From Matt Carroll
It is intriguing that the sex ratio at birth of human populations might be affected by psychological stress, but I find Ralph Catalano’s evolutionary interpretation hard to swallow (30 April, p 19).
The study demonstrated a statistically significant increase in pre-term mortality of male fetuses in California following the 11 September attacks. Catalano is quoted as suggesting that this might be an evolved mechanism for culling weak males “in the interests of the herd”.
For very good reasons, “group selection” explanations have fallen drastically out of favour among evolutionary biologists since the 1960s, and have been replaced by theories working at the level of individual or gene selection. A strategy of selective miscarriage might evolve if it pays mothers to produce strong, healthy sons, or if mothers carrying male fetuses are more likely to suffer in times of stress.
Nottingham, UK
