It's official: It's not a marriage penalty, it's a singles penalty. The evidence was presented in a law review article written by Lily Kahng, who served "three years as attorney advisor in the Office of Tax Legislative Counsel in the U. S. Department of the Treasury." Her article about "the single taxpayer in a joint return world" can be downloaded for free.
Here is her most important conclusion:
"There is never a single person's bonus - that is, a single person never pays less relative to a couple, whether married or unmarried, with the same amount of income as the single person (p. 660)."
Of course, that's not what we hear in the public conversation. Instead, the fight over the so-called marriage penalty is about the differences in taxation between an unmarried couple and a married couple. Quoting Kahng again:
"In recent years, the debate regarding the joint return has not questioned the primacy of the couple as taxpayer, focusing rather on whether and to what extent couples should suffer marriage penalties or enjoy marriage bonuses, and who among the universe of couples ought to be eligible for the marriage bonus. The treatment of single individuals has received almost no attention (p. 663)."
Kahng distinguishes between an "unmarried couple's penalty or bonus" and a "single person's penalty." An unmarried couple can, under certain circumstances, end up paying more in taxes if they marry - that's what we usually think of as the marriage penalty. But they can also pay less. A single person (not part of a couple) never pays less on the same income as a couple.
The most common rationalization for single people paying more is that the single person is just one person and the couple is two people, so the same amount of money should go further for the single person than for the couple. Kahng explains that this is not necessarily so. For example (continue reading here).