Alamy

IT HAS been known for a while that stressful conditions such as famine result in more girls being born than happens in good times. The shift in the sex-ratio is tiny—around 1%—but in a large population that is still noticeable. A possible evolutionary explanation is that daughters are likely to mate and produce grandchildren regardless of condition, whereas weedy sons may fail in the struggle to have the chance to reproduce at all. In hard times, then, daughters are a safer evolutionary bet. Regardless of why the shift happens, though, it has long been argued that the moment when it happens is conception—or, more probably, implantation. A womb exposed to stress hormones, runs the hypothesis, is less likely to accommodate a male fetus.

A recently published study, however, suggests this ain't necessarily so. According to Ralph Catalano of the University of California, Berkeley, and his colleagues, writing in the American Journal of Human Biology, stress-induced sex selection can take place long after conception and implantation.

Famines being rare in America these days, Dr Catalano and his colleagues used unemployment as their stressful event. They studied the birth records of the state of California from April 1995 to December 2007, and compared these with the number of new claims for unemployment insurance. Based on hints from earlier work, they looked specifically at unemployment claims that had wider social resonance than the firing of a few individuals—namely those in which an employer sacked 50 workers or more in one go. These mass lay-offs, it might be hypothesised, are more like natural catastrophes, such as famines, than isolated accidents that cause a few people to fall on hard times.

The researchers discovered that mass lay-offs did, indeed, lead to fewer boys being born. Over the whole period 52.4% of births were of boys. In some months, though, that fell as low as 51.2%. Teasing out the statistics suggested that the stress of mass lay-offs probably caused these drops, but that the lay-offs in question could happen months after conception. Male fetuses were, in other words, being spontaneously aborted—presumably as a consequence of stress.

That does not mean the original hypothesis is wrong. But it is not the whole truth. The ruthless winnowing of inappropriate offspring can, it seems, also take place well after a fetus has started developing. The next step, according to Dr Catalano, is to measure in pregnant women the levels of hormones known to predict spontaneous abortion, and to work out if these levels vary with stressful events.