In 1980, the American rich and middle class and most of the poor had higher incomes than their counterparts almost anywhere in the world. But incomes for the middle class and poor in the United States have since been growing more slowly than elsewhere. Why? Among the reasons: This country has lost its once-wide lead in educational attainment. Other countries have increased their workers’ skill levels more quickly, helping create well-paying jobs. The United States also tolerates more inequality: The minimum wage is lower here. Executives make more money. The government redistributes less of it. By 2010, the poor in several other countries had pulled ahead. And Canada’s median income had reached a virtual tie with that of the United States. Since 2010, other data suggest Canada has moved ahead.