One of the least discussed parts of America’s income tax is how progressive it is, and the tax overhaul didn’t change that fact. In 2018, top earners will pay a higher share of income taxes.

The individual income tax matters—a lot—because it is the largest single source of U.S. revenue. And its share has risen in recent years. For 2018, it could raise 50% of total federal revenue, according to estimates from Congress’s Joint Committee on Taxation, up from about 48% last year.

So...