"When you have fewer employees doing compliance work, you end up leaving tax revenue on the table," Mr. Koskinen added, explaining how budget cuts have damaged the agency and cost the government at least $20 billion over the last five years.
Americans acknowledge as much. While people surveyed overwhelmingly say they believe everyone has a responsibility to pay their fair share, a majority admit that it is fear of an audit or third-party reporting that ultimately spurs them to hand over the money.
Still, there is some striking evidence that the rich are different: They are more tempted to push back against the government. In a classic study, Joel Slemrod, a tax expert at the University of Michigan, found that the amount of reported income increased among low- and middle-income individuals after they were told their returns would be "closely examined." Higher-income individuals had the opposite reaction — their reported income went down.
The results, Mr. Slemrod suggested, may be explained by the resources available to the wealthy. Backed by an array of legal and accounting experts, the rich view the early warning of an audit as just an opening gambit by the I.R.S. "They talk to their accountant, and he says, 'Calm down, it's a negotiation,' " Mr. Slemrod said. And so they respond in turn with a lowball offer of payment.
Mr. Slemrod said that what distinguishes the latest wave of research are the field experiments, a version of studying animals in the wild instead of in the laboratory. For example, researchers in the United States and elsewhere have sent different taxpayers different notices, comparing a warning that they have a 50-50 shot of being audited with one that says an audit is a certainty.
It turns out that just receiving any kind of alert from tax officials — a subtle reminder that they know about you, yes, you — can spur an uptick in payment.
And while moral appeals are often not very effective in getting people to reveal hidden income, there is some evidence they may work better when it comes to collecting a tax debt.
A study conducted in conjunction with the British tax collection agency found a significantly different response to a letter reminding people that nine out of 10 people pay their taxes on time with a second one that not only repeated the message but also warned: "You are currently in the very small minority of people who have not paid us yet."