The Price of a Child (2013)

There's a huge compounding economics of scale effect for kids. Not only do consumables become cheaper in bulk but there's more opportunities to reuse. Three boys two years apart can all ride in the same car seats, wear the same clothes, ride the same bikes and use the same baseball gloves. I recall my brother (7yr younger) wearing shirts I wore in grade school after getting them from cousins that weren't the same cousins we gave them to. If the time between children is very little or they're not all the same gender (preferably boys, hand me down everything doesn't fly with girls for the most part) the "unit cost" goes up.

In a lot of middle class families teenage sons wind up learning skills that their fathers don't have so the net loss can go down in those years.

The way financial aid for college "works" (quotes indicate sarcasm) is you basically get told what you can pay and schools make at least a token effort (depending on the marketability of your kid, e.g black jewish straight-A D1 athlete majoring in something trendy gets more aid than an average white dude getting average grades in an average major) to try and not exceed that. If you can pay X and have Y kids in school then supposedly you pay X/Y for each kid in college. After adjusting for reality it's probably north of X/(Y+something) but the point is that it's cheaper than if you had two kids 4+yr apart and are only ever paying tuition for one at a time.

They aren't absolutely infertile, but in hunter gather societies this is handled by infanticide. When it isn't possible for a mother to support two children under the age of four, the only solution was to kill one, which is what they did.

In the _Wandering God_ by Morris Berman, there is a story of a story of a girl (maybe 19th or early 20th century?) in a nomadic society in Africa. Her mother got pregnant again and was going to kill the baby, but the three year old child protested and agreed to go with being breast fed to try to save her newborn sibling. It wasn't possible to breastfeed two children at once, so her mother was going to kill the infant.

Miraculously the child managed to survive, but it was by no means guaranteed and she had to figure out to be self sufficient to some degree. It was considered such an odd occurrence that the relevant anthropologist reported the story.

I think we forget that infanticide wasn't uncommon that long ago. It was one of the major 'moral' victories of Christianity to make infanticide uncommon. And it could only do that because it wasn't necessary anymore.