It began as a retort and became a fear. For years, when liberals would accuse conservatives of cutting taxes for the rich, our main argument was that low marginal tax rates on high earners were good for the economy. But we would also respond that rich people actually pay a large share of all income taxes. Over time, many conservatives grew convinced that the true fairness issue raised by the tax code is that this share is too large — and, even more, grew alarmed by how many people were not paying income taxes.

That 47 percent of all tax filers …