As prosperity increases, growth booms but then predictably falls below replacement rate. So humans kind of do this on their own if you can get through the boom period without breaking things too badly.
Maybe I don't understand the question, but you can see that in population statistics - certain populations reproduce at an above replacement rate, some below. In a lot of the developing world, however, and no different 150 years ago in the developed world, you'd have a lot of kids because you'd expect several to die early. Once a certain level of prosperity and peace is reached, this doesn't become as much of a biological imperative.
I am aware that some countries reproduce above replacement rate and some below, and that this rate is normally tied with country economic prosperity.
My point was an evolutionary one: wouldn't those couples who choose to reproduce more (assuming you can afford to raise your kids) be selected for, evolutionarily, vs couples who chose to remain childfree?
Thus, even in a few generations, I would assume a moderate cultural shift towards having kids, since now only those who want kids, have them.
The first hormonal birth control method (The Pill) was invented in 1960; we're 60 years beyond that (~3 generations). Maybe my theory isn't panning out as fast as I assumed.
> My point was an evolutionary one: wouldn't those couples who choose to reproduce more (assuming you can afford to raise your kids) be selected for, evolutionarily, vs couples who chose to remain childfree?
Keep in mind that the incentive to have kids, evolutionarily speaking, is weak enough that we have had to be bribed with sex to keep things going.
Once sex and reproduction were decoupled, you're definitely looking at much more than a couple of generations before biological selection pressures start having the effect of an increase in the desire to have children per-se, as distinct from the desire to have sex.
OTOH, cultural incentives are an entirely different matter: whether we're talking about religions decrying or forbidding the use of contraceptives, or advocacy for those who want children at all to have larger families, these can have a snowballing effect especially since culture is passed down to offspring even more readily, though not as reliably, than genetic traits (eg. you can pass your culture on to an adopted child, or to a spouse, etc.).
This selects for cultures that are good at spreading themselves and good at encouraging reproduction, and selects humans for a general tendency toward susceptibility to indoctrination (at least during childhood) including the imperative to eventually pass the cultural package onward. To a certain extent it also selects for acceptance of authority and resistance to indoctrination with beliefs that conflict with already held ones as an adult.
Actually, all these particular biological traits are selected for and reinforced by so many other different feedback loops, that cultural packages can mostly just assume their presence and focus on leveraging them.
