Not too skinny, not too fat, but just right

Last winter, Half Sigma pointed to an online FAQ where it was revealed that commercial turkeys have, through selective breeding, developed breasts too large to allow for natural breeding to occur. Consequently, they must be artificially inseminated. He wondered if there was something applicable to humans in this:

People have also been, allegedly, getting fatter. I think we need to look into whether there’s a genetic explanation. Are fat people having more children than thin people? It’s well established that married people weigh more than single people, and traditionally married people have children and single people don’t. Maybe people are getting fatter for the same reason that turkeys have gotten too fat to mate. It’s all in the breeding.

In 2004, GSS interviewers were asked to assess respondents' weight and place each into one of four categories. The following table shows the mean number of children by weight class. To exclude those whose reproductive life is still ahead of them, only respondents who were at least 40 years old at the time of the interview were included:

Weight	Kids
Thin	1.93
Average	2.31
Heavy	2.22
Really Heavy	2.11

This provides a pretty crude measure of heftiness, but it doesn't support the assertion that humans are getting fatter because the corpulent have more kids than healthy folks do. Those who are noticeably slender, a designation that applies to less than 7% of the population (probably indicating skinniness to the point of apparent emaciation), aren't popping out a lot of kids, either.

I find the table above to be encouraging!