Girls on top

摘要

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.