In times of trouble, multiple studies have shown, more girls are born than boys. No one knows why, but men need not worry about being overrun by women. An analysis of old church records in Finland has revealed that the boys that are born in stressful times survive better than those born during less challenging periods. The work helps explain why women may have evolved a tendency to abort certain males and could lead to a better understanding of miscarriages.

Males are more likely to die than females while in the womb. Bouts of severely cold weather, earthquakes, natural disasters, even the 9/11 terrorist attack on New York City exacerbate this difference, as months later the ratio of boys to girls born can decline to well below the typical ratio of 105-to-100.

Biologists have long thought that the women spontaneously abort male fetuses that are frail, making room for new pregnancies and, possibly, a healthier baby. (Females are thought to have a better chance of reproducing than males in tough times, so aborting them doesn’t make as much evolutionary sense.) Because of the large investment required to raise children, “there would be a big payoff to being able to select which fetuses to raise and which not,” says Ron Lee, an economic demographer at the University of California (UC), Berkeley.

One outcome of such “culling” should be that healthier sons are born during rough periods. Indeed, in 2006, population health researchers Ralph Catalano and Tim Bruckner of UC Berkeley found this trend in population data from the Human Mortality Database. In those rough years, a greater percentage of boys survived infancy. But Bruckner wanted to get beyond these statistics to see not just if those boys born during stressful times were healthier but also whether they would produce more children than boys born during less stressful times. Such a pattern would provide an evolutionary explanation for such culling. It “might be adaptive,” Lee says.

Bruckner turned to Virpi Lummaa, a biologist at the University of Sheffield in the United Kingdom who had digitized centuries of Finnish church records that documented family histories and recorded other data about every Finn born at those times. The researchers looked at the sex ratios of newborns from 1790 through 1870 and tallied how many males survived infancy, an indication of how healthy the fetus was, and how many children they subsequently had that in turn reached puberty. They found 16 years where the percentage of male infants surviving plunged, with one in the late 18th century dropping to 79 males for every 100 females. Those males did do better than their peers born in normal years, with about 12% more of them surviving past age 1, Bruckner, Lummaa, and their colleagues report online this week in the Proceedings of the Royal Society B. The data also indicated that in the most extreme case, survivors produced 8.7% more offspring than males born in years where the numbers of male and female babies were about equal.

The finding “offers further confirmation of the suspicions that male [fetuses] are more vulnerable to miscarriage than females and that the aborted fetuses are frail,” says psychologist William James, an honorary research associate at University College London who was not involved with the work.

The researchers don’t yet know the stress, such as hunger, that may have been experienced by pregnant women in those unusual years. Still, these data show that “the ambient environment during pregnancy shapes the quality of the males over their lifetime,” Bruckner says. In general men die 5 or 6 years earlier than women. “We’re trying to understand the causes of gender imbalance in life span and what contributes to the variance” in how long individuals live, he adds. Pinpointing what happens to frail males in utero is a first step in that direction.

The study is very intriguing, says Ken Robert Smith, a biodemographer at the University of Utah in Salt Lake City, because “it covers a time when parents had no control over the sex of their children,” and thus “the sex ratio reflects the basic interplay of biology and environment stress.” Comparing the survival of girls during normal and female-dominated sex ratio years would help ensure that males were really more robust and not just benefiting from an environment that was better for all infants, Smith says. Also assessing the survival of siblings would have strengthened the results. And Lee wonders if there’s any way to assess whether the aborted fetus resulted in better care of the siblings or a healthier mother. Regardless, he adds, the work “demonstrates that basic forces that arise early in life have [important] consequences.”