That's the wrong question. The competition is a family with one earner vs the counterfactual /same/ family with two earners. The second one will have more mone

That's the wrong question. The competition is a family with one earner vs the counterfactual /same/ family with two earners. The second one will have more money in the modern context so they're better off. At some times in the past, they wouldn't have been better off because their expenses would increase more than their income. Basically it's about cost of childcare.