So now that Mitt Romney has released another tax return, we're going to get a rehash of stories on the fact that Mitt Romney's tax rate is lower than the average American's. But let's take a look at whether this is true; and, to the degree it is, why it's true.

To start, we'll use the TPC estimates of 2011 tax

rates. Now Mitt Romney's tax rate was reported to be 14.1%. So how does that compare? Well, first we have to choose what to compare it to. One approach would be to compare Romney's rate to the sum of the income and payroll tax rates. It's clear that neither the corporate tax nor the estate tax can be discerned from Romney's tax return. It's also true that Romney's FICA tax is unknowable from his return but we can assume that Romney's FICA tax is pretty close to zero as a percentage of his income.

So if we look at income plus payroll taxes by income group, we find that middle income taxpayers (the middle quintile of earners) have a lower tax rate than Romney (12.1% vs 14.1%) and the second quintile has a slightly higher rate (16.0% versus 14.1%).

So what accounts for the difference? Well, many people have claimed that the difference is driven by "preferential rates on capital gains." But this just doesn't seem to be true. Taxation on capital gains is different from ordinary income in two ways. One, it's taxed at a flat 15% and two, there is no FICA tax on capital income. So let's pull these two things apart.

To look at the effect of the rate differential, let's just look at the effective federal income tax rate. By this measure, Romney's effective tax rate is higher than every group of taxpayers below the 96th percentile, hardly lower than most Americans.

So what's accounting for the difference is the fact that FICA taxes aren't applied to capital gains and (for most of them) are capped at an income threshold. But these two things have been true of FICA taxes for as long as they have existed. So, in effect, the root cause of all of this is the fact that FICA taxes don't apply to capital income. Nobody has proposed changing this and it has always been this way.

Just thought it worth pointing out.