How Many Baby Boys Did the Clean Air Act Save?

(Brand X Pictures)

Our latest Freakonomics podcast, “Misadventures in Baby-Making,” includes a discussion of how sex-selective abortion has led to 160 million missing females in Asia. Closer to home, however, researchers Nicholas J. Sanders and Charles F. Stoecker are focusing on a different problem: missing baby boys. In an effort to evaluate the effects of environmental policy on fetal health outcomes, the authors examine the “gender ratio of live births.”  From the abstract:

We present the gender ratio of live births as an under-exploited metric of fetal health and apply it to examine the effects of air quality on fetal health. Males are more vulnerable to side effects of maternal stress in utero, and thus are more likely to suffer fetal death due to pollution exposure. We demonstrate this metric in the context of the Clean Air Act Amendments of 1970 (CAAA) which provide a source of exogenous variation in county-level ambient total suspended particulate matter (TSPs).

The authors find surprisingly significant results:

We find a statistically and economically significant association between ambient TSP levels and the fraction of live births that are male: a one unit increase in annual ambient TSP levels is associated with approximately a 0.088 percentage point change in the probability of a live birth being male, and a standard deviation increase in the annual average TSPs (approximately 35 micrograms per cubic meter) is associated with a 3.1 percentage point change. These effects are larger when considering particularly vulnerable subgroups, such as less educated mothers, single mothers, and black children.

We convert this gender ratio change into a potential measure of fetal deaths prevented by the TSP reductions caused by the CAAA. We discuss a number of possible metrics, and estimate a range of 21,000 to 134,000 avoided fetal deaths, or 2 to 13 percent of the birth population in those counties.

That’s a pretty big range, but even the lower-bound is pretty striking: a minimum of 21,000 avoided fetal deaths as a result of the CAAA?  It certainly makes you wonder how many fetal deaths are currently being caused by ongoing air pollution — in the U.S. and elsewhere.