How well do we know ourselves? The intuition that you are your own best judge is strong, yet flawed. The prevailing wisdom in social psychology today is that we are incorrigible self-enhancers. We tend to think we are more attractive, intelligent and agreeable than we really are. Conversely, depressed people have been found to make more realistic estimates of their own abilities, a phenomenon that suggests a touch of optimism may build our resilience to the bumps and bruises of everyday life.

Yet psychologist Simine Vazire at Washington University in Saint Louis and her colleagues believe this theory is overly simplistic. “A large group of people seems to have very accurate views of themselves,” she says. And that’s probably a good thing.

Although overconfidence can make us seem influential and knowledgeable, it can also backfire. In one set of experiments, by psychologists Elizabeth Tenney and Barbara Spellman at the University of Virginia, people compared two applicants for a job in day care. One of them claimed to be extremely patient and alert whereas the other was more circumspect. Initially the recruiters preferred the highly confident applicant—until they learned from a personality test that this person had overstated her abilities. Then the more cautious candidate came out ahead.

These findings and others suggest that self-enhancement becomes an impediment once an acquaintanceship deepens past first impressions. In Vazire’s presentation at the American Psychological Association’s annual meeting in Honolulu last week, she noted that two things have stood out in her data as the key components of a strong friendship: thinking our friend is funny, and believing she knows herself well.

Vazire and her colleagues wondered how self-knowledge might affect relationships. Measuring the accuracy of a person’s beliefs about himself, however, is devilishly tricky. So they focused on how aware people are of their own typical behaviors. First, they asked their 77 study participants to nominate a parent, romantic partner or very close friend who could offer insight into how the participants spent their days—whether they devoted more or less time than the average person to socializing, watching television, listening to music and so on. The participants provided assessments of their own time use, too.

Then the subjects donned a digital recorder programmed to record half a minute of sound every 12 minutes over the course of four days. From these 30-second segments the researchers could extrapolate what their participants did during the day and see how well their estimates held up. The team also asked the participants and their informants to rate the quality of their relationship.

As the researchers reported in the journal PLOS ONE last week, they found that the people with better self-knowledge—those whose estimates matched the sound-sampling data most closely—had the strongest relationships. The team used the friend, parent or partner’s time-use estimates to rule out one potential confounding factor: that perhaps what strengthened the relationship was being highly predictable. The accuracy of the informant’s time-use estimates did not correlate with relationship quality, which suggests that something unique to self-knowledge drives the connection.

If relationships thrive on self-knowledge, that would suggest that the strongest bonds are the ones most grounded in reality. If so, it would challenge that old aphorism that love is blind. To investigate this possibility, Vazire and graduate student Brittany Solomon looked at the nuances of how two people in a romantic relationship view each other and themselves.

Participants in this study and their partners rated themselves and each other on attributes such as attractiveness, intelligence and likability. They found that across the board, people tended to view themselves in a harsher light than their partners did. But there’s more. The partners were not as blindly doting as they seemed at first.

To understand their findings, consider one representative, fictional couple, Jack and Judy. Let’s say Judy rates herself as a 10 out of 15 in intelligence. Jack rates her as a 14. But if you ask Jack how he thinks Judy would rate herself, he’ll tell you that she thinks she is an 11. And if you ask Jack what Judy’s friends might say, he would guess they give her a 12.5. Ask the friends directly, and they would say that Judy is an 11.5.

In short, Jack seems to be juggling three views of Judy: he maintains his own belief, but he also recognizes that his assessment may not be shared by the world at large, and not even by his dearly beloved Judy. As this study illustrates, our thinking about our own beliefs—metacognition—reveals layers of bias and realism.

In order to maintain an intimate bond, “we need uniquely positive feelings toward our partnership,” Vazire say. Yet accuracy, too, fuels affection. “Maybe we can have both, by having positive illusions about our partner but also recognizing they’re somewhat idiosyncratic.” To this end, Vazire cites the well-known song “You Are So Beautiful,” the next two words of which are telling: “to me.”