FOR MOST AMERICANS, income tax returns are a private matter, and federal law protects that privacy. For those who would be president, a different standard applies. The modern presidency demands so much of one individual — decisions of immense complexity, consequence and difficulty — that the candidates’ characters must be thoroughly examined.

The exploratory process is often unpleasant for candidates, especially when it is stimulated or exploited by their opponents. But it is essential for voters. The probing and investigating is a chance to examine all the ups and downs of a career, the critical moments and life experiences that might foretell how a president will make decisions.

This is why, as we said months ago, Mitt Romney’s tax returns are important. He has described himself as a successful capitalist who took risks and created wealth, a laudable credential. Voters would benefit by seeing and evaluating the details of that story, including through his tax records.

So far, Mr. Romney has made public one year of his federal return, for 2010, and promises to release the 2011 return when it is ready. He has declined to disclose any earlier years. This is an error that he should quickly reverse. He does not need to go as far as his father, who made public 12 years of tax returns when he was running for president in 1968, but more than two years would be informative. The last Republican presidential nominee, Sen. John McCain (R-Ariz.), made public only two years of his tax returns before the election; others have provided many more.

A president often has to decide how much to disclose about events or decisions that are not particularly pleasant. We’ve long argued that openness and transparency are critical in building public confidence and in the functioning of democracy. If Mr. Romney is not willing to open up his tax returns, what does that say about his instinct for leveling with the American people from the Oval Office? His refusal to release the names of his chief campaign bundlers compounds the concern.

On Tuesday, Mr. Romney brushed off demands to make public more tax returns, saying they would be used by the Obama campaign’s opposition research. “And I’m simply not enthusiastic about giving them hundreds or thousands of more pages to pick through, distort and lie about,” he said. Given both sides’ campaign so far, his expectation of distortion may be realistic.

But Mr. Romney surely is capable of responding to any distortions. For voters, the documents are essential exhibits in an application for the presidency. It is insulting to voters for Mr. Romney to keep them under wraps and will only fuel suspicions that he has something else to conceal.

He should prove otherwise.