A couple days ago, Harvard economist Gregory Mankiw tried his best to defend the carried interest tax loophole by blowing smoke at the debate and hoping no one would notice. The carried interest loophole allows hedge fund and private equity managers to reclassify their compensation for management services—a hefty slice of the return on their investors’ capital—as capital gains, which are taxed at a preferential 15 percent rate instead of the top marginal income tax rate of 35 percent. Mankiw is an economic adviser to former Massachusetts Gov. Mitt Romney, who inadvertently thrust the carried interest loophole into the spotlight with his 13.9 percent effective tax rate. But no amount of smoke or sand can cover up Romney’s tax return or a tax code that throws fairness out the window for the millionaires and billionaires in high finance.

Rather than defending carried interest outright, Mankiw muddies the water by leading readers through five examples of varying business arrangements and their respective tax treatment, attempting to illustrate that the line between labor and capital income is often blurred. Fair point. The tax code is complicated and similar modes of economic activity are often taxed differently, violating the principle of horizontal equity. Indeed, the tax code grossly violates the principle of horizontal equity when compensation is reclassified as investment income, as the carried interest loophole allows. Inadvertently, Mankiw is making a strong case for again equalizing the tax treatment of income derived from wealth and income derived from labor (as was done under the Tax Reform Act of 1986). After all, why should the tax code incentivize one compensation arrangement over another?

And Mankiw brushes off the second half of the fairness question: The carried interest loophole and the preferential treatment of capital gains it confers also violate the principle of vertical equity (the basic tenet of a progressive tax code that effective tax rates should rise with income). Instead, he compares Romney to a carpenter specializing in business fixer-uppers, implying that this tax question is about fairness to the middle class. But this is about someone with a net worth between $190 million and $250 million paying less than 15 percent on $21.7 million in income and the principle of vertical equity being undermined where it is most needed (where the money is).

This flaw in the tax code spans well beyond the presidential campaign trail: Private equity firm The Carlyle Group recently disclosed that its three founders each received in 2011 a $275,000 salary, a $3.5 million bonus, and roughly a $134 million share of investment profits, much of which is carried interest. Additional returns on their personal investments in Carlyle ranged from $57 million to $78 million. With so much income taxed at a 15 percent rate, it’s hard to imagine their effective tax rates landing in a different ballpark than that of Romney. The carried interest loophole helps the wizards of high finance to undermine the basic principles of fairness in the tax code. And unless repealed, this Wall Street subsidy will cost taxpayers $13 billion to $24 billion over a decade (the range of estimates in President Obama’s four budget requests, all of which have proposed repealing the carried interest loophole to no avail).

As Alec MacGillis notes, Mankiw’s half-hearted defense of the carried interest loophole is odd because he had previously concluded that, “Deferred compensation, even risky compensation, is still compensation, and it should be taxed as such.” But that was before he became an economic adviser to Romney. One of Mankiw’s famous “10 Principles of Economics” is that people respond to incentives, as he’s aptly proving.

The carried interest tax loophole is simply indefensible, as demonstrated by Mankiw’s fickle muddy-the-water defense. There are certainly gray areas in the tax code, but no amount of smoke can shroud this particular loophole as anything but an egregious subsidy to high finance.