Some high-income business owners could face marginal tax rates exceeding 100% under the Senate's tax bill, far beyond the listed tax rates in the Republican plan.

That means a business owner's next $100 in earnings, under certain circumstances, would require paying more than $100 in additional federal and state taxes.

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As lawmakers rush to write the final tax bill over the next week, they already are looking at changes to prevent this from happening. Broadly, House and Senate Republicans are trying to reconcile the bills they passed, looking for ways to pay for eliminating the most contentious proposals. The formal House-Senate conference committee will meet Wednesday, and GOP lawmakers may unveil an agreement by the end of the week.

The possible marginal tax rate of more than 100% results from the combination of tax policies designed to provide benefits to businesses and families but then deny them to the richest people. As income climbs and those breaks phase out, each dollar of income faces regular tax rates and a hidden marginal rate on top of that, in the form of vanishing tax breaks. That structure, if maintained in a final law, would create some of the disincentives to working and to earning business profits that Republicans have long complained about, while opening lucrative avenues for tax avoidance.

As a taxpayer's income gets much higher and moves out of those phaseout ranges, the marginal tax rates would go down.

Consider, for example, a married, self-employed New Jersey lawyer with three children and earnings of $615,000. Getting $100 in business income beyond that amount would force the lawyer to pay $105.45 in federal and state taxes, according to calculations by the conservative-leaning Tax Foundation. That is more than double the marginal tax rate that household faces today.

If the New Jersey lawyer's stay-at-home spouse wanted a job, the first $100 of the spouse's wages would require $107.79 in taxes. And the tax rates for similarly situated residents of California and New York City would be even higher, the Tax Foundation found. Analyses by the Tax Policy Center, which is run by a former Obama administration official, find similar results, with federal marginal rates as high as 85%, and those don't include items such as state taxes, self-employment taxes or the phase-out of child tax credits.

The bill as written would provide incentives for business owners to shift profits across calendar years, move personal expenses inside the business and engage in other economically unproductive maneuvers, said David Gamage, a tax-law professor at Indiana University.

"I would expect a huge tax-gaming response once people fully understand how it works," said Mr. Gamage, a former Treasury Department official, who said business owners have an easier time engaging in such tax avoidance than salaried employees do. "The payoff for gaming is huge, within the set of people who both face these rates and have flexible enough business structures."

The analyses "raise a valid concern" that lawmakers are examining, said Julia Lawless, a spokeswoman for the Senate Finance Committee.

"With any major reform, there will always be unusual hypotheticals delivering anomalous results," she said. "The goal of Congress's tax overhaul has been to lower taxes on the American people and by and large, according to a variety of analyses, we're achieving that."

Marginal tax rates are different from average tax rates. A marginal rate is the tax on the edge, or margin, of one's earnings, and so it reflects what would be the next dollar of income. The average rate is a way of measuring a taxpayer's total burden.

The Republican bills are trying to reduce both marginal and average tax rates, and for many taxpayers, they do. The marginal tax rates above 100% affect a small slice of households with very particular circumstances. Similar, though smaller, effects occur throughout the tax system.

"This is a big concern," said Scott Greenberg, a Tax Foundation analyst. "It would be unfortunate if Congress passed a tax bill that had the effect of making additional work and additional income not worthwhile for any subgroup of households."

Here's how that New Jersey lawyer's marginal rate adds up to more than 100%:

-- The household is paying the 35% marginal tax rate on their income range. Or, they are paying the alternative minimum tax, which operates at the same marginal rate in that income range.

-- The household is paying New Jersey's highest income-tax rate, which is 8.97%, and now has to pay all of that because the Republican tax plan wouldn't let such state or local taxes be deducted from federal income.

-- The household is also losing a deduction the Senate created for so-called pass-through businesses such as partnerships and S corporations. That 23% deduction is fully available to owners of service businesses like law firms, but only if income is below $500,000 for a married couple.

The deduction then phases out over $100,000 in income, according to a complex formula, disappearing entirely once income reaches $624,000. Up to that point, each additional dollar of business income faces progressively steeper tax rates because the deduction and its benefit are shrinking rapidly as income goes up.

The provisions also interact with each other in ways that drive up marginal rates. "The central problem here is that there is a large benefit phasing out over a short range," Mr. Greenberg said.

-- The Republican bill doubles the child tax credit to $2,000 but phases it out beginning at $500,000 income for joint filers. The credit shrinks by $50 for every $1,000 in income above that, so a married couple with three children faces a higher marginal tax rate when they're in that phase-out range.

-- The analysis assumes that the New Jersey lawyer is paying a 3.8% tax on self-employment income.

Pushing marginal tax rates lower on these households wouldn't be easy and would require tradeoffs. Republicans could make the phase-out of the business deduction more gentle, spreading it over, say, $200,000, as opposed to $100,000, of income above $500,000.

But that would make the tax cuts bigger, and Republicans are already looking for money to offset other changes they are planning.

They could lower the threshold for the child tax credit, but that would reduce tax cuts for households below $500,000.

Under current law, there are some high marginal tax rates for some lower-income households. Some families just above the poverty line can see their earned income tax credits and food stamps going down as their federal and state taxes go up. That combination can create marginal tax rates of around 75%, according to the Congressional Budget Office.

Write to Richard Rubin at richard.rubin@wsj.com

(END) Dow Jones Newswires

December 10, 2017 19:01 ET (00:01 GMT)