The habit of candidates universally releasing tax returns runs back to the 1970s. Even before then, there’d been some releases. George Romney famously released 12 years of returns ahead of the 1968 election. During the 2012 election, George’s son Mitt dragged his feet on releasing returns, earning some unflattering comparisons. At the time, Politifact investigated and found that since 1972, only seven presidential nominees had refused to release their returns: Democrat Jerry Brown (1992); Republicans Pat Buchanan and Dick Lugar (1996), Mike Huckabee, Rudy Giuliani, and Romney (2008), and Green Party candidate Ralph Nader (2000).

One thing sticks out about those candidates: None of them won a major-party nomination, or for that matter really came especially close. Trump, as the presumptive nominee of the Republican Party, is a different situation.

But Trump is a different situation for other reasons, too. He’s far wealthier than any other candidate to run for president, and he has a long history of questionable finances, and faces other allegations. His companies have declared bankruptcy four times. He’s been fined by the Federal Trade Commission for improper behavior. He incorrectly received a tax break for people making less than $500,000 per year. All of this means that people might have legitimate questions about what Trump is doing with his supposed vast sums: what he does with it, whether those things are legal, and further whether the techniques he likely uses to reduce his tax obligations (like many wealthy people) are appropriate, even when they are legal. Given Trump’s repeated attacks on companies that move their profits offshore, or hedge-fund managers who use the carried-interest loopholes, voters have a right to know whether he practices what he preaches.

It is true that candidates are all required to file a personal financial disclosure as part of post-Watergate reforms from the 1970s, but tax experts say returns offer a more complete view. Trump released a disclosure in 2015, claiming that he was worth $10 billion. But many finance experts greeted that estimate with feelings ranging from skepticism to derision. The disclosure form allows for ranges of values, so that Trump could simply say certain holdings were worth more than $50 million—and then claim the top-line value. The Wall Street Journal offered a more sober estimate of “at least $1.5 billion.”

The journalist Tim O’Brien was especially savage in mocking Trump’s rather inflated claims of value for his brand. If it seems a little personal for O’Brien, that’s understandable—and the backstory explains why it’s wise to be skeptical of Trump’s claims and push for more disclosure. In a 2005 book, O’Brien sought to determine just what Trump was really worth. He concluded that the Donald was really only worth $150 to $250 million. Trump, outraged, sued O’Brien for $5 billion for libel. (Perhaps he doth protest too much!) It didn’t work: The suit was thrown out. That doesn’t prove that O’Brien was right—it only proves that there weren’t grounds for a libel case—but the proceedings offered more reasons to doubt the face value of Trump’s claims, as O’Brien writes.