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Imagine the uproar *I can't wait!!* when some innocent researcher who's done his homework says, "gee, all the evidence points to abortion being the main cause."
It will happen.
I don't think abortion is the reason for depopulation or declining populations. It has more to do with the wealth of these societies - Japan, U.S., Canada, Europe. The richer a society becomes the birth rate drops. The main reason seems to be women in the workforce. These women tend to have fewer children - either because they have children later in life and therefore have fewer kids or they do not want children to interfere with their careers. In poor countries, including most Mulsim societies, women rarely work outside the home. Women who live this way tend to have more kids. As opportunities slowly open up for women in these societies they will have fewer kids. It's already happening in some countries.