Girls, Girls, XXs...

October 27, 2004 9:34 AM

Girl Power or: Partnership status and the human sex ratio at birth: a paper by Karen Norberg

Could the sex of a child be influenced by the status of the parents' relationship at the time of conception? In a sample of 86,436 births in the United States, we find a small excess of sons among births to parents who were married or living with an opposite sex partner before the child's conception, compared to births to parents who were not. This is the first evidence that household arrangements can affect the human sex ratio at birth, and could explain the fall in the proportion of male births in some developed countries over the past thirty years.

(Data published on FirstCite registration required) via The Economist

(special note for mathowie: No word yet as to whether or not those single moms can also reliably produce offspring with an astigmatism.)
posted by lilboo (12 comments total)

A number of related links here.
posted by Kwantsar at 10:39 AM on October 27, 2004

Perhaps women are just more likely to have an abortion if the baby is a male. Something like 1/4 of all pregnancies in the U.S. are terminated, so it wouldn't surprise me if women just prefered to raise girls, and if they were sitting on the fence, they would be more likely to have the child if it were a girl. If the woman is in a committed relationship, she's less likely to have an abortion, and so the sex ratio would go toward the natural number.
posted by cameldrv at 10:40 AM on October 27, 2004

Throughout the course of the human species, male children have been more desirable than females, so I don't think that theory holds much water, cameldrv.
posted by jpoulos at 10:56 AM on October 27, 2004

Huh? Doesn't everyone know that male sperm swim faster, die quicker, but female sperm swim slower and last longer.. So anyone who's "getting it" often is more likely to have sons.

I was planned with that knowledge many years ago, Mum wanted a girl.
posted by dabitch at 11:00 AM on October 27, 2004

jpoulos, the data cited in the FPP isn't on "the course of the human species" but at a particular historical moment in time.

Most of the people I know in the US who have one child want a girl; single mothers especially express strong preferences for girl children. However, I don't think that very many women in the US have abortions for sex-selection.

One hypothesis I have heard from many people, including doctors, is that the older women are at the time of their first pregnancy, the more likely they are to have girl children and the less likely they are to have boy children. I don't know what, if any, research has been done on this question.
posted by Sidhedevil at 11:16 AM on October 27, 2004

Yeah, I don't think so cameldrv. By the time you can reliably assess the sex of a fetus, it's usually too late for anything but a second trimester abortion. Second trimester abortions only make up 10% of all abortions in the US, so of the 25% of all terminated pregnancies, 90% of them are terminated before a woman would even know if the fetus were male or female.

[Cite; determining gender] [Cite; CDC Abortion Surveillance]
posted by headspace at 11:22 AM on October 27, 2004

is that the older women are at the time of their first pregnancy, the more likely they are to have girl children

I've heard that too, and I've often wondered what implications it could have. Older first-time mothers probably skew more feminist on average, so it could be that girls are more likely to be raised with more nontraditional ideas about gender roles.
posted by transona5 at 11:48 AM on October 27, 2004

I once heard in a biology class that male children are slightly more likely to be conceived, but female children are slightly healthier and are more likely to survive to birth. Normally, this balances the birth ratio out pretty well.

It's possible that as the average age of American mothers rises, the health advantage of female fetuses becomes more important, causing the ratio to adjust in that direction. Male children would be at a disadvantage in a middle-aged mother because of the accumulated genetic damage to their single X chromosome.
posted by Mitrovarr at 11:53 AM on October 27, 2004

Are couples who have a boy more likely to have another boy? I remember reading an article in Slate about how parents of boys are less likely to divorce. So if the couple already has one son, maybe they're more likely to stay together and to have another son?
posted by transona5 at 12:11 PM on October 27, 2004

In Robin Baker makes the argument in "Sperm Wars" that males are an evolutionary gamble. He writes that statistically theres a much bigger difference between genetically successful and unsuccessful men (i.e. those who impregnate more or less females) than between women.

As being rich and powerful affects a mans sexual opportunities more than it does for women, then a male is a better genetic bet when the family is rich and powerful .

Statistically today, almost every woman has at least one child, whereas many men do not reproduce at all. Therefore, having a female child is a safer bet genetically for the woman when the environment is uncertain, because the odds are that her daughter would have at least one child even if she comes from a poor family. On the other hand if the environment is secure, e.g. rich, powerful man as the provider, then having sons are a better genetic gamble, as if the son turns out to be a millionaire playboy who impregnates lots of women, then the genetic payoff (i.e. successful copies of the mothers genes) is potentially much bigger than a daughter could be.
posted by tranceformer at 4:07 PM on October 27, 2004

Along with the tendancy for women to have girl children when the environment is uncertain, women also tend to have girl children when they aren't socially dominant. It's the same power/gender thing that tranceformer brings up. Girls can marry up, but boys have to marry across or down. So I'd say that single women don't tend to be as socially dominant as married women, which would probably come from the fact they don't tend to be as rich.
posted by stoneegg21 at 5:34 PM on October 27, 2004

Many persistent environmental pollutants are estrogenic.
posted by troutfishing at 1:04 AM on October 28, 2004