Sunday, March 04, 2007

Economists refuse to stay within the lines: The causes of autism

Not long ago there was a car commercial showing a kindergarten classroom with the teacher saying "stay within the lines" as the children filled in their coloring sheets and staying within the lines. Except who was expressing his creativity and imagination. That was the truly productive one.

Economists - as much as they are accused of making accountants look alive by comparison - tend not to stay within the lines. Economics is an imperial social science and its practioners think they can apply its tools just about anywhere.

The latest example is found on the front page of Friday's Wall Street Journal ($ req.):
Prof. Waldman's willingness to hazard an opinion on a delicate matter of science reflects the growing ambition of economists -- and also their growing hubris, in the view of critics. Academic economists are increasingly venturing beyond their traditional stomping ground, a wanderlust that has produced some powerful results but also has raised concerns about whether they're sometimes going too far.

Ami Klin, director of the autism program at the Yale Child Study Center, says Prof. Waldman needlessly wounded families by advertising an unpublished paper that lacks support from clinical studies of actual children. "Whenever there is a fad in autism, what people unfortunately fail to see is how parents suffer," says Dr. Klin. "The moment you start to use economics to study the cause of autism, I think you've crossed a boundary."

Prof. Waldman, who thinks television restriction may have helped rescue his own son from autism, says many noneconomists don't understand the methods he used. His paper recommends that parents keep young children away from television until more rigorous studies can be done. "I've gotten a lot of nasty emails," he says. "But if people aren't following up on this, it's a crime."

Such debates are likely to grow as economists delve into issues in education, politics, history and even epidemiology. Prof. Waldman's use of precipitation illustrates one of the tools that has emboldened them: the instrumental variable, a statistical method that, by introducing some random or natural influence, helps economists sort out questions of cause and effect. Using the technique, they can create "natural experiments" that seek to approximate the rigor of randomized trials -- the traditional gold standard of medical research.
But as enthusiasm for the approach has grown, so too have questions. One concern: When economists use one variable as a proxy for another -- rainfall patterns instead of TV viewing, for example -- it's not always clear what the results actually measure. Also, the experiments on their own offer little insight into why one thing affects another.

"There's a saying that ignorance is bliss," says James Heckman, an economics professor at the University of Chicago who won a Nobel Prize in 2000 for his work on statistical methods. "I think that characterizes a lot of the enthusiasm for these instruments." Says MIT economist Jerry Hausman, "If your instruments aren't perfect, you could go seriously wrong."
In principle, the best way to figure out whether television triggers autism would be to do what medical researchers do: randomly select a group of susceptible babies at birth to refrain from television, then compare their autism rate to a similar control group that watched normal amounts of TV. If the abstaining group proved less likely to develop autism, that would point to TV as a culprit.

Economists usually have neither the money nor the access to children needed to perform that kind of experiment. More broadly, randomized trials seldom lend themselves to studying economic questions, particularly the more traditional ones. It would be unfair to randomly subject some people to a higher tax rate just to see how it affects their spending.

Instead, economists look for instruments -- natural forces or government policies that do the random selection for them.
Prof. Waldman and his colleagues had such studies in mind when they approached autism and TV. By putting together weather data and government time-use studies, they found that children tended to spend more time in front of the television when it rained or snowed. Precipitation became the group's instrumental variable, because it randomly selected some children to watch more TV than others.
Waldman et al. available here (pdf).

Thanks to Steven Levitt for the pointer. Levitt wonders:
As an aside, isn’t it strange that we live in a world where I’m puzzled as to why the WSJ won’t give away their product for free? In general, it doesn’t seem like a good idea to give your product away if you are a company, but given that most newspapers do, why doesn’t the WSJ?



Post a Comment

Links to this post:

Create a Link

<< Home