The absolute nuclear family only survives under special conditions: when all men are in the militia, all families are culturally homogenous and near kin, and where there is little migration.

In other words, it evolved under the middle class version of farming: manorialism, and the middle class version of farms: producing excess for market sale, and the middle class marketplace that results from the production of excess for market sale.

The nuclear family and government pensions are a very, very, very destructive cancer.