Our readers know that the statutory U.S. corporate tax rate of 35% (plus another 4.1% average state rate) is the highest among developed nations. Yet the myth persists that most American companies somehow pay little or no tax. Now comes a new study showing that the tax burden is as punitive as advertised.

The tax evasion fable got new legs in July when the Government Accountability Office released a study concluding that the effective U.S. corporate tax rate is only 12.6%. The effective rate is what corporations actually pay...