I should note that the strongest case against the argument I mounted in these posts - that the relationship between abortion and out-of-wedlock births is too murky to draw any real conclusions - is a version of the famous Levitt-Donohue argument about abortion cutting crime. This theory holds that abortion has reduced out-of-wedlock births (to teenage mothers, at least), but over generations rather than immediately, by culling away children conceived by unwed mothers who would otherwise have been statistically likely to grow up to be unwed parents themselves. Here's a recent paper on the subject (its conclusions are - I imagine - as open to debate as the Levitt-Donohue thesis); meanwhile, here's an older paper from the 1990s making the case that legalizing abortion raised the illegitimacy rate, at least over the short run.
 

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