 50-50% to 60-40% range. however twin pairs share identical environments before they’re born which makes it difficult for studies like this to separate genetic from environmental factors that occur before birth. which brings us to another study this one based on autopsies that studied the brains of children with autism versus matched neurotypical kids. researchers looked closely at brain structure and found that children with autism had far more neurons packed into certain areas of the brain than children who are unaffected. this difference in fundamental brain architecture had to arise very early probably within the first one or two trimesters of pregnancy—that’s when neuronal development and cell growth takes place. if confirmed this study supports the idea that very very early influences are critical and that children who are destined to have autism may well already have the disorder at birth even if manifestations aren’t seen yet. there are many potential influences on brain development that occur early in pregnancy and many factors to consider: stress infections nutrition genetics and other exposures. an example of one such environmental trigger was revealed in another recent study which showed an increase in autism among children whose mothers took antidepressant medications early during their pregnancies. while medication exposures couldn’t account for all cases they could certainly be one of many potential exposures that may interact with genetic factors to cause autism.
what is the relationship between genes and environment with regard to autism?