The Supreme Court meets the fallacy of the one-sided bet

by Andrew Gelman on April 8, 2013 · 3 comments

in Judicial,Methodology,Science


Doug Hartmann writes (link from Jay Livingston):

Justice Antonin Scalia’s comment in the Supreme Court hearings on the U.S. law defining marriage that “there’s considerable disagreement among sociologists as to what the consequences of raising a child in a single-sex family, whether that is harmful to the child or not.”

Hartman argues that Scalia is factually incorrect—-there is not actually “considerable disagreement among sociologists” on this issue—-and quotes a recent report from the American Sociological Association to this effect. Assuming there’s no other considerable group of sociologists (Hartman knows of only one small group) arguing otherwise, it seems that Hartman has a point. Scalia would’ve been better off omitting the phrase “among sociologists”—-then he’d have been on safe ground, because you can always find somebody to take a position on the issue. Jerry Falwell’s no longer around but there’s a lot more where he came from. Even among scientists, there’s a large enough minority with traditional moral values, that it shouldn’t be hard to find some who feel strongly that it’s harmful to raise a child in a single-sex family.

But what I really want to talk about here is not scientific consensus (which, after all, can be wrong) but a little-noticed (as far as I can tell) aspect of Scalia’s statement, which is that it’s an example of the fallacy of the one-sided bet, an argument that is artificially restricted to go in just one direction. Scalia’s saying that raising a child in a single-sex family might be “harmful to the child” or maybe not. But by framing it this way, he’s implicitly excluding a third possibility, which is that being raised in this way may be helpful to the child. What do I really think is happening? I think that the same-sex-parents environment will be helpful to some kids, harmful to others. Assuming the American Sociology Association report is correct and the research doesn’t find any aggregate effect, that would suggest that some kids are helped and some are hurt, with no clear evidence that the average effect is positive or negative. So, from the data, the effect could be positive, or it could be negative, or it could be small enough on average to consider as zero. And, just from prior reasoning, I could imagine an effect that is positive (gay parents try harder and they could be less likely to have unwanted children) or negative (maybe it’s better to have parents of both sexes and not to have to face prejudice from outsiders). I don’t know, apparently the data don’t know either. But by framing his statement the way he does, Scalia is excluding the possibility entirely.

In all this discussion I’m sidestepping the causal questions, how one might consider formulating hypothetical interventions, whether one would want to consider the “treatment” at the level of individual families or state-level policies, and the difficulties of statistical identification. These issues are important—-indeed, central to any discussion of this issue—-but here I want to focus on the one-sided argument, which is such a pervasive fallacy. I keep hoping that, by giving this error a name, I can reduce its incidence.


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