Examining Environmental Factors in Sex Ratio

Could gender be determined by anything other than biological factors? One medical researcher says that when women have a child without a live-in male partner, the odds are tipped slightly against having a boy.

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RENEE MONTAGNE, host:

A few weeks ago we aired a report on a 60-year-long decline in the ratio of boys to girls born in the US. Richard Knox reported the new data and he mentioned a possible explanation: women who are not living with a man at the time of conception are less likely to have sons. Many asked how can this be. Here's NPR's Richard Knox with a follow-up.

RICHARD KNOX reporting:

Many people wrote in to tell us they just couldn't believe this story. One woman says she knows many single mothers who have sons. But scientists say it's easy to be misled by small numbers.

(Soundbite of baby crying)

KNOX: For instance, more babies are born here at Brigham and Women's Hospital in Boston than anywhere else in New England.

(Soundbite of baby crying)

KNOX: Last year, for every thousand girls born here, there were 1,067 boys. That's way more boys than the national ratio, so the sex ratio can deviate from the norm in a given hospital or a given family. It takes big numbers and many years to see what's really going on. Many listeners were puzzled by the notion that a baby's gender could be determined by anything other than the father's sperm. Dr. James Zuckerman of Brigham and Women's has an answer to that. It's true that if an egg is fertilized by a sperm bearing a Y chromosome, that makes for a boy. If it's an X-bearing sperm, a girl. And, Zuckerman says, studies indicate that males make equal numbers of X and Y sperm. But he points out that in populations, there are always more boys born than girls.

Dr. JAMES ZUCKERMAN (Brigham and Women's Hospital): The observable, demonstrable fact is that there is a male bias at birth, so there has to be an explanation for that.

KNOX: Many specialists think the woman somehow influences the baby's gender.

Dr. ZUCKERMAN: And that really is the key issue, I think, for the future is to try to understand what happens in the intrauterine environment, in the environment that the mother nurtures the baby.

KNOX: Many things could influence that environment. Dr. Karen Norberg of Washington University came up with the single-mother hypothesis. It's based on 80,000 births over 40 years. She found that boys were less likely to be born to mothers who were not living with a man at the time of conception.

Dr. KAREN NORBERG (Washington University): That's such an unfamiliar concept, that people are suspicious when they hear it first, and probably rightly so.

KNOX: But Norberg had a lot of information about the parents during each conception and pregnancy.

Dr. NORBERG: For parents who had one child under one set of circumstances and then had another child under another set of circumstances, they got the same pattern of results.

KNOX: If a woman had a child without a live-in male partner, the odds were tipped slightly against having a boy. For children born later to the same mothers when fathers were living in the household, the odds of a boy were higher. Some listeners wondered if abortion could be a factor. Norberg says she found the same pattern prior to Roe vs. Wade. Some think environmental chemicals like pesticides are likely to be a more important factor. Pete Myers of Environmental Health Sciences is a leading exponent of that theory. He says cases in Italy, Russia and Canada suggest certain chemicals that disrupt hormones reduce the proportion of boy babies.

Mr. PETE MYERS (Environmental Health Sciences): But you're left with a very large challenge. We live in a multi-causal world. There are many things operating simultaneously, and it's almost impossible to tease out the individual effect of different factors.

KNOX: Norberg favors the single-mother hypothesis because...

Dr. NORBERG: There's been a very dramatic rise in the number of women who are single at the time of a child's birth and not living with a male partner.

KNOX: From 5 percent in the 1940s to more than 20 percent today. But one thing the numbers can't tell us is how would a woman's living situation affect a baby's gender. Norberg thinks it has to do with feast and famine and family resources acting over thousands of generations. It takes about 10 percent more calories to gestate a male fetus, and it probably takes more food to raise a boy to adulthood.

Dr. NORBERG: Yes, boys cost more than girls, and if the single mother sort of almost by definition only has her own resources to put into rearing the child, it may be more risky for a single mother to be undertaking the rearing of a son than a daughter.

KNOX: Norberg doesn't think it's necessarily good or bad the sex ratio is changing. It just shows that humans adapt to their circumstances in surprising ways. Richard Knox, NPR News, Boston.

MONTAGNE: You're listening to MORNING EDITION from NPR News.

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