President Donald Trump and the GOP leadership have vowed to reduce U.S. corporate taxes, arguing that the statutory 35 percent rate is far too high.

But thanks to loopholes and tax-minimization strategies, few large, profitable corporations actually pay that rate. Among the roughly half of Fortune 500 companies that were consistently profitable between 2008 and 2015, the effective federal income tax rate was 21.2 percent over that eight-year period, according to a study from the Institute on Taxation and Economic Policy. Eighteen of them, including General Electric (GE) and Priceline.com (PCLN), paid no federal income taxes during that time.

The findings come amid pledges by Republican leaders to slash corporate taxes, arguing that American companies are at a competitive disadvantage compared with countries such as China and Mexico, which have top rates of 25 percent and 30 percent, respectively.

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Studying an eight-year period addresses how taxes impact corporations during an entire business cycle, rather than shorter periods previously analyzed by ITEP, a liberal-leaning think tank focused on tax policy.

“The effective tax rate for the sample is a little bit higher, but it’s closer to half of the legal tax rate than to 35 percent,” said ITEP senior fellow Matthew Gardner. “It was still pretty astonishing when you put all the buckets together for these 18 companies that the taxes on their profits would add up to zero.”

He added, “These companies have a lot of tax avoidance strategies at their disposal.”

Although the strategies may be legal, their usage raises questions of fairness given that large corporations employ tax experts and large tax advisory firms to help them minimize their taxes. That’s not typically accessible to smaller businesses, which means they may be saddled with higher taxes while larger, more profitable firms are in some cases able to completely eliminate their tax burden.

“If you want to put blinders on you can view this as a level playing field, but when it requires such an investment of time, it makes [the strategies] unavailable for smaller businesses,” Gardner said. “It shifts the tax load away from these big corporations and onto smaller business and middle-income families. It’s a real fairness problem.”

Other studies have found that U.S. corporations aren’t paying the 35 percent rate, including a March report from the Congressional Budget Office, which found the average U.S. corporate tax rate stands at 29 percent.

Common tax-reduction strategies include issuing stock options to executives so they can buy stock in the company at a discount. When options are exercised, the company takes a deduction for the difference between the market value of the stock and what the executives pay. Facebook (FB) has been the largest benefactor from this strategy, saving $5.8 billion in taxes over eight years, ITEP calculated.

Other industry-specific tax breaks include write-offs for business activities such as building NASCAR race tracks or research, as well as “accelerated deprecation,” which effectively lets companies defer taxes on equipment and other assets.

Companies can also receive a deduction for domestic manufacturing, a tax break created under the American Jobs Creation Act of 2004. The goal was to help revitalize American manufacturing, but companies have pushed the meaning of “manufacturing” to mean, well, almost anything. For instance, companies that are using the tax break include Starbucks (SBUX), Hollywood film producers and online reservation system OpenTable, ITEP said.

Some GOP lawmakers want to lower the corporate tax rate to 20 percent and institute a so-called border adjustment tax (BAT), which would add a tax to imported goods sold in the U.S., a cost that would likely be passed on to consumers. The border adjustment tax would raise more than $1 trillion in revenue over a decade, according to Goldman Sachs.

That would likely shift the burden of taxes onto consumers, while allowing corporations to continue to employ loopholes and tax strategies to lower their taxes on a 20 percent rate rather than 35 percent, Gardner said.

“The bottom line is [the BAT] is a straight-forward money grab,” he said. “It’s a way of filling a money hole if the corporate tax rates are reduced. There’s a very high likelihood much of it would be passed onto consumers. That is a sharp difference from the way our corporate income tax works. It’s thought, by and large, the corporate income tax is passed onto shareholders of the corporation.”

Gardner, who said a fairer system would eliminate loopholes and tax-avoidance strategies, said the point of ITEP’s report is not to “badmouth all corporations,” but to point out the discrepancies between different industries and businesses. For instance, gas and electric utilities paid the lowest effective federal tax rate over eight years, at 3.1 percent. By contrast, retailers paid one of the highest effective rates, at 31 percent, according to the analysis.

“There are a lot of people with a bone to pick with the current system,” he said.

Below are the 18 companies that paid no federal income tax from 2008-2015, according to ITEP. Many of them are in the energy sector.