The Alternative Minimum Tax is quite possibly the least popular part of the federal tax code. One Forbes columnist calls it “the normal tax system’s evil twin.” The Tax Policy Center has labeled it “the epitome of pointless complexity.” Nine presidential candidates – ranging from Marco Rubio to Bernie Sanders – have pledged to eliminate it.

Nevertheless, the AMT remains a part of the U.S. tax code, and over 3.9 million households are subject to it every year. What do we know about these households?

First, if a household makes less than $200,000 a year, it is very unlikely that it will be subject to the AMT. The graph below shows the share of households in each income group that fell under the AMT in 2013:

In 2013, 62.5 percent of households with income between $200,000 and $500,000 were subject to the AMT, the highest rate out of any income group. Among households making between $100,000 and $200,000, only 3.8 percent fell under the tax.

There are a couple other important characteristics of households that fall under the AMT:

Households that live in high-tax states are much more likely to be subject to the AMT. In New Jersey, 81.6 percent of households making between $200,000 and $500,000 fall under the AMT. In Wyoming, only 26.3 percent of households in the same income range are subject to the AMT. This is because the AMT does not provide a deduction for state and local taxes paid, as the regular tax code does.

are much more likely to be subject to the AMT. In New Jersey, 81.6 percent of households making between $200,000 and $500,000 fall under the AMT. In Wyoming, only 26.3 percent of households in the same income range are subject to the AMT. This is because the AMT does not provide a deduction for state and local taxes paid, as the regular tax code does. The more children a household has, the more likely it is to fall under the AMT. This is because the AMT does not provide a larger exemption for households with more children, as the regular tax code does.

a household has, the more likely it is to fall under the AMT. This is because the AMT does not provide a larger exemption for households with more children, as the regular tax code does. Almost all households that are subject to the AMT are ones that itemize deductions (3.8 million out of 3.9 million). This makes sense, as one of the chief purposes of the AMT is to prevent Americans from reaping too much benefit from itemized deductions.

As these statistics show, the AMT basically functions as a surtax on high-income taxpayers in high-tax states with children. This data is important because it provides context for recent political debates about the Alternative Minimum Tax:

Bernie Sanders and the three leading Republican candidates have all proposed eliminating the AMT. Meanwhile, Hillary Clinton would keep the AMT in place and add a second minimum tax to the U.S. code – the Buffett rule.

Critics of the AMT often point to the overwhelming complexity of the tax. The AMT is essentially a parallel tax system, in which taxpayers are required to calculate their tax bill a second time over, using different rules and definitions. Even though only 3.9 million taxpayers end up owing AMT, over 9.7 million taxpayers are required to go through the steps of calculating their AMT liability each year.

On the other hand, supporters of the AMT often point to the ways in which it is designed better than the regular tax system. The AMT disallows many of the deductions and preferences that clutter the U.S. tax code. Furthermore, the AMT levies a lower marginal tax rate (28 percent) than the regular tax system (39.6 percent), and is therefore more favorable to work and investment. Indeed, the Tax Foundation’s economic model finds that repealing the AMT would decrease the long-run size of the U.S. economy by roughly 0.25 percent.

Of course, a fundamental reform of the U.S. tax code would alleviate many of the problems that the AMT was created to fix. So, proposals to eliminate the AMT should not be evaluated in isolation, but as part of broader tax reform plans.