I wonder if it comes from the whole 'nuclear family' concept from the '50s. Just the idea that each person, plus a partner and offspring, must be self-sufficient, living on their own little kingdom in the 'burbs. I often think about how if we took human differences and ability to specialize to apply to living conditions, and not just your job (which we aren't that great at either, but better), then we might be using bigger groups of people with more sophisticated living arrangements. But is that all just hypothetical, or do people actually live like that anywhere in the world or history?