The Other Gender Gap: How the Weather, the Economy, and 9/11 Affect the Birthrate

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It wasn't just air travel and the Dow that fell after the September 11 attacks. So did birth rates for male babies, and a group of scientists say that's because the communal trauma of the attacks led to high rates of miscarriage of male fetuses, according to a new study.

Scientists have long known that stress can depress the male-to-female birth ratio, but the study in BMC Public Health zones in on miscarriage, rather than several other potential factors, as the culprit for the 9/11 drop. To make the analysis, researchers compiled data on fetal death from 1996 to 2002, for a total of some 156,000 fetal deaths of both genders. in September 2001, the rate of male fetal deaths increased by 12 percent over September 2000.

But male fetal death, on average, is higher than females: 995 to 871. Why? Ralph Catalano, a professor of public health at the University of California at Berkeley and one of the study's authors, says it's simple natural selection: males are more likely to die before reaching the age of reproduction and require greater resources, so it makes sense for the mother's body to spontaneously abort them when there are environmental risk factors that could make an already uphill battle even steeper.

In fact, there a wide range of factors that can affect the ratio of male to female births in any given circumstance. Here are a few examples:

1. Natural and social catastrophes: It's not just 9/11. Lower male birthrates have been recorded following other events that create stress across a society. Examples include the London Great Smog in 1952, the collapse of Communist East Germany in 1991, Jerusalem after the Six-Day War, and Kobe, Japan following a catastrophic 1995 earthquake. A study of the Kobe case suggests the problem was paternal, rather than maternal, stress: sperm of Kobe's men appears to have been affected by the quake, leading to more female conceptions than male ones.

2. Unemployment: As another source of stress, unemployment drives down male birth rates. Catalano found that to be the case over several cycles of the California economy. Broadly speaking, male fetal death rates increased linearly with rising unemployment rates. So does that mean researchers should expect to see a sudden drop-off in male births between 2008 and 2010? Catalano says probably not, because the collapse happened slowly over a long period of time, which might help to cushion mothers' stress levels.

3. Timing: Male fetuses are conceived over a smaller window of the menstrual cycle than females (a hint to would-be parents hoping for one or another). Conception in the early cycle tends to produce boys, while later conception is more likely to create girls.

4. Weather: Some like it hot—but baby boys apparently aren't among them. A study released in April showed that the boy-to-girl ratio is significantly lower in warmer climates than elsewhere. Near the equator, males comprise 51.1 percent of births, as opposed to 51.3 percent in temperate and subarctic areas. The science behind it isn't known—author Kristen Navara wrote that it was unclear whether miscarriage rates were higher or sperm quality was altered.

5. Diet: Mothers who are in the higher percentiles for energy intake are more likely to bear boys than girls, according to a British study released in 2008, which broke expectant mothers into three groups based on calorie consumption. In the high-energy third, 56 percent of mothers had sons, while only 45 percent of the lowest third did. The mothers in the first third were also more likely to have consumed more and more varied nutrients. Since male offspring (as noted above) are more resource intensive, the study's authors suggested that high intake indicated plentiful resources to support a baby boy.