And yet, as I say, higher U.S. productivity is not the main reason for this prosperity gap. Comparing America with the richest European countries, output per hour worked is not that different. In levels of productivity, Europe’s most successful economies have caught up. Then why are they still so much poorer? Because Europeans work less. A higher proportion of the U.S. population is employed, and Americans work longer hours. Effort, not efficiency, is why Americans are richer.