Original URL: http://www.theregister.co.uk/2006/11/03/the_odd_body_birth_rates/

Is it true that fewer boy babies are born in hard times?

Survival of the fittest

By Stephen Juan

Posted in Science, 3rd November 2006 14:20 GMT

Also in this week's column:

Is it true that fewer boy babies are born in hard times?

Asked by Jack O'Connor of Dublin, Ireland

The sex ratio of boy and girl births is affected by severe stressors such as earthquakes, tsunamis, environmental toxin contamination, political and social upheavals, and even serious downturns in the economy.

When society gets such a severe shock, the number of boy births falls and the number of girl births rises. A reduced number of boys are conceived and a higher number of boys die before birth.

The theory behind this is that natural selection favours female births when times are hard because, on average, females have a better chance of mating successfully than males. This principle seems to be followed in animals.

In 2003, evidence emerged that the same principle applies to humans. In an article in Human Reproduction that year, Dr Ralph Catalano, professor of public health at the University of California, Berkeley, presented an examination of birth records from Germany. Birth records reveal that in 1991, immediately following German reunification, the ratio of boys to girls in the former East Germany dropped to its lowest point since 1946, but then bounced back a year later. In the rest of Germany, where conditions were more stable, the boy/girl ration was unaffected.

In the same journal in September 2006, Dr Catalano has shown that the same odd occurrence happened in New York City after 9/11. The sex ratio of male births in New York City dropped in the period 1 January to 28 January 2002.

In addition, Dr Catalano has suggested that a male's life expectancy is affected by whether or not he was born in stressful times.

In the American Journal of Human Biology (e-published 12 October 2006), Dr Catalano and colleague Dr T Bruckner evaluate two rival theories accounting for this reduced male fetal morbidity. The first is the "damaged cohort" theory. This theory holds that a mother's response to the shocks of stressful times can trigger "stress reactivity" in the fetus and thereby shorten the lifespan of males in utero.

The second is the "culled cohort" theory. This theory holds that shocks of stressful times induce spontaneous abortions of frail male fetuses, but hardy male fetuses survive. Drs Catalano and Bruckner examine data from several northern European nations and conclude that there is more support for the "culled cohort" theory.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au