Warren Buffett is fond of saying his tax rate is lower than his secretary’s. He does not publicize his tax returns, but for the tax year 2010, he paid $6.9 million on taxable income of $39.8 million, according to partial disclosures he made in 2011.

What is astounding about those numbers is not the 17.3% tax rate, but that Buffett’s $39.8 million of taxable income is only about 0.05% of his reported net worth ($71 billion according to Forbes, which put him third on its list of the 400 wealthiest people in the world for 2015)....