Where do the children play?

Basic population dynamics. For a population to remain steady, a breeding pair can only have on average two surviving children that procreate themselves.

If you want to get into the weeds, there's obviously some "fudge factors" that bring this a little bit up above two.

1. Not every kid that survives to adulthood will go on to procreate themselves, so the remainder need slightly more than two to make up the slack.

2. During periods of population growth, the average survivorship has to be higher.

3. The percentage surviving depends on how many were born per family to begin with. I didn't state a percentage, I said two. Okay, fine 2.4 or whatever, but not a fraction, that "depends" on too many variables.

> For the majority of human history the childhood mortality rate has hovered around 50% not 80%.

RECORDED history, which is a short blip in our evolutionary history as a species. I said megayears, a.k.a.: millions of years, for most of which we have scant evidence. Extrapolating from our wild animal cousins and just observing how these "uncontacted" tribes live, it's pretty obvious that for 99% of the time we could be called human, we had five+ kids per couple, and ten+ wasn't uncommon... of which two-point-something survived.

That's just the way it is, for essentially all species. It has to be, otherwise populations would explode in numbers until it's standing room only for the entire surface planet.

PS: Next time you watch some BBC documentary about some species giving birth to hundreds of offspring, well... now you know. They didn't make it. Certainly, statistically, most of them must not have, because if they could and did, then that species would have their population numbers grow astronomically fast!

PPS: You hand-waved away a 50% loss rate as if it's a detail. That in no way undermines my argument that if you have an only child, or even two or three, that losing half of them is not considered acceptable parenting in this day and age. There is absolutely no way anyone I know would trade half of their children so that they can have a wild, carefree, and unsupervised childhood like "nature intended"!

Human population has been increasing for at least thousands of years. Our best estimates put the human population 30000 years ago to about 8 million people, at about 1 ad the population is estimated at about 200 million people, in the 17 hundreds the population is estimated at about 600 million.

That is not a stable population, that's a growing population. You also have to take into account pandemics, widespread violence, etc. The black plague killed 1/3 of the population of Europe, you can be pretty sure that the reproduction rate was above 2 both before and after the pandemic. Millions of people died when Europeans colonized the Americas again the population growth rate would have to be much larger after the event for the population to bounce back.

Additionally adult mortality before procreation does not factor in child rearing behavior. Because why would it?

And lastly all that is pretty much irrelevant especially the habits of prehistoric people, because the change in how Americans raise their children happened in the last 20 years. Not in the 1900s when child mortality went way down.

The thing that changed between the 80s and now was not the acceptability of losing children, what changed was how Americans in particular assess risk, the Satanic Panic of the 80s, the disappearance of Madeleine McCann in 2007, the revelation of how widespread childhood sexual abuse is (before the 90s it was estimated at about 1% of the population turns out it's over 16%), all of these factors made the American public incredibly afraid about their children safety.

It's not that people in the 70s and 60s were okay with harm coming to their children it's that they didn't believe harm would come to their children when they were with other children alone running around. Which by the way was mostly a correct assessment.

In Japan the fertility rate is 1.2 per woman and yet 6 and 7 year olds got to school on their own. Because the culture there believes that is a safe practice, mostly because it is.

Go watch Old Enough, obviously it's a television show exaggerated and not how life really works in Japan, the production team and the parents are essentially watching over the children, but it clearly demonstrates that the culture there expects children to roam the cities safely.

Oh and by the way on average preindustrial families had 5 to 7 children not 10 so 50% survival rate is 2.5 to 3.5. Which taking into account occasional widespread population collapses fits much better with the observed long term growth.