If Democrats Hillary Clinton and Barack Obama ever took a break from fundraising to bone up on psychology, they might realize the need to talk up . . . John Edwards.

The same goes for front-runners John McCain and Rudy Giuliani in the race for the 2008 Republican presidential nomination. They ought to be drawing attention to Mitt Romney, or to "Law and Order" star Fred Thompson, who could be running third in the race if he declared.

Front-runners are usually focused on racing each other. They often do not realize that when people cannot decide between two leading candidates -- and it doesn't matter whether we are talking about politicians or consumer appliances -- our decision can be subtly swayed by whoever is in third place.

Psychologists call this the decoy effect: In a perfectly rational world, third candidates should only siphon votes away from one or both of the leading contenders. Under no circumstances should they cause the vote share of either front-runner to increase. In the actual world, however, third candidates regularly have the unintended effect of making one of the front-runners look better than before in the minds of undecided voters.

Joel Huber, a Duke University marketing professor, showed how the decoy effect works with restaurants. Huber asked people whether they would prefer to eat at a five-star restaurant that was far away or at a three-star restaurant nearby. As with many choices in life, each restaurant had different advantages. If the better restaurant was also nearby, there would be no dilemma. But the question forced people to compare apples and oranges -- trade off quality against convenience -- which ensured no automatic answer.

The human brain, however, always seeks simple answers. Enter the third candidate. Huber told some people there was also a choice of a four-star restaurant that was farther away than the five-star option. People now gravitated toward the five-star choice, since it was better and closer than the third candidate. (The three-star restaurant was closer, but not as good as the new candidate.)

Another group was given a different third candidate, a two-star restaurant halfway between the first two. Many people now chose the three-star restaurant, because it beat the new option on convenience and quality. (The five-star restaurant outdid this third candidate on only one measure, quality.)

What the decoy effect basically shows is that when people cannot decide between two front-runners, they use the third candidate as a sort of measuring stick. If one front-runner looks much better than the third candidate, people gravitate toward that front-runner. Third candidates, in other words, can make a complicated decision feel simple.

How would this work in the context of the current political race?

Let's say you are a centrist Democratic voter who cannot decide between Clinton and Obama because you want a candidate who is strong on national security but also someone fresh. You like Clinton on one measure and Obama on the other. Enter Edwards, whom you see as more dovish than Obama but part of the same establishment as Clinton. Obama looks better than Edwards on both counts, whereas Clinton beats Edwards on only the national security issue.

On the other hand, let's say you care about experience but are wary of policies such as universal health care. You like Clinton's experience but are worried about her track record on health care. Enter Edwards, whom you perceive to be as untested as Obama but even more likely to pursue a traditionally liberal agenda. Clinton now looks better than Edwards on both counts.

What this means is that Obama and Clinton stand to gain by drawing attention to those qualities of Edwards's that make each front-runner look much better than the other. Clever front-runners, in other words, can turn third candidates into their wingmen.

"Many people lavished hate on Ralph Nader for presumably taking votes away from the Democratic front-runner in the 2000 presidential election," said Scott Highhouse, who has studied the decoy effect at Bowling Green State University. "Research on the decoy effect suggests that Nader's presence, rather than taking votes away, probably increased the share of votes for the candidate he most resembled."

Suzanne Fogel, head of the marketing department at DePaul University, conducted a study of the 1992 presidential election, where Ross Perot provided the psychologist with a third candidate and a national laboratory. She and colleagues Yigang Pan and Robert Pitts found that, contrary to the conventional wisdom about which candidate Perot would hurt, undecided voters who focused on different qualities of Perot tended to gravitate toward George H.W. Bush or Bill Clinton.

"People are not very thorough information processors," she said. "People try to distill the essence from things, and if someone calls attention to one attribute or another, you make your choice based on that attribute because it is in the foreground of your attention."

I asked her whether the 2008 front-runners could take advantage of the phenomenon.

"You can manipulate that by what you draw attention to," she said. "You would find someone who is sort of close to you but that you are better than, and talk about how you are different from that person."

And what should people do to avoid being manipulated by marketers and pols? Don't let salespeople tell you what issues to care about, and don't let candidates define one another. More simply, think for yourself and be wary if a difficult choice suddenly feels simple.