While the Occupy Wall Street movement may be waning, the perception of growing income inequality in America is not. For those on the left, the widening gap between the top 1% of earners and the remaining 99% is proof that American capitalism is unjust and should be traded in for an economic model more closely resembling the social democracies of Europe.

But an examination of changes in income distribution over nearly 100 years, not just in the United States but elsewhere in...