Economic stress in Germany linked to decline in male births
BERKELEY – What influences a baby's gender? Forget the myths about the lunar cycle or the location of the parents at conception. A new study by a researcher at the University of California, Berkeley, says a country's dramatic, faltering economy may be a powerful determinant in the number of boys that are born.
Ralph Catalano, professor of health policy and management at UC Berkeley's School of Public Health, analyzed birth records in East and West Germany from 1946 to 1999. He found that in East Germany in 1991 - a time when the region was reeling from the collapse of Communism and the transition to a free market economy - the ratio of male to female births dropped to its lowest levels since World War II.
"There has been a long-standing theory in biology that says stressed populations yield fewer males than otherwise expected," said Catalano, whose study appears in this month's issue of the journal Human Reproduction. "It's a phenomenon that has been reported in herd animals responding to famine or drought. The change in the ratio of male to female births in Germany suggests that similar mechanisms may be at work in humans."
Catalano notes that lower male to female sex ratios have been observed in other populations after traumatic events, such as a temporary decline in male births in Japan's Hyogo prefecture after a devastating earthquake hit the region's capital, Kobe, in 1995. But he points out that his study is the first analysis to look at sex ratios in a human population over a long span of time. He compared male and female births - recorded in the Human Mortality Database - in East and West Germany over 53 years.
In 1990, the Berlin Wall was torn down and East Germany had reunited with West Germany. It was a rocky transition as workers and firms in East Germany waited for an infusion of investment from West Germany. In 1991, an estimated 20 percent of the labor force in East Germany was unemployed, and an additional 20 percent of the labor force worked only a few hours a week. Industrial production dropped 50 percent, and inflation shot up 6 percent.
"I used data from Germany because there is no doubt among historians and economists that East Germany essentially had no economy in 1991," said Catalano. "What we found when we looked at the birth records was that approximately 800 fewer boys were born in East Germany than expected."
No significant differences were seen in the sex ratio of babies born in West Germany, where the economy was humming along smoothly in comparison. Furthermore, the number of male births in East Germany bounced back to expected numbers in 1992, when times improved.
Even with the decline in the number of expected male births, 104 boys were born for every 100 girls in East Germany in 1991. The figure is significantly lower than the expected ratio of nearly 106 boys born for every 100 girls.
Catalano explains that natural selection normally favors the birth of baby boys when the environment is stable and the population is healthy. More males need to be born so that enough survive to reproduce, and they are able to produce more offspring than females, he said.
According to a controversial population theory, however, the reproductive success of males declines when the population becomes stressed.
"The chances of males successfully mating with females drop when they are physically weak," said Catalano. "Over eons of time, natural selection supposedly starts to favor those who give birth to fewer boys in times of stress."
Scientists have suggested that male fetuses are more susceptible than female fetuses to stress-induced hormones and miscarriages early in pregnancy, though the exact reasons for that are unclear. Another contributing factor may be that stress reduces sperm motility, resulting in fewer males conceived, said Catalano.
Prior studies have looked at the effects of poverty on sex ratio with mixed results. But Catalano distinguishes sudden, dramatic events with ongoing sources of stress. "We adapt to chronic stress," he said. "It's very different when you experience a sudden, unexpected decrease in security."
Despite the intriguing results, Catalano cautions that this analysis is not a definitive test. "What happened in East Germany is so unusual, it may not be applicable to other situations," he said. "The people were experiencing economic stress, but they were also grappling with dramatic cultural, political and societal changes as well."
Catalano points out that these population-level observations do nothing to help individual parents trying to influence their chances of conceiving a boy or girl.
"The influence of sudden stress doesn't work by changing the gender of babies conceived," he said. "But those who do conceive baby boys may be less likely to carry them to full term."