Tough Times Spur Miscarriages of Boys, Study Suggests
Brian Handwerk, for National Geographic News, February 2, 2006
During times of widespread stress, a mother's body may be more likely to spontaneously terminate weak especially male fetuses, a recent study suggests.
The finding adds new perspective to a long-standing debate in population science: Why do male birthrates decline during periods of major societal stress?
The phenomenon has been observed in many animal populations-including humans.
Previous studies have linked the decline in male births to political unrest, natural disasters, environmental changes, and economic recessions.
But scientists are undecided about how and why the decline occurs.
Ralph Catalano and Tim Bruckner of the University of California, Berkeley, School of Public Health reported their findings in last week's issue of the journal Proceedings of the National Academy of Sciences.
Their data support the idea that stressed mothers' bodies may develop a mechanism to reject especially weak male offspring whom they might have otherwise carried to term.
The authors caution, however, that the research has little to do with individual births.
"This is not going to affect anything that you do in the clinic," Catalano said. "There's no impact on how to affect the gender of child. This is about an old and often bitter controversy in population biology."
Females the Stronger Sex?
In general, male fetuses are slightly weaker than females and a bit less likely to survive to birth.
What's more, changes in a woman's body chemistry when she is subject to stress can affect males in the womb more than females. (Watch new 4-D ultrasound video of a baby in the womb.)
One theory holds that a decline in male births during stressful periods is a side effect of these factors. The mother's body is more likely to abort a male fetus because her physical tension too heavily damages her unborn child.
A second theory argues that outside stresses cause a mother's body to develop a lower tolerance for especially weak males.
Catalano and Bruckner believe they have evidence for this second theory based on studies of the life spans of babies born during times of societal upheaval.
Recent observations have suggested that surviving infants born in these times would have shorter life spans as a result of the stresses they underwent in the womb.
"Fetal-origins theory [suggests] that much of the health trajectory of an individual is determined in utero," Catalano said.
But his data, based on amazingly complete records of births and deaths in Sweden dating back to 1751, tell a different tale.
By examining male and female birthrates, Catalano and Bruckner first identified periods of social stress by the correspondingly lower male births.
The researchers then checked the life span records of all babies born during those times to see if children subjected to stresses in the womb did in fact have shorter life spans.
"We found support that times of higher [male] sex ratios, those of benign gestations, showed shorter life spans than you'd expect," Catalano said.
In more stressful times, when more weak males had been aborted, they found relatively long lifespans among the survivors.
"The surviving [males] turn out to be hardy," Catalano said.
The results support the notion that the bodies of mothers under pressure reject weaker babies in favor of ones that can survive in a harsh environment.
Devra Lee Davis is the director of the Environmental Oncology Center at the University of Pittsburgh Cancer Institute in Pennsylvania. She calls the study fascinating.
"[Catalano is] showing once more that the patterns that affect [the sex ratio of] a population reflect social patterns," she said.
What Factors Determine Sex?
Catalano and Bruckner's research contributes to the broader goal of understanding why babies are born male or female.
Davis notes that recent literature has been exploring a host of factors, such as workplace environment, that may affect the sex of unborn offspring.
For example, a recent study of submariners revealed that they have significantly more girls than boys, although researchers are not sure why.
In addition, environmental pollution may be affecting male birthrates in animal populations.
"In the past decade England, Germany, and Denmark have all issued reports on the decline of males in wildlife," Davis noted. "Some of those reports suggest links to levels of pesticides, solvents, and metals."
"We're not sure what it means," Davis continued.
"But I think we need to look at these animals and understand what factors can affect such an exquisitely sensitive thing as whether we're born female or male-or whether we're born at all."
[related story: Too Many Males: A Fast Lane to Extinction?]
During times of widespread stress, a mother's body may be more likely to spontaneously terminate weak especially male fetuses, a recent study suggests.
The finding adds new perspective to a long-standing debate in population science: Why do male birthrates decline during periods of major societal stress?
The phenomenon has been observed in many animal populations-including humans.
Previous studies have linked the decline in male births to political unrest, natural disasters, environmental changes, and economic recessions.
But scientists are undecided about how and why the decline occurs.
Ralph Catalano and Tim Bruckner of the University of California, Berkeley, School of Public Health reported their findings in last week's issue of the journal Proceedings of the National Academy of Sciences.
Their data support the idea that stressed mothers' bodies may develop a mechanism to reject especially weak male offspring whom they might have otherwise carried to term.
The authors caution, however, that the research has little to do with individual births.
"This is not going to affect anything that you do in the clinic," Catalano said. "There's no impact on how to affect the gender of child. This is about an old and often bitter controversy in population biology."
Females the Stronger Sex?
In general, male fetuses are slightly weaker than females and a bit less likely to survive to birth.
What's more, changes in a woman's body chemistry when she is subject to stress can affect males in the womb more than females. (Watch new 4-D ultrasound video of a baby in the womb.)
One theory holds that a decline in male births during stressful periods is a side effect of these factors. The mother's body is more likely to abort a male fetus because her physical tension too heavily damages her unborn child.
A second theory argues that outside stresses cause a mother's body to develop a lower tolerance for especially weak males.
Catalano and Bruckner believe they have evidence for this second theory based on studies of the life spans of babies born during times of societal upheaval.
Recent observations have suggested that surviving infants born in these times would have shorter life spans as a result of the stresses they underwent in the womb.
"Fetal-origins theory [suggests] that much of the health trajectory of an individual is determined in utero," Catalano said.
But his data, based on amazingly complete records of births and deaths in Sweden dating back to 1751, tell a different tale.
By examining male and female birthrates, Catalano and Bruckner first identified periods of social stress by the correspondingly lower male births.
The researchers then checked the life span records of all babies born during those times to see if children subjected to stresses in the womb did in fact have shorter life spans.
"We found support that times of higher [male] sex ratios, those of benign gestations, showed shorter life spans than you'd expect," Catalano said.
In more stressful times, when more weak males had been aborted, they found relatively long lifespans among the survivors.
"The surviving [males] turn out to be hardy," Catalano said.
The results support the notion that the bodies of mothers under pressure reject weaker babies in favor of ones that can survive in a harsh environment.
Devra Lee Davis is the director of the Environmental Oncology Center at the University of Pittsburgh Cancer Institute in Pennsylvania. She calls the study fascinating.
"[Catalano is] showing once more that the patterns that affect [the sex ratio of] a population reflect social patterns," she said.
What Factors Determine Sex?
Catalano and Bruckner's research contributes to the broader goal of understanding why babies are born male or female.
Davis notes that recent literature has been exploring a host of factors, such as workplace environment, that may affect the sex of unborn offspring.
For example, a recent study of submariners revealed that they have significantly more girls than boys, although researchers are not sure why.
In addition, environmental pollution may be affecting male birthrates in animal populations.
"In the past decade England, Germany, and Denmark have all issued reports on the decline of males in wildlife," Davis noted. "Some of those reports suggest links to levels of pesticides, solvents, and metals."
"We're not sure what it means," Davis continued.
"But I think we need to look at these animals and understand what factors can affect such an exquisitely sensitive thing as whether we're born female or male-or whether we're born at all."
[related story: Too Many Males: A Fast Lane to Extinction?]