[dagw]

I suspect the big factor is that the opportunity costs have gotten a lot higher over that past 80-100 years. For the median woman in the 1930s, her expected lifetime earnings if she didn't have kids probably wouldn't be that much higher compared to if she took 10-20 years off to raise some kids. Equally the relative quality of life a single income family could expect wasn't too different from that of a dual income family. Today a dual income, no kids, household have opportunities and possibilities simply not available to such a household in the 30s.