Verified by 
I have studied the psychology of deceiving and getting deceived. My colleagues and I did some research in which we asked people to tell us, in detail, about the most serious lie they ever told to anyone—and the most serious lie anyone ever told them. (You can read some of their stories here.) The most serious lies were about affairs more than anything else. And there was a certain theme I found intriguing in the stories of the people whose partners were cheating on them: They suggested that once they became certain their partner was cheating, all sorts of things seemed to fall into place. It was as if they "knew it all along," but had not let themselves acknowledge what was going on until they had to.
There was no way to know from that study alone what was actually happening: Did people just tell themselves that they knew it all along so they would not feel like complete and total dupes? Or at some level, did they really know?
People in close romantic relationships often believe that they know best when their partner is lying. Sometimes they can even describe to you what they believe to be the telltale clues. But thinking that you can tell doesn't mean that you can tell. (In fact, in a review of all of the available studies of accuracy and confidence, my colleagues and I found that people who were confident about their judgments of deceptiveness were no more or less likely to be correct in their judgments than people who were not confident.)
The intuitive case for believing that you know when your partner is lying better than anyone else does is that you spend so much time with that person and observe that person so carefully, at least in theory. Perhaps that greater exposure and experience could help—if it were the only thing factoring into your insights into your partner's deceptiveness. But it is not. One of the most important additional factors is motivation. What would it mean to see through to the truth instead of the fantasy that your partner is selling to you? If you worry that you might not like what you find, then you may not be so great at recognizing what is right in front of you.
My former graduate student, Eric Anderson, did a clever dissertation (described here) in which he had people answer the dreaded question, "Do you think that person over there is attractive?" Romantic partners did worse at knowing whether the answers were truthful or deceptive than total strangers did. At some level, though, they did seem to know the truth. When they were asked indirect questions (for example, "Did you get enough information from the person's answer?"), the partners were actually better than the strangers at distinguishing the lies from the truths. That is, they realized that they had not gotten enough information when their partner was lying (even though they didn't know for sure that their partner was lying) and they felt that they did get enough information when their partner was telling the truth.
Probably because of my interest in single life, I've always been more interested in what friends know and don't know than what romantic partners do. As friends get to know each other better, do they become more insightful about each other's honesty and dishonesty? My colleagues (Eric Anderson and Matthew Ansfield) and I assessed the deception-detection abilities of 52 pairs of friends who had known each other for just one month, and then again about five months later. (The study is here.) When we compared the deception-detection accuracy of all 52 pairs the second time around to their accuracy when they only knew each other for a month, it looked like nothing happened: They got no better or worse over time.
As friendships progress, some deepen and others don't. We thought that might matter. So we looked separately at the pairs of friends who had become closer after five months, and those who had become less close. The friends who were becoming closer also became better at separating each other's lies from their truths. Not so for the friends who were growing apart; they just became more distrusting overall, and more inclined to believe that their friend was lying even when that person was telling the truth.
Does a friend's insightfulness depend on what it is that the other person is trying to hide?
It does indeed. In another study (also here), Weylin Sternglanz and I videotaped people as they honestly described experiences that made them feel happy, sad, or angry. They also described other experiences that made them feel happy or sad or angry, but tried to conceal their true feelings rather than express them clearly. Close friends, less-close friends, and strangers watched the tapes and tried to figure out how the people on the tapes really did feel.
When no one was trying to hide anything, friends were always better than strangers at knowing how the people on the tape really did feel. The closer friends were better than the strangers, but the less close friends were better, too.
If you want to test your psychological intuition, take a guess about the relative accuracy of the close friends, the less-close friends, and the strangers when the people on the tape were trying to conceal negative feelings (sadness or anger).
When you've told yourself your answer, continue reading.
The result: People who did the worst at discerning how the people on the tape were feeling when they were trying to hide sadness or anger were the close friends. The close friends were even a shade worse at the task than complete strangers. The people who did really well were the less-close friends.
It will take additional research to nail down a definitive explanation for these results, so I'll just mention a few of the possibilities Sternglanz and I considered: What if the feelings of sadness or anger had something to do with the friend? It might be more threatening to a closer friend than to a less-close friend to realize that a friend is mad at you or disappointed with you. So if you are that closer friend, maybe you just don't see that negativity.
Or maybe closer friends are more inclined to allow one another a zone of privacy. Maybe they can tell when the other person doesn't want to talk about something, and they leave it alone. Perhaps closer friends—if they recognized the anger or sadness—would feel more responsible for doing something about it than less-close friends. If they don't want to do what it would take to deal with a close friend's distress, then maybe the easy answer is not to recognize it. (That doesn't fit the ideal of a close friend, and it is not my preferred interpretation, but it is a possibility.)
The bottom line: Things are complicated with friends, as they are with lovers. Your special closeness and added experiences offer the potential to make you more insightful. That's just potential, though. If you might not like what you see, maybe you just won't see it. There may come a time when denial is no longer an option. It is then that you may realize that, at some level, you knew it all along.
