One of the most controversial issues surrounding inequality is work effort. Some on the right argue that top earners are successful in part because they work harder than others. Many on the left argue that the middle class and poor work just as hard – maybe even harder, with multiple jobs -- but that the economic deck is stacked against them.

A new study offers evidence that higher-educated (and therefore higher-earning) Americans do indeed spend more time working and less time on leisure than poorer income groups. In...