Our default is "lots of babies"; women getting more education and economic liberty decrease that number. Poor places don't generally have significant educational budgets or opportunities for women, but as that specific set of things changes — education and freedom, not even always money — there is a corresponding change to number of births per woman.
It also means that rich couples in societies that lack women's lib still have more kids, even where you can infer high intellect without the convenient proxy of the women being allowed to go to university and giving you a test to score against.
Our default is "lots of babies"; women getting more education and economic liberty decrease that number. Poor places don't generally have significant educational budgets or opportunities for women, but as that specific set of things changes — education and freedom, not even always money — there is a corresponding change to number of births per woman.
It also means that rich couples in societies that lack women's lib still have more kids, even where you can infer high intellect without the convenient proxy of the women being allowed to go to university and giving you a test to score against.