The Buffett rule is rooted in the fairy tale that taxes on the wealthy are lower than on the middle class. In fact, the Congressional Budget Office notes that the effective income tax rate of the richest 1% is about 29.5% when including all federal taxes such as the distribution of corporate taxes, or about twice the 15.1% paid by middle-class families. (See “How Much the Rich Pay,” January 23, 2012.)

This is because wealthy tax filers make most of their income from investments. Such income is taxed once at the corporate rate of 35% and again when it is passed through to the individual as a capital gain or dividend at 15%, for a highest marginal tax rate of about 44.75%.

This double taxation is one reason the U.S. has long had a differential tax rate for capital gains. Another reason is because while taxpayers must pay taxes on their gains, they aren’t allowed to deduct capital losses (beyond $3,000 a year) except against gains in the current year. Capital gains also aren’t indexed for inflation, so a lower rate is intended to offset the effect of inflated gains.