A new epidemiological study linking autism with pesticide exposure is expected to surface soon, according to our sources in the scientific community. The study was commissioned by Childhood Autism Risks from Genes and Environment (CHARGE).
A new epidemiological study linking autism with pesticide exposure is expected to surface soon, according to our sources in the scientific community. The study was commissioned by Childhood Autism Risks from Genes and Environment (CHARGE). We anticipate that the mainstream media and some in the scientific community will latch onto the study because the rising rate of autism is alarming, and the public is understandably searching for answers. But we would caution the scientific community, the media, and the public to approach the study with skepticism instead of automatically buying into the results.
Speaking about the results, ahead of publication in a recent video, (posted on June 10, 2014) the senior author, Dr. Hertz-Picciotto, states: “there were associations with several classes of pesticides.” She goes on to state: “this is actually the third study to show some link with the organophosphates and autism risk.” I was surprised to see a study author talking publicly about the results of an embargoed study, since other scientists and journalists are precluded from talking about the study until after its publication.
Furthermore, using epidemiological terms of art such as “associations” and softer, more vague terms such as “link” can be misleading to a layperson (presumably the audience to which this video is aimed). An “association” is a statistical correlation between two variables, not a causal link. As we discussed in a recent educational video, the randomized controlled trial (RCT) is the gold standard for uncovering causal relationships. But RCTs are not feasible for studying causal relationships between environmental factors and chronic diseases, such as autism. As an alternative, epidemiology studies observe people in their natural surroundings and draw inferences about associations. But epidemiological studies are often plagued by confounding factors and biases.
Unlike Dr. Hertz-Picciotto, I cannot comment on the CHARGE study until it is published. But I can offer a set of generally accepted principles and guidelines that the study must follow for the results to be considered valid. Any case control study evaluating a putative role for these sorts of environmental factors should adhere to the following characteristics:
Of course, no single epidemiological study can meet all of these characteristics and that is why it is unreliable to draw cause and effect conclusions based on the findings of a single epidemiological study. Replication and consistency of results are essential, as Sir Austin Bradford Hill explained in his 1965 address to the Royal Society of Medicine in which he set forth a set of criteria to evaluate bodies of scientific evidence. This is the role of well-conducted, comprehensive systematic reviews and meta analyses.
I’m prepared to scrutinize the CHARGE study’s design and execution because its predecessors are replete with flaws that make reliable interpretation difficult or impossible. The rising rate of autism in the US has led to a proliferation of research on the possible causes, including the possibility that it is due to increased diagnostic scrutiny. But much of this research is poorly conducted and improperly interpreted by the media. For example, a study published in March purporting to link autism to environmental factors received widespread media coverage, including suggestions that it supported a role of pesticides in autism. But the study did not actually measure any environmental exposures, rendering any conclusion about a link between pesticides and autism impossible.
The possible role of environmental factors in autism and other neurodegenerative disorders is an important issue that should be addressed with rigorous studies based on sound scientific principles. It is incumbent upon epidemiologists to design, conduct, and interpret studies with this in mind if we are going to uncover meaningful answers. And it is the responsibility of the media to critically scrutinize these studies instead of instantly accepting unsubstantiated conclusions.
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