Comment by bradleyjg

In certain cultures rearing by grandparents already happens. I’d be interested in any longitudinal studies, though it’s hard to disentangle confounders.

That said, I don’t know that economic factors are important as social ones. If they were you’d expect to see something different in couples where the husband earns much more than the wife, but in my observation those women wait about as long as their friends in marriages where the salaries are more similar. The social context seems far more determinative.