An oft-asked question in biology classes used to be “Is it nature or nurture?” Loosely translated, the question means, Are genes or the environment responsible for what you see? No human condition may have split people more on this question than has autism. Are genes responsible, or is the environment behind it? Recent research published July 4, 2011, in Archives of General Psychiatry suggests that, as with so many answers to the nature vs nurture question, it’s a fairly even interaction of the two, one that probably starts in the womb.
The researchers with the California Autism Twins Study, led by Standford’s Joachim Hallmayer, looked at 192 pairs of twins, at least one of whom was diagnosed with an autism spectrum disorder (ASD). The twins were all born between 1987 and 2004. Fifty-four of the twin pairs were monozygotic, meaning they were genetically identical. The remaining 138 twin pairs were dizygotic (fraternal), meaning they were only 50% genetically related. All of the twins obviously shared a womb before birth.
The study split ASDs into “strict autism,” meaning a classic and intense form of autism, and “broad autism,” a more expansive definition that encompassed what some people consider to be “higher-functioning” autism. The researchers used statistical modeling to determine that the environment was responsible for just over half of the traits among the twin pairs with strict autism and for almost 60% among the pairs falling within the broader definition.
They concluded that the genetic contribution to either strict or broad autism was just under 40%. Based on their evaluation, they found that “Susceptibility to ASD has moderate genetic heritability and a substantial shared twin environmental component.” The “shared” environment at work in the autism of the twin pairs, according to the authors, was most likely the womb and probably explains about 55% of what underlies autism. They describe genetic factors as playing an “important” role, but emphasize that shared environment in the womb.
What in the womb could influence brain development and lead to the spectrum of differences called autism? The possible answers to that question could literally fill an entire book, but they include parental age, infections the mother may have had, the fact of the multiple pregnancy, or about one million things that have yet to be identified. The womb is a strange and mysterious place where we first start to take shape thanks to an understructure of DNA upon which environment begins to act from the moment the sperm and egg meet. Thanks to this careful assessment from Joachim Hallmayer and colleagues for the California Autism Twins Study, scientists at least may have narrowed down the location – if not the timing or much else – of the environment itself.
Dr. Emily Willingham came to EarthSky from The Biology Files. Her background includes a PhD in biological sciences, a bachelor's degree in English, and a published book: The Complete Idiot's Guide to College Biology. She is a scientist, writer, editor, teacher, autism & ADHD parent, and "all around opinionator." Says Emily: "Got an English BA & biology PhD, & I'm not afraid to use them, often together."