Among most couples, romantic bonds often unravel when one person begins to doubt the other's affection. The threatened party typically starts behaving in ways that sabotage the union, studies show, whether or not the initial worries are founded.

Narcissists, however, come equipped with a belief that may head off this downward spiral: "They think, I know I'm wonderful, and therefore, my partner must think I'm wonderful," says Joshua Foster, a University of Georgia graduate student.

Foster led a group of female subjects to temporarily question their partners' devotion by having them list reasons why their partners may not be committed to them. After making their lists, the women tended to score high on a test about their relationships' dysfunction—they reported a willingness to accept a date from a stranger, for example.

The narcissists of the group, not surprisingly, found the first task difficult to complete. But they went on to score much lower on the next part of the exercise, portraying themselves as committed to their boyfriends, even though self-centered people are well known to have a propensity for cheating. Narcissists essentially balked at the suggestion that their partners might not think highly of them, whereas the "normal" subjects were unsettled by the thought.

A narcissist's inability to be affected emotionally derails his or her relationship over the long run, says Foster. "Narcissists seem great at first—they are confident, exciting and seem to be free of hang-ups. But then, Mr. Cool turns into Mr. Doesn't Care."