For years we’ve been told that we’re becoming (to borrow the title of one gloomy book) “overworked Americans,” working longer and longer hours each week and reversing the trend toward shorter hours that dominated the early 20th century. In a paper released last December by the National Bureau of Economic Research, two economists—Peter Kuhn of the University of California at Santa Barbara and Fernando Lozano of Pomona College—combed through census data on employed men ages 25 to 64 to determine which groups were most likely to work long hours (defined as 50 hours per week or more).

They found a clear pattern. Long work weeks were more common, and increased more, among affluent workers than poor ones, among college graduates than less educated workers, and among older than younger workers. Between 1979 and 2002, long hours became 14.4 percentage points more common among the fifth of workers with the highest incomes but dropped by 6.7 points for the bottom quintile of wage earners—the opposite of the trend seen in the early 20th century.

In other words, the trend is being driven by incentives that encourage the most skilled and affluent salaried workers to put in a longer day at the office. Most of us who are working longer and harder—God help us—are choosing to.

Graph (not available online): Percentage of Men Working At Least 50 Hours a Week