American tax policy must stand as one of the great mysteries of the global political economy.

In 1969 Neil Armstrong walked on the moon, Jimi Hendrix played “The Star-Spangled Banner” at Woodstock, and federal, state and local governments in the United States raised about the same in taxes, as a share of the economy, as the government of the average industrialized country: 26.6 percent of gross domestic product, against 27 percent among the nations in the Organization for Economic Cooperation and Development.

Nearly 50 years later, the tax picture has changed little in the United States. By 2015, the last year for which the O.E.C.D. has comparable data, the figure was 26.4 percent of G.D.P. But across the market democracies of the O.E.C.D., the share had climbed by an average of more than seven percentage points.

Citizens of many countries that were poorer and little taxed in the 1960s, like Spain and Japan, today pay a much larger share of their incomes in taxes than their American counterparts. In some rich countries like Denmark, where taxes were already high in the 1960s, taxpayers now contribute almost 20 percentage points of G.D.P. more to the public purse than Americans do.

Wagner’s Law, named for the 19th-century German economist Adolph Wagner, states that government spending as a share of the economy will increase as nations get richer and their citizens demand more and better public services. This may approximate public policy in other industrialized nations. In the United States, it fails.