Fat people have more children

In a comment to my previous post about how weight is genetic, Dennis Mangan wrote:

The environment is just as, if not more, important than genetics. The gene pool hasn't changed over the last thirty years, but the number of overweight and obese has massively increased.

Well, if fat people were having more children than skinny people, then this would easily explain why the number of overweight and obese people is increasing.

Doing some research on the web, I discovered that the question was answered by a Duke University study:

The researchers examined data from the Health and Retirement Study (HRS), a national survey designed to study health, social, and financial issues in middle-aged Americans. The HRS study includes information on more than 12,600 Americans, collected from 1993 to 2000. Most respondents were age fifty-one to sixty in 1992. The study is a cooperative agreement between the In-stitute for Social Research at the University of Michigan and the National Institute on Aging. For the Duke analysis, the researchers used data only from a baseline survey completed in 1992. Only married respondents and spouses between the ages of forty and seventy with children were included in the analysis, which comprised a total of 9,046 men and women (4,523 couples). Single or divorced individuals with children were excluded. The number of children reported by the couples ranged from one to nineteen, and included both biological and adopted children. Of the sample, 79 percent were white, 12.5 percent were African American, and 8.5 percent were Hispanic.

Because the researchers focused their analysis exclusively on the connection between number of children and obesity, they controlled for age, household income, race and ethnicity, work status, physical activity, and tobacco and alcohol use. "After adjusting for all these factors, the number of children played a statistically significant role in the obesity of both men and women," says Truls Ostbye, senior author on the paper and professor of community and family medicine at Duke.

Ostbye says that, while the increased risk for obesity in women was 7 percent for each additional child and only 4 percent for men, the difference between these two figures was not statistically significant. "Obesity associated with number of children is not just a problem linked to physiological changes in women during pregnancy," he says. "There are social, cultural, or psychological mechanisms that bring about this weight gain, and this is illustrated by our results that showed men were also at a greater risk of obesity."

The researchers are implying that having children somehow causes obesity, but I strongly suspect that they have the cause and effect wrong, and actually what is happening is that heavier people have more children.

UPDATE

After posting the above, I found the more detailed study, and the authors cite many other studies showing the link between weight and the number of chidren:

Several investigators have found an association between reproductive history and obesity among women and, in one cohort, among men but we did not locate any studies that have examined the association in couples. A study published in 1956, which included 583 women attending a diabetic clinic in Oxford, England, found an increasing prevalence of overweight women with increasing numbers of pregnancies.[24] In a Swedish study of twins, an average 2 kg increase in weight after pregnancy was observed in women with two or more children compared with childless controls.[25] A population study from Finland found that number of children among women aged 25–84 was closely related to the prevalence of obesity, independent of marital status, occupation, and smoking habits.[17] The strongest relationship between number of children and BMI was in the youngest age group (25–34), although the relationship persisted even among the women aged 75–84. In a more recent study from Sweden, number of children was also associated with obesity among 5464 women ages 45–73.[26]

Similar associations between number of children and obesity have been observed in the United States. In the Nurses' Health Study, an increase in BMI was found with increasing number of children among women aged 42–67.[27] Among 41,000 Iowa women, BMI increased with number of children in a linear fashion after adjustment for age, education, marital status, and smoking status.[9] In a case-control study, women (n = 1716) from Massachusetts (age range 45–69) with five or more births were more frequently obese (defined as a BMI of >/=29) compared with women with no births.[28] Among U.S. women aged 45–74 who participated in the first NHANES follow-up study, as well as among women aged 35–68 who participated in the Framingham Heart Study, investigators found a significant increase in BMI with an increase in number of children.[3]

The Rancho Bernardo Study found an association between number of children and obesity among women and men many years after child-bearing.[29,30] In this cohort study of women and men ages 55–84, the mean age-adjusted BMI was positively associated with number of biological children.[29] Among men 50–89 years of age (n = 1039), those who had five or more biological children were significantly more obese (as estimated by BMI and waist/hip ratio) than men with no biological children.[30] Several investigators have proposed socioeconomic status (SES) as a significant confounder in the number of children and obesity relationship.[31,32] Men and women with less education or of lower social class are at higher risk for obesity,[33] and this effect may be related to personal habits, such as excessive caloric intake, as well as number of children.

The evidence is pretty clear. Heavier people have more children. Weight is genetic. Thus the reason for the "obesity epidemic."

Comments

[Nancy Lebovitz]

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

[Mark Seecof]

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.)

[APH]

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:

http://www.cdc.gov/nccdphp/publications/factsheets/ChronicDisease/images/ne_04.gif

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 ??

[Half Sigma]

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.

[SFG]

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

[Amanda]

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.

[Douglas Knight]

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.

[bbartlog]

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 :-).

[Half Sigma]

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.

[Amanda]

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.