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.
Some have suggested Trump's tax returns could reveal his wealth to be less than what he says. Fortune's Shawn Tully made such an argument in March. David Cay Johnston, a columnist for The Daily Beast and author of 2016 title The Making of Donald Trump, said in a Slate podcast this week that he and other observers may be able to back engineer Trump's tax returns to decipher how much his buildings are actually worth.
However, most experts agree Trump's tax returns would not somehow blow Trump's net wealth cover.
"We are discussing 'income' tax returns, not 'estate' or 'wealth' returns. Trump's tax returns would reveal very little about his wealth," said Steve Rosenthal, senior fellow at the Urban-Brookings Tax Policy Center.
Federal income tax returns show only taxable income from property (often a category of income that is aggregated across many different pieces of property), and one cannot back into the value of Trump's properties through the taxable income they generate (which reflects deductions, for example). State and local property tax returns will report property value, but no one is arguing Trump release his property tax returns.
Perhaps Trump's own paranoia about speculation surrounding his net wealth -- or his concerns that media conjecture, however unfair, will try to tie his tax returns to his wealth -- is enough for him to keep his returns under wraps.
"Donald Trump, in his dealings with us, was very frank, he was always gracious and generous with his time with us, but he would often slip in casual references to the fact that he might sue us if he didn't like the book, and especially on that topic of how much is he really worth," Kranish told NPR.