Earlier quoted context omitted.
People aren't making more children because they largely can't afford to, either in actual resources, or in time spent. And, unlike in a rural community, children don't have any "return on investment" - plus, you generally have to raise them alone even when they're no longer toddlers, rather than relying on an entire community to do it together. There's no great mystery, and a colony on Mars will not change this even…
> because they largely can't afford to, either in actual resources, or in time spent But that's absolutely not true. People are so wealthy right now that they can afford practically an unlimited amount of children. The avoidance of having children is almost entirely shaped by the cultural environment and not economic environment. > relying on an entire community to do it together I am not sure that it ever was a case…
Cultural factors are certainly a part of it, but probably mostly in the sense that we are (rightfully, I believe) far less willing to let children die young or get maimed. But economic factors are a huge thing; I don't know where you live that you thing young people "can afford practically an unlimited number of children", but in most of Europe and the USA it is in fact impossible for the bottom 50+% of young couples to even support themselves, not to mention a child, on a single income. And it's also very hard to raise a child 0-4 in a city while working full time+.
> "Relying on an entire community" to raise children generally meant that children were raised mostly by themselves.
No. Children, historically, after they became capable of roaming around, were raised by grandparents, neighbors, their older siblings etc. Even when they were away from adults, they'd be roaming near the village in groups with older children.