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Thursday, November 5, 2015

The Riddle of Causation

In The Politics of Autism, I discuss various ideas about what causes the condition.
Here is just a partial list of correlates, risk factors, and possible causes that have been the subject of serious peer-reviewed studies:
Air pollution and proximity to freeways;
Maternal thyroid issues;
Autoimmune disorders;
Induced labor;
Preterm birth;
Birth by cesarean section;
Maternal and paternal obesity;
Maternal and paternal age;
Maternal post-traumatic stress disorder;
Smoking during pregnancy;
Antidepressant use during pregnancy. 
At Slate, Sarah DeWeerdt writes:
Progress has also been hampered by history unique to autism research: the theory, thoroughly discredited by the scientific community, that childhood vaccines cause autism. Bitterness over the way this falsehood has shaped the public view of autism has contributed to scientists’ skepticism about other potential environmental factors, says Irva Hertz-Picciotto, professor of environmental and public health sciences at the University of California–Davis MIND Institute. “I think in the autism field that actually has been a bit of an obstacle because people equate vaccines and environment.”

Epidemiologists are a cautious bunch to begin with. In order to establish a risk factor as causative, epidemiologists typically apply nine criteria, including whether independent data sets agree on the associations, whether epidemiological studies agree with laboratory findings, and whether a plausible physiological mechanism for the proposed effect exists. Any one of these criteria is difficult to meet, and satisfying all of them—or at least, enough of them to endorse a causal link—is a high bar to clear.

Nailing down who has been exposed to how much of a particular environmental factor is another distinct challenge. To find genes that contribute to autism, blood samples are all you need. Nongenetic risk factors, by contrast, are difficult to measure. There’s no way to quickly and easily scan someone’s blood for a comprehensive record of past environmental exposures. “Molecular biology technology has not provided that huge boost that it has provided on the genomics side yet,” says Craig Newschaffer, director of the A.J. Drexel Autism Institute at Drexel University in Philadelphia.
Due to these obstacles, the list of environmental risk factors that autism researchers generally agree upon is short. One of the best known and most widely accepted is maternal infection during pregnancy.