I argue that fertility may be a strategic choice for ethnic groups engaged in redistributive conflict. I first present a simple conflict model where high fertility is optimal for each ethnic group if and only if the economy’s ethnic diversity is high, institutions are weak, or both. I then test the model in a cross-national dataset. Consistent with the theory, I find that economies where the product of ethnic diversity and a measure of institutional weakness is high have increased fertility rates. I conclude that fertility may depend on political factors.I'm skeptical of the cross-national empirics without having looked at them, because ... they are cross-national empirics (see here for further explanation). But the theory is interesting and sounds plausible.
(And here's the link for the paper by Thorsten Janus, forthcoming in Public Choice)