Is Autism Linked to the Environment?
Despite new study, development comes down to genes
Last week, when a large, high-profile twin study came out suggesting that the environment plays a significant role in autism, a lot of people were shocked.
The research, published online in the Archives of General Psychiatry, looked at 192 sets of twins, in which at least one twin was diagnosed with an autism spectrum disorder. The scientists found that in identical male twin pairs, when one had autism, 77 percent of the time, the other one did too. For identical girls, the other had the diagnosis 50 percent of the time. In fraternal twins, when one had autism, 31 percent of boys and 36 percent of girls did as well.
Plugging that into a statistical model (that considers the percentage of genes shared for each kind of twin set – 100 percent for identical and roughly 50 percent for fraternal), the researchers came to the conclusion that genes only play a 38 percent role in autism. The twins shared conditions in the womb and early babyhood accounted for 58 percent of the disorder.
The conclusion of this study? Autism is almost 60 percent environmental. That’s a result that apparently surprised even the researchers themselves because it diverged so far from previous work.
But before you start to panic about your family and the plethora of scary chemical hazards out there, let’s step back and reconsider the actual meaning of the word “environment,” because these findings play into our misconceptions about how nature and nurture really intertwine.
When we think environment, we tend to imagine toxins, diet, or unhealthy family conditions swooping in and altering a child’s biology or behavior. But it’s much more intricate and subtle than that. Anything that affects the expression of our underlying genetic code is considered the environment. It’s a loose term that encompasses even the most subtle (and less sexy, news-making) molecular signals in the womb that alter the expression of our genes. During all fetal development – healthy or otherwise – chemical switches are constantly turning off and on certain pieces of DNA.
In other words, it still comes down to genes. Most likely, people with autism have inherited many characteristic pieces of DNA for the disorder, making them more susceptible. Then, a series of signals – not necessarily anything catastrophic but, more likely, subtle changes to the baby’s environment – amp up or silence those genes that already exist. Even in the current study, with 77 percent of identical twins sharing the diagnosis (compared to the general population’s less than one percent incidence) autism still comes across as heavily inherited.
The takeaway? The findings are really not surprising. We already knew the environment has a significant impact on gene regulation. The real question is which environmental signals play on genes and edge a baby’s brain development towards the characteristics of autism.
There isn’t one answer to that question – there could be many small chemical twists involved. They most likely occur in the womb or the newborn months, given the explosion of brain growth and the fact that autism signs appear so early on. Some possibilities have been floated before (and in some cases, with data behind them): parental age, low birth weight, multiple births, closely-spaced siblings, maternal infections, and more.
But again, it’s not only one of these factors. Just as the genetic blueprint of autism varies from child to child, it’s likely that so do the environmental signposts and detours that crop up along the way in development. In fact, just this week, researchers reported in Pediatrics that after reviewing 40 autism studies, they found 16 factors associated with autism, including umbilical cord complications, fetal distress, birth trauma, multiple births, summer births, low birth weight, congenital malformations, and meconium aspiration.
We’re waiting for one magic answer to the root cause of autism – but that answer may never come. It’s an endlessly complex disorder involving hundreds of pieces of DNA (which aren’t the same from one child to the next). The mix of molecules that guide those genes could vary just as widely. And, in the end, it doesn’t make sense to argue over nature versus nurture, because neither really works without the other.
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Great explanation of a confusing study, Heather! Thanks for decoding the word “environmental” that scientists interpret much more broadly than everyone else. I wish we (they?) could come up with another word that doesn’t conjure toxic cleaning products and the like. I also am curious what could happen in the first couple years of life, environmentally speaking that sets the twins on different paths. I think it’s also fascinating that even identical twins have different DNA later in life – obviously the environment “works” on genes too. Also we might be interpreting “genes” too narrowly also – as we’re starting to learn how mutations might occur in the womb that create differences in twins with the same genes so to speak. Twins may be less identical than we think.
Anyhow, I will be eager to see if this study will be replicated. In any event, thanks for reminding us how complicated the roots of this disorders and also the concepts involved. Sounds like we all need to think less narrowly about genes and environment, especially in light of epigenetic research highlighting the subtle interaction between the two. It might be a clunky dichotomy in a decade.