The debate over autism’s cause has been raging for several decades, with most medical personnel favoring nature (genetics) over nurture (environment).
Last week, however, an extensive study of twins published in the Archives of General Psychiatry demonstrated that the link may be, if not predominately environmental, then at least as much so as the heritable genes through which disease and traits are passed down.
Researchers came to this conclusion after observing that – while identical twins (one egg, one sperm) are diagnosed with autism more often than are fraternal twins (two eggs, two sperm) – the neurological problem is surprisingly common among fraternal twins as well.
This means that genetic factors no longer take a front seat, since fraternal twins are essentially siblings born at the same time. In fact, the data suggests that something in their common environment is responsible for the autism rather than a shared genetic heritage.
The news is somewhat alarming, given that twinness is on the rise, with 74 percent more women giving birth to twins than did so before 1980. This increase, 118,916 sets of twins out of about 4 million births, or 1 in 33 (2.97 percent) is statistically significant, as is the fact that 33 percent of these twins are identical.
Why are women having more multiple births now than ever before? For one thing, women are waiting longer to start families, and the incidence of twinning rises by one percentage point for every five years that elapse after age 25. Thus, a woman at 25 has only a 3 percent chance of having twins; by the time she reaches 40, that possibility has risen to 5 percent.
Because women are waiting to start families, fertility clinics are used more often, and the medicines used to stimulate ovulation, and thus pregnancy, actually encourage the release of several eggs at once as opposed to the normal release of a single egg every month.
It should come as no surprise to readers that the researchers conducting the study were taken aback by their findings. As Dr. Joachim Hallmayer of Stanford University in California noted, the evidence suggests not only an environmental cause early in development, but a case where these environmental factors may be interacting with genetic factors.
Translation for the rest of us: it’s a lot more complicated than anyone thought, and the 192 sets of twins studied (54 pair of them identical) clearly demonstrate that the real balance between nature and nurture may easily be 58 percent environmental and only 38 percent genetic (with the remaining two percent representing an uncertainty factor).
According to Dr. Gary W. Goldstein, president and CEO of the Kennedy Krieger Institute in Baltimore (and a member of the board of Autism Speaks), the fact that environment has been shown to weigh so heavily may, in part, be the result of what he calls a “vulnerability” gene.
Some experts, and some parents of autistic children, are convinced the vulnerability actually occurs in the form of Thimerisol-based childhood vaccines. Others think the neurological deficit might relate to environmental toxins/poisons in baby formula and even breast milk. Still others think the link is Lyme disease.
And a study from the University of Cambridge on July 12 shows that autism originates in the same area of the brain devoted to reading emotions from facial expression (e.g., empathy), which has caused some researchers to label autism “the ultimate male brain”.
Whatever the cause, and there may be many – all working together to delay the development of speech, social relationships and imagination – there is clearly a long road ahead to discover it. For more information on autism, please visit WebMD’s autism pages
Photo credit: reshealth.org
For other great stories on all aspects of climate change, check out Celsias: