“Yes, in France the TFR rose from 1.67 to 1.78 for a few years.”

?

Fertility rates rose from 1.66 in 1993/4 to 1.78 in 1999, yes. They then continued to keep rising, to 1.87 in 2000, and 2.00 in 2010. They hardly fell.

As a matter of fact, the birthrate for the US in 2011 hit a historic low of 63 live births per female.

WOW. And that’s low?

I’m reminded of the standard line regarding population control: “Somewhere in the world, a woman gives birth four times every second. This woman must be found and stopped.”

And from a purely statistical point of view, their projected growth rates are almost impossible to arrive at, for the number of fertile women will be decreasing, not increasing. In China, a plunge in fertility and hence population is almost assured; the ration males to females there is 123 to 100. In China, Europe, Russia, and Japan as the populations age and the number of women in their prime fertility years decrease, future women would need to bear between 2 and 3 children just to stop the hemorraging.

End genetics have little to do with it. It’s all behavior.

Note that the tendency to have a big family depends on 1) biological capacity, 2) capacity to attract a mate, and 3) social/psychological factors, such as a desire to have a big family. I suspect this last variable – the desire to have a big family – is the dominant factor for determining the “heritability of fertility.” That is, I suspect that the propensity to have a big family is more a social than a biological variable.

But imagine it weren’t. That is, imagine that family size were mostly a function of biology.

Attributes that tend to result in genes being passed to the next generation get reinforced in future generations; attributes that tend to impede genes being passed to the next generation tend to get suppressed. All else being equal, high fertility would tend to help genes get passed to the next generation.

But all else may not be equal. The authors cite studies suggesting that households with fewer kids are able to expend more resources on each kid, and that these kids acquire social advantages that make them more likely to reproduce than would otherwise be the case. Hypothetically – and this is the big assumption here – the reproductive advantage of having fewer, but more advantaged, kids could offset the reproductive disadvantage of having fewer kids.

If this hypothesis were accurate, then in a world without effective contraception – or among the strict Catholics in our own world — evolution might come to favor less fertile people. That would be the ultimate in Natural Family Planning!

“As the first law of behaviour genetics is that all human behaviour is heritable”

I’m an evolutionary partisan, and even I don’t grant that assumption.