What restrains politicians and businesses from acting dishonestly? A lot of people would answer: nothing.

Periodic political and corporate scandals have created a popular image of politicians and businessmen as little more than a collection of cheats, liars and crooks. However, while there will always be some dishonest people in any profession, the vast majority of American politicians and businessmen do not end up being frog marched out of their offices in handcuffs with their heads held low in shame before a gaggle of news cameras.

What helps keep companies honest is the threat that if they cheat customers, people won’t buy from them again. But that won’t work for politicians. Politicians don’t always have the incentive of re-election because eventually they all face a last term in office. Politicians retire at some point. They can’t live forever.

So, if it isn't the threat of facing the voters, what could ensure that politicians keep their promises?

There has been a lot of academic work studying this question, and the way to solve the problem is to elect politicians who inherently value the policy positions that they take.

Of course, voters won’t exactly figure out whether a politician really values what he claims to value, but there are signals that voters can look for. One is how politicians flip-flop on issues. If a politician can easily change what he claims to believe during a campaign, voters will have a lot less confidence that he really believes in what he is telling them. If he doesn’t hold his views strongly, voters can’t trust the politician to keep his promises.

We see this in other areas of life as well. For example, with lifetime appointments, Supreme Court justices are removed from any equivalent of re-election pressures. When a president nominates justices, he looks for a judge who will provide a reliable vote for his political orientation. The president analyzes potential nominees’ career voting records and looks for other signs of an intrinsic commitment to shared ideals. Judges may be able to hide or misrepresent their true philosophy in hopes of getting a Supreme Court nomination, but it is difficult to do this over a long period of time. Supreme Court nominees will usually have years of consistent rulings that testify to their reliability. It is a major reason why presidents prefer older nominees over younger ones, even though the younger ones can spend a much longer time on the bench.

In the last week, charges of flip-flopping got into high gear against both presidential campaigns.

For Senator McCain, the flip-flop everyone focused on is his proposal to resume offshore drilling. As the Washington Post notes: “McCain's announcement is a reversal of the position he took in his 2000 presidential campaign . . .”

While the change isn’t making environmentalists happy, McCain notes that a policy that made sense when gasoline was about a dollar a gallon doesn’t make much sense with gas now at $4.08 a gallon. Obviously the price has risen dramatically even compared to what it was a few months ago.

The charges against Senator Obama over the last week have been more numerous and apparently much more difficult to explain away.

-- NAFTA. During the primaries, an Obama campaign slogan was “Only Barack Obama Consistently Opposed NAFTA” and that he would use the threat to “opt-out” as a “hammer” to force the Canadians and Mexicans to "renegotiate" NAFTA. But last week in an interview with Fortune magazine, Obama reversed course saying that he was “not a big believer in doing things unilaterally’ and that he wasn’t going to force a renegotiation of NAFTA. Obama’s explanation for the switch now that the primaries were over was that "Sometimes during campaigns the rhetoric gets overheated and amplified."

The flip-flop was all the more difficult to explain because Obama’s senior economics adviser, Austin Goolsbee, was caught right before the Ohio primary telling Canadians that Obama didn’t really mean his promise to renegotiate the treaty. Whether Obama really believed what he was saying during the campaign became an important question, and Obama went to great length on multiple occasions to assure voters that he would stand by his promise. Both Obama and Goolsbee completely denied the Canadian story, with Goolsbee claiming that the reporting by Canadian TV was “a totally inaccurate story.”

-- Foreign Intelligence Surveillance Act (FISA). For Democrats during the primary, FISA was something for which no compromise was allowed. Back in September, Obama’s campaign vowed he would "support a filibuster of any bill that includes retroactive immunity for telecommunications companies." But this last week Obama reversed course and supported the FISA compromise granting the companies amnesty. Democrat party activists such as the Daily Kos are describing Obama’s switch as a “disaster.” Politico reports that “Obama's allies at MoveOn are asking supporters to 'call Sen. Obama today and tell him you're counting on him to keep his word.' ”

-- Public Financing of Presidential Campaigns. A year ago Obama gathered favorable headlines and media coverage for his dedication to saving public financing of presidential campaigns. Editorials chided Obama’s opponents for not making the promise that he had made. To Obama, the decision likely came down to how much money he can raise for the general election. If Obama can raise as much for the general election as he has for the primaries, he could easily exceed what McCain will be allowed to spend by 4-to-1. Even that might be an underestimate, because he will now have the resources of a united Democratic party.

Obama’s explanation for breaking this promise is that “The public financing of presidential elections as it exists today is broken, and we face opponents who have become masters at gaming this broken system.” Yet, he hasn’t explained why the system is any more broken now than it was late last fall.

With the primaries over, both candidates may want to be in the political center. But that is a much more natural place for John McCain with his maverick voting record than it is for Barack Obama.

If Obama’s awkward movements to the center continue, even his massive money advantages during the general election might be unable to undo the damage to his credibility.

John Lott is the author of Freedomnomics and a senior research scientist at the University of Maryland.