one interesting model of maintaining a sub-population with high fertility rates is israel's. their ultra-orthadox sects have well above replacement fertility rates and can replenish the secular jews who have near replacement (or below replacement i haven't looked at the data in a while) fertility rates, the problem is that they consider it against their religious beliefs for the secular society to be able to draw from their pool as a resource while also disproportionately receiving government benefits and not contributing labor. it's an unsustainable situation as their demographic increases in size. if you were to attempt to engineer a religion you would have to implement a mechanism to allow the below replacement industrial technological society to draw members from the above replacement religious society. maybe it would be beneficial to have a religion similar to judaism to draw from for scholars and a religion similar to the amish to draw from for laborers. this is all well above my paygrade and i'm just out in the weeds rambling at this point.
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