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Societal Stress Leads to Fewer Male Births

Posted on: Monday, 30 January 2006, 15:00 CST

By SUSANNE RUST

For years, researchers have observed that when women are faced with periods of economic or environmental distress, the number of male babies born, in comparison to females, declines. But why this happens is not known.

Now, researchers think they have figured it out and they say it has more to do with the promise of the future than the stressful realities of the day.

Ralph Catalano, a professor of public health at the University of California-Berkeley and an economist by training, has long been interested in the way sex ratios seem to coincide with current events.

In 2003, he compared the number of male live birth rates in East and West German hospitals from 1946 to 1999.

He discovered that while the sex ratios remained pretty stable for most of the 45 years, in 1991, there was a sudden drop in the number of East German boys born.

The sex ratio is the number of males per 100 females.

Under normal circumstances the sex ratio should be just above or just below 100.

According to Catalano, the East German dip was the probable result of the country's 1991 economic collapse. West Germany, which didn't experience the collapse, didn't show a corresponding anomaly.

In 2005, he made a similar observation when he looked at data collected in California after the terrorist attacks of Sept. 11, 2001.

The birth cohort, born four to five months after the attacks, had fewer males.

This was an observation, he said, that corresponds with what biologists know about stress, pregnancy and sex ratios.

Researchers have long known that males tend to be weaker than females, as fetuses, embryos, children and adults. Indeed, it's not until people reach their 90s that men start outliving females, Catalano said.

It's also been shown that more male fetuses and embryos are spontaneously aborted than females. This becomes particularly apparent after six weeks of gestation.

The biological mechanisms behind this are not known.

Some speculate that males are extremely sensitive to stress in the mother's body.

If a woman is producing and circulating high concentrations of stress hormones, called glucocorticoids, the male fetus is going to react badly, and possibly die.

If that is true, then the observation that the male sex ratio was lower in the birth cohort four or five months after the Sept. 11 terrorist attacks, then the timing would be right.

But why a woman's body might spontaneously reject a male fetus or embryo has been hotly debated for years.

And although two reigning hypotheses fuel the fight, until now no one had put them to the test using human population data, Catalano said.

The first hypothesis, called the "culled cohort," argues that women's bodies will abort weak male fetuses and embryos during stressful times because weak sons will produce fewer offspring than weak daughters.

Therefore, aborting a weak male should allow the mother to begin a new pregnancy, with the possibility of yielding a daughter or a more robust son.

The hypothesis assumes that during stressful periods the criteria for a weak male and an acceptable son will change.

In a healthy environment, a slightly weak male may not be so bad off. However, that same male in a stressful or harsh environment might not survive.

Therefore, during stressful times, the mother's body will abort males that in happier times might be deemed acceptable.

The second hypothesis, called the "damaged cohort," argues that fetuses simply die as the by-product of a mother's internal stress response.

It suggests that both weak males and females will be aborted, but presumably will affect more males because they are generally weaker.

And as a result of the mother's internal response, every exposed fetus will become weaker.

Catalano wanted to determine which best explained the demographic data. Do these deaths have a selective value, he wondered, or are they just the result of women's biology gone wrong?

If the "culled cohort" hypothesis was correct, he said, you'd expect the sex ratios of live births to fall as a result of a higher number of spontaneously aborted male fetuses: Fewer males at birth, because more males died in utero.

You'd also expect that males born in birth ratios with relatively low sex ratios would survive longer, on average, than those born in other cohorts. That's because the shift in criterion what constitutes a normal vs. weak male should have culled all males except for the strongest and healthiest.

On the other hand, if the "damaged cohort" hypothesis explains the dips, he said, then you'd expect to see just the opposite: The survival of males born in these cohorts should be lower than males born in other cohorts.

The reasoning: As the mother's body experiences stress, her fetus becomes damaged and weakened by it. Every child born during such a time, therefore, will be weaker than children born in more benign periods.

Analyzed lifespans

Catalano and his co-author Tim Bruckner, also of UC-Berkeley, looked at data collected on Swedish men from 1751 to 1912. They chose 1912 as the end date because most men born during that year are now dead, enabling the researchers to look at the men's lifespans.

Figuring that the live birth sex ratio should predict survivability, and therefore, lifespan, the researchers analyzed the data and discovered that men born in low sex ratio birth cohorts lived longer than other men.

This evidence, Catalano said, supports the "culled cohort" hypothesis, indicating that natural selection is playing a role in the spontaneous abortion of weaker males.

Rick Nordheim, a professor of biostatistics at the University of Wisconsin-Madison who was not involved in the study, said Catalano's analysis and conclusions seemed reasonable. "However, most papers of this type do not provide that kind of detail," he said. But added that he saw no " smoking guns' to question the conclusions."

Robert Trivers, the co-author of the "culled cohort" hypothesis, was tickled by the results.

"This is a very clever, very interesting paper. It's another example of how productive that little paper was," he said, referring to his 1972 hypothesis.

Trivers' one reservation was that there were likely alternatives, other than the "culled cohort" hypothesis, to explain the author's findings.

JoAnne Solchany, professor of family and child nursing at the University of Washington, said that while she doesn't know of any literature to support the idea that there are higher miscarriage rates, of either sex, during periods of environmental stress, she said stress can cause other trauma to a developing fetus, including prematurity and low birth weight.

Copyright 2006, Journal Sentinel Inc. All rights reserved. (Note: This notice does not apply to those news items already copyrighted and received through wire services or other media.)

Source: Milwaukee Journal Sentinel

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