I don't see any evidence that the heritability of desire to have large families (or the heritability of factors linked to not accidentally having large families) is anywhere near as strong as the effect of socioeconomic trends which have decimated the cultural and rational reasons for having large families in developed countries. Looking at a different aspect of heritability, modernity did far more damage to the intake of celibate religious orders and celebration of virginity than aeons of selective removal from the gene pool of those especially vulnerable to the idea that chastity was a virtue.

Populations of people that think birth control is a good idea and are quite happy to defer their children until later in life still reproduce in sufficient numbers to remain the majority in developed countries, especially assuming that many offspring of reproductive fanatics will end up "defecting" and behaving according to the prevailing social norm rather than their parent's preferences or inherited instincts.

Given the continued reproduction of people who think birth control is a sensible idea, women should have careers and more than three kids is a headache, I think heritability of desire to have large families would have to be almost deterministic for us to start to worry about hyper-fertile people taking over.

I think we can keep it going for a while, as you say, by continuously absorbing defectors from more fertile populations. But I think if you admit to any genetic component to this at all, there will definitely be genetic flow, with the more fertile populations eventually taking over.

Being "reasonable" and "planning" are just extremely maladaptive in the current world. For example, reproduction in this population is completely dwarfing reproduction of highly educated, self-controlled, non-criminal groups: http://www.amazon.com/Promises-Can-Keep-Motherhood-Marriage/...

I think you're placing too much emphasis on genetics. Genetics play a major role in determining the traits of an individual, but the way the individual was raised and the culture they grew up in also play a major role. The exact balance between the two is different depending on the trait, but an argument to the tune of "treat the likelihood of reproducing solely as a genetic trait" isn't going to have enough predictive power to say that people who don't want to have children are going to die off.

The same argument (falsely) applies to homosexuality. Since homosexual people don't pass on their genes, surely they must be bred out of the population? Well, estimates put the rate of homosexuality between 1% and 10%, so that didn't really turn out the way the purely-genetic argument would predict. It's just more complex than pure genetics.

The things I mentioned aren't just genetic - culture is also very heritable. So selection will work on that level, too.

The argument about homosexuality is similar to asking why amazon hunters keep getting killed by jaguars - you could "disprove" evolution by asking "why hasn't evolution just selected the ones resistant to jaguars?" But it's because getting as close to danger as possible is an effective strategy. That's one of the more reasonable genetic explanations for for why homosexuality still exists, too.

Astoundingly enough, the birth rates in countries where women are given equal liberty as men are much lower than those where culture or law prevent women from exercising free agency.