Autism is a disorder that typically manifests itself in early childhood and is characterized by "markedly abnormal or impaired development in social interaction and communication and a markedly restricted repertoire of activity and interests," according to the Diagnostic and Statistical Manual of Mental Disorders. Diagnoses of autism have risen dramatically over the past few decades, from an estimated one in 2500 children thirty years ago to one in 150 today. As a result, autism has become a subject of great concern for parents, medical providers and researchers, and policy makers alike.
Despite the growing interest in autism, its causes are not well understood. It is widely accepted that genetics or biology plays an important role in the development of autism. However, many in the medical community believe that the increasing prevalence of autism points to a role for an environmental "trigger" that is becoming more common over time. Yet there is little consensus as to what the trigger (or triggers) might be.
With the recent explosion in television programming and videos aimed at very young children, exposure to electronic media may be one possible trigger. One study found that on a typical day, four out of five children aged 6 months to 6 years old use screen media (TV, videos and DVDs, computers, and video games), for an average of two hours per day. While similar statistics for earlier periods are hard to come by, it seems likely that young children are spending more time in front of the television today than they did in the past.
In "Does Television Cause Autism?" (NBER Working Paper 12632), researchers Michael Waldman, Sean Nicholson, and Nodir Adilov explore the hypothesis that "a small segment of the population is vulnerable to developing autism because of their underlying biology and that either too much or certain types of early childhood television watching serves as a trigger for the condition."
This theory has received little attention in the medical literature. It may seem like an unusual topic for a trio of economists to tackle, not only because of the subject matter but also due to the difficulty of identifying a causal relationship between television watching and autism. If watching more television is associated with higher rates of autism in the data, this does not prove that television is an autism trigger. There could be a third factor - for example, the child's diet - that is correlated with both television watching and autism and is the real trigger. It could also be the case that children who are vulnerable to developing autism have a predilection for watching lots of television, so that the direction of causality runs from autism to television watching rather than the reverse.
The authors' key contribution is to identify "natural experiments" that can be used to help establish a causal relationship between television watching and autism. The authors reason that children are likely to watch more television if they live in an area that gets more precipitation If that is the case, then a finding that areas with higher levels of precipitation have higher autism rates would be strongly suggestive of a role for television watching as an autism trigger, particularly if precipitation levels essentially vary randomly across areas that are otherwise quite similar.
The authors first use data from the American Time Use Survey to confirm the link between precipitation and television watching. The results suggest a strong relationship - a child under age 3 watches an average of 27 additional minutes of television on a day with one inch of precipitation (which is equivalent to a day of heavy rain) than on a day with no precipitation.
Next, the authors construct a county-level data set for California, Oregon, and Washington, three states with high precipitation variation across counties. The data set includes information on autism rates among young school-age children and the average annual precipitation level when those children were ages 0 to 2, as well as control variables such as the county's per capita income and racial makeup.
Using these data, the authors find that higher levels of precipitation are strongly associated with higher autism rates. To guard against the possibility that the correlation is coincidental (e.g., families with children prone to autism happen to locate in high precipitation areas for some unknown reason) the authors include county fixed effects. This amounts to asking whether within a given county, cohorts of children who experienced relatively heavy precipitation before age 3 subsequently had relatively high autism rates. The authors find that this is indeed the case.
While the results indicate that "there is a trigger for autism where exposure to this trigger is positively related with the amount of precipitation in the child's community prior to the age of three," it does not prove that television watching is the trigger, since there could be other indoor activities that children are also more likely to engage in when it rains. As another way to test their hypothesis, the authors explore whether the share of households in a community with subscriptions to cable television is positively correlated with autism rates. They find that it is, and that this correlation cannot be explained simply by the fact that both cable subscriptions and autism rates were rising over the study period, since communities where subscription rates grew faster experienced faster growth in autism rates as well.
The study's findings suggest a quantitatively important role for television viewing in autism diagnoses. The authors estimate that 38 percent of autism diagnoses can be attributed to the additional television watching that occurs due to precipitation and that 17 percent of the increase in autism rates over a twenty-year period is due to the growth of cable households and subsequent increase in early childhood television watching.
The authors caution "although our findings are consistent with our hypothesis, we do not believe our findings represent definitive evidence for our hypothesis. We believe the only way to establish definitively whether or not early childhood television watching is a trigger for autism is to more directly test the hypothesis." Nonetheless, they suggest that until more research can be conducted, it may be prudent to place additional emphasis on the recommendations of the American Academy of Pediatrics that early television watching should be eliminated or at least quite limited. The authors note "we see little downside in taking this step and a very large upside if it turns out that television indeed causes autism."