Wednesday, June 2, 2010

The weaker sex? Communal bereavement and fetal mortality

Did the stress of 9/11 lead to increased male fetal loss? A new paper by Tim Bruckner et al., which is making a splash in the press (e.g., on radio and in print), makes the case that it did, indicating that 'communal bereavement' does happen, and that male fetuses suffer for it. They don't know why this might be, but they are sure it must have an adaptive advantage (as they might say: mustn't everything?).
Although the biological mechanism remains unclear, male fetuses appear more sensitive than female fetuses to maternal corticosteroids produced after the twentieth week of gestation. This elevated stress reactivity apparently jeopardizes the viability of males in utero. Consistent with the theory of natural selection, humans may have conserved this male fetal sensitivity to maximize the mother’s total yield of grandchildren.
The authors looked at fetal death statistics for pregnancies over 20 weeks from 2006 through 2001 (of which it's estimated that only 20 - 30% are actually reported) and found that the number of deaths of male fetuses was higher than expected in September 2001. However, relative to the total number of pregnancies, the numbers were very small indeed (in the hundreds).

Now, the authors also cite reports in the literature that suggest that major pollution events or economic downturns are correlated with male fetal loss as well, which means that the effect--if it's real--could be confounded. And indeed, in the study population Bruckner et al. used for their analysis, more than half of the mothers had a high school or lower education level, suggesting that economic stressors would be chronic for these women. If the reported effect on sex ratio is real, why isn't the sex ratio among poor women always skewed more in favor of girls, then?

It's also interesting that the authors don't report an increase in the total number of later-term miscarriages, just the sex ratio. It's hard to believe that an effect of stress that's strong enough to skew the sex ratio wouldn't increase the number of miscarriages as well. To determine whether there was an increase in total fetal deaths, we looked at reported vital statistics and see no increase in miscarriages in either 2001 or 2002. If the effect of communal bereavement is as the authors report, this strikes us as unexpected. Indeed, this would reflect an increase in male fetal deaths but with no increase in total deaths, there must have been a corresponding decrease in female deaths. Or, a trivial effect of 'communal bereavement' on fetal deaths.

According to demographic theory, there are typically more males conceived than females, more boys born than girls, and higher male fetal death rate in general. One explanation for this is that the Y-bearing sperm are lighter and hence swim faster than X-bearing sperm (the X is a much larger and hence heavier bit of baggage to carry). But risk of death for male embryos is higher in the best of times. At birth, the sex ratio is about 107:100, but male infants die at a higher rate than females so--the theory and some data go--the sex ratio is about even at puberty when matings start.

But, this brings us to the evolutionary importance of this paper. While claiming such significance, the authors cite just one paper on this, a 1973 Trivers and Willard paper in Science that argued that natural selection favors deviations away from equal parental investment in boys and girls, "rather than deviations in sex ratios per se." But this is based on a model applied primarily to deer. They say it's complicated in humans, and suggest that in humans the "sex ratio at birth correlates with socioeconomic status." They also suggest that differential mortality takes place early in pregnancy, and that it's chronic poor maternal condition that makes the difference.

Now, whether or not the Trivers and Willard model has held up over 40 years we don't know because we don't know that literature, but it clearly doesn't apply to the Bruckner et al. paper. It's not surprising though that an adaptive explanation for the finding--itself so tenuous--is tenuous at best, but it's not unusual that the authors try to claim one anyway. In addition, there is a large literature on an increase in sex ratio--more boys per birth--after traumas like war; the sociobiological explanation has been that this allows a society to make up for valiant heroes lost in combat; whether this idea has stood the test of time we can't say.

The problem here is the usual one of making claims that may be plausible, but go far beyond the data, and the data are inconsistent. But if you simply must have an evolutionary reason, then of course you can always make one up. The problem is that these things are so often empty of seriously rigorous content.

11 comments:

  1. I didn't quite have enough coffee yet, but I am shocked by their claims. Wow.

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  2. If more coffee helps you makes sense of this, please let me know!

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  3. Holly, I'm sure it shows that you're the weaker sex. Women who get pregnant ought to be counseled about watching, or not watching, movies about 9/11....depending on whether they want a son or a daughter.....

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  4. If I understand this correctly, since this has nothing to do with miscarriage, then this is all about natural selection and environmental effects of XX sperm versus XY sperm. What else could this mean?:)

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  5. Actually, this _is_ about miscarriage -- late term fetal loss (over 20 weeks gestation). The natural selection argument they invoke has something to do with male infants born under stress being able to give the mother fewer grandchildren as adults than males born in better times. So these fetuses are miscarried, to give the mother the chance to have a male under less trying circumstances, or a female. I think.

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  6. Oops, make that X sperm versus Y sperm.:)

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  7. If sociobiological adaptation leads to increased numbers of boys after periods of stress like wars, to make up for valient hero loss doesn't it follow that boys are therefore biologically more aggressive and warlike and that women are boilgically unsuited for war? Also, doesn't the idea that there is a reason for such kinds of adaptations imply that there is a God/ess of some sort planning it all? I mean, if there is a reason, who's reason?

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  8. At second glance, I suppose I focused too much on the following: "It's also interesting that the authors don't report an increase in the total number of later-term miscarriages, just the sex ratio." Okay, I didn't carefully remember what was written prior to that before I posted.

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  9. Jennifer,
    The test would be whether in the society of the Amazon warrior women, was there an excess of female births after they had a war? We'll never know, I guess.

    The problem with a lot of thinking about evolution is that too many people think of it, perhaps informally, in teleological terms--as if it had goals it was trying to get to. Instead, it's just trying to get to tomorrow.

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  10. if evolution is just trying to get to tomorrow, then why should any of the rest of us have to set long term goals?

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  11. Because we have language and a means to envision the future, we know things that happen (kids grow up, we have to die, we might want to retire or save money to buy a big-screen TV, etc. Long term usually means our own lives or our relatives', still rather short in evolutionary terms but very long relative to what other animals can do.

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