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May 08, 2007


There's some reason to think that sleep deficiency causes weight gain, and extended sleep deficiency is typical for people with babies.

I don't think you've distinguished correlation and causation yet, even with all the extra citations.

What you need is a study that shows obesity predicts subsequent childbearing, and you don't have that. (E.g., higher weight of childless teenagers to predict their subsequent childbearing after controlling for SES/IQ; or failure to lose weight after first baby to predict bearing second one, etc.) Basically, you need a study with time-series data.

As you describe them, some studies show that people who have (already produced) children are fatter. Possible explanations abound, including: (a) people who already have children are less eager to find a (new) mate and reproduce, so neglect their own health and appearance; (b) men with fertile wives prefer them to neglect their looks so fewer rival men will find them attractive; (c) people with children eat their kids' leftovers; (d) people with kids get less exercise (because they spend their time fooling with kids instead of playing racquetball); (e) women are affected by pregnancy to make them feel more hungry; (f) social pressure or just social license for pregnant women to "eat for the baby" causes them to gain weight, which they then retain because everybody tends to retain weight once gained; etc-etc.

I'm not discounting reasons things might break your way, e.g., (a) men secretly find fat women more desirable; (b) due to fat womens' elevated estrogen levels [fat produces estrogens] they are more fertile; (c) plump women are less likely to miscarry due to some shortage of micronutrients; etc. I'm just pointing out that your correlations don't prove any hypotheses either way.

Obesity correlates inversely with SES/IQ, and so does childbearing, but just observing that doesn't prove that obesity causes childbearing.

In places/times where food costs discourage low-SES/IQ people getting fat, they may still produce more children than high-SES/IQ people, even though (in those times/places) more high-SES/IQ people are fat due to affluence.

I think we can look to TV, falling food prices, reduction in physical labor (and welfare payments), and changes in the US ethnic mix to explain obesity rates.

(And there's one more thing: American black women may be trying (unconsciously) to manipulate the mechanism of the Sailer Hypothesis. That is, competing with higher body-fat-percentage women for mates may have prompted American black women to gain weight themselves. In combination with other factors, many of them may be overshooting their marks.)

Have you ever seen those line charts with the percentage of obese americans over time?

Here is one with BMI, which is similar enough for this purpose:


A cursory analysis suggests the rise in obesity has been happening WAY faster than your genetic explanation (even with the "compound" effect) - The graph for kids is even worse - fat parents + playstation/wii = fatter kids ??

APH linked to a chart showing that the percent of people in NE with BMI > 30 has increased a lot. Why is this?

Unfortunately the chart doesn't break this down by race. The Hispanic population of Nebraska has exploded, from 36,969 in 1990 to 94,425 in 2000. Hispanic people are more likely to be heavy. This is surely contributing to part of the increase.

Enough with the racism. People are sitting in front of the TV and eating more junk food. This doesn't make you gain weight?

I agree with Mark Seecof that you're confusing correlation and causation. For instance, in the US, poor people are significantly fatter; poor people have significantly more children; and once people have many children, they are much more likely to become poor. I notice the cited study does not take income level into consideration.

Of course the effects of socio-economic class are only one explanation, but your explanation of sudden genetic changes is most unlikely. Genetic changes generally take a very long time to take effect, barring a sudden catastrophe resulting in intense evolutionary pressures (something that most certainly has not happened in modern America). This is especially the case for a long-lived, slow-breeding species like humans.

Even controlled genetic changes (breeding of domestic animals, for instance) takes at the minimum several generations. 30 years is only one human generation, and no one is deliberately selecting for fat people: in fact, the fat have considerable disadvantages (poverty, poor health, social stigma). Your argument betrays a basic lack of understanding of evolutionary change.

an average 2 kg increase in weight after pregnancy

Maybe I'm misinterpreting this, but it looks like the claim that causality runs from children to weight.

the fat have considerable disadvantages

Well, yes. But evidently they aren't *evolutionary* disadvantages that reduce their number of offspring.

...(c) people with children eat their kids' leftovers;...

This hypothesis is my favorite :-).

It's likely that in the pre-modern society, fat people faced evolutionary disadvantages that they no longer face.

The same thing is happening with intelligence.

the fat have considerable disadvantages

Well, yes. But evidently they aren't *evolutionary* disadvantages that reduce their number of offspring.

The point is that experiencing widespread genetic changes in only thirty years requires either a situation where all the unfit die or fail to reproduce (hasn't happened in the US) or a breeding program selecting for a particular characteristic (not happening since everyone hates fat people). There's no obesity gene which has suddenly appeared; people are getting fatter for environmental reasons.

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