also this is global, so I thought there was a strong correlation between agrarian societies, having low or depressed wages, and having lots of kids to help with the farm labor, or because there isn't much else to do for fun.

This could be a regressive imperialist view, idk, but its what I was always told about the correlation between wages and children, especially on a global scale - would be interested to see this same chart for singular countries, the USA for example.

Other commenter pointed out that typically higher incomes generally come with rights for child bearers... So that also could have something to do with it too.

Anecdotal, but partner and I have put off having children multiple times due to affordability, IE who can take that much time off work and still pay food/rent costs?