Newly elected Rep. Alexandria Ocasio-Cortez spent her first few weeks on Capitol Hill calling for a 70% top marginal income-tax rate, and suddenly the debate over optimal rates has reopened. To support her charge, some liberals are citing a 2011 study by economists Peter Diamond and Emmanuel Saez, which advocates for confiscatory upper-range tax rates. But a quick look at their analysis reveals grave caveats that only an advocate of higher taxes could possibly overlook.

Messrs. Diamond and Saez admit that taxes can have detrimental...