I think what happened was that more and more women entered the workforce, either out of necessity or out of personal desire (I think largely the latter) and the market adjusted accordingly. Because the work pool roughly doubled, the wages decreased over time. Add to that the fact that one of the two largest expenses a family typically has, a car, doubled because both parents work, and also the fact that we generally live in houses that are larger and better equipped than the previous generations, and I think it makes sense that we have adjusted our lifestyles to using up the full capital we receive when both parents work.