Donald Trump has repeatedly refused to release his tax returns this election cycle, despite a push from both sides of the aisle. He claims an audit by the IRS is preventing him from keeping with a protocol followed by major presidential nominees since the 1970s. But that’s not really true: No law prevents Trump from releasing his returns during an audit, and the IRS has already completed audits of his returns from 2002 through 2008.

Like all other citizens, U.S. presidents enjoy this protection of their privacy. Since the early 1970s, however, most presidents have chosen to release their returns publicly. In the hope of making this information more widely available, the Tax History Project at Tax Analysts has compiled an archive of presidential tax returns. Knowing this, it is clear that any serious candidate for President of the United States would willingly hand over their tax records–unless something ugly is hiding inside their returns.

Here are five things that could actually be hiding in Trump’s taxes, according to

1. He doesn’t pay any taxes at all.

“I think the biggest likelihood is that the returns show him with relatively little income, and paying little in tax. This might be a result of him losing money in some years, and taking advantage of tax breaks on real estate investment,” said Joe Bankman, professor of tax law at Stanford University. “He couldn’t argue that he is a lot richer than he appears on the tax return without calling attention to the fact he pays tax at a lower rate than, say, a high school principal.”

New York Times column recently outlined such a scenario, pointing out that Trump could take advantage of tax breaks available to (and commonly used by) real estate developers to reduce his reported income to near-zero, or even report a loss. Real estate LLCs can generate enormous losses because of depreciation, interest payments, real estate taxes and operating costs that they can use to offset taxable income.

There is evidence he has done so in the past. A financial disclosure made in 1981 to New Jersey gambling regulators revealed Trump had in 1978 and 1979 claimed negative income, allowing him to pay no taxes. There are also indications he did not pay taxes in 1984.

“He might be worth gazillions of dollars, and he might have gazillions of dollars in income. But usually in the real estate ventures…those losses might be deductible then against his other income, indeed resulting in a lower taxable income than one might think someone in his position would have, and low taxes,” said Don Williamson, professor of taxation at American University.

2. His taxes would reveal lies and hypocrisy.

His taxes could reveal that he is running some personal expenses through his companies instead of his personal bank account. Or, they could tell a different story than the one he tells the bankers, lenders or casino operates with whom he works. A financial analyst for one of Trump’s companies working on a $200 million project currently underway in Washington, D.C. admitted in a deposition reported by BuzzFeed News that the firm prepared financial estimates for its lenders and investors that were “rosier” than those kept internally.

A recent Vanity Fair article speculated that Trump may have transferred the foreign rights to his brand to a company headquartered in an offshore tax haven, copying a move commonly used by tech giants like Apple (AAPL) and Alphabet (GOOGL) to lower tax burden. That kind of revelation could undercut Trump’s “America First” campaign message.

3. He doesn’t give to charity.

Charitable donations — or a lack thereof — could be at issue as well. The Washington Post has made attempts to delve into Trump’s charitable giving record, in public records finding the candidate has given significantly less to his Donald J. Trump Foundation than he has claimed (prior to his recent $1 million veterans’ donation, $2.8 million in a 15-year period, less than one-third of the pledged amount).

Trump’s camp has argued that he has given away much more privately, and his tax returns would likely provide insight into the veracity of such accounts.

4. He doesn’t really want to be president.

Ed Kleinbard, professor of law and business at the University of Southern California, has come to believe that what is most concerning about Trump’s refusal to release his tax return is not what might be in them but instead what it reveals about his true ambitions. What if the reason Trump isn’t complying with presidential tradition is that he doesn’t really want to be president at all?

“At some level, consciously or unconsciously, he is still focused on the fact that he is a businessman and doesn’t want to reveal the intimate details of his profitability to his competitors, to his prospective partners or to prospective customers,” Kleinbard said in a radio interview last week. “He’s not thinking about being a statesman for the rest of his life the way Mitt Romney did when he released his returns. He’s thinking bout still being a businessman, whether January 2017 or whenever.”

If his next act is as president, why would it matter that he doesn’t pay much in taxes, that he might sour his business relationships or that he doesn’t give as much to charity as he says?

5. He’s not worth as much as he says.

Washington Post reporter Michael Kranish, who, along with his colleague Marc Fisher, recently authored Trump Revealed: An American Journey of Ambition, Ego, Money and Power, told NPR that Trump “frequently gives wildly different figures for how much he’s worth” and that “he’s actually admitted in court documents that he makes up the value of many of his properties.”

Trump says he is worth $10 billion, but others peg it at far less. He once sued Timothy O’Brien, author of the 2005 book Trump Nation: The Art of Being The Donald, for calling him merely a “millionaire,” writing that Trump’s worth was somewhere between $150 million and $250 million (the suit was eventually thrown out).

The suit was material, and not just a matter of ego, according to Trump’s lawyers, because his “ability to close deals and secure financing for his projects depends on investors trusting his reputation and net worth,” said NPR host Robert Siegel during his interview with Kranish, quoting directly from the lawsuit.

After careful consideration of the possible reasons Trump will not release his tax returns, one can only think that whatever is hiding deep inside his history with the IRS, it is something so ugly, even Trump can’t handle it, and perhaps he really does have a lot to lose.