That one ranks high on my list. If you're a farmer in the countryside children are an economic asset. They can feed the animals, plant, weed the crops, maintain fencing, etc. Once you move to the city, whatever else they are in terms of fulfillment, they're an economic liability.

Oddly enough, it's likely the US and Europe defused (and then some) Erlich's population bomb with farm subsidies which pushed people off farms and in to Nike factories in the developing world.