Krugman shows that Americans are working longer than they used to, but it's more prevalent in higher income brackets. He attributes it to an increased desire to make money (contra David Brooks). That may be true in the top-most brackets, but i suspect the discrepancy between the lowest and middle brackets is because the lowest paid workers are probably hourly, and companies do their best to keep those workers at 35-40 hours (to avoid having to pay overtime) or under 20 (to avoid paying any benefits at all). Salaried workers in the middle tiers are more likely to be FLSA exempt, etc., and therefore companies can "ask" them to put in more than 40 hours, stay late, work weekends, etc., without having to pay them any more.