Americans now work 50 percent more than do the Germans, French, and Italians. This was not the case in the early 1970s, when the Western Europeans worked more than Americans. This article examines the role of taxes in accounting for the differences in labor supply across time and across countries; in particular, the effective marginal tax rate on labor income. The population of countries considered is the G-7 countries, which are major advanced industrial countries. The surprising finding is that this marginal tax rate accounts for the predominance of differences at points in time and the large change in relative labor supply over time.