Story highlights College students were asked to rate others on a friendship scale

Only 53 percent of the relationships were reciprocal

Here's a fun exercise: Take a minute and count up all your friends. Not just the close ones, or the ones you've seen recently -- I mean every single person on this Earth that you consider a pal.

Got a number in your mind? Good. Now cut it in half.

Okay, yes, "fun" may have been a bit of a reach there. But this new, smaller number may actually be more accurate. As it turns out, we can be pretty terrible at knowing who our friends are: In what may be among the saddest pieces of social-psychology research published in quite some time, a study in the journal PLoS One recently made the case that as many as half the people we consider our friends don't feel the same way.

The study authors gave a survey to 84 college students in the same class, asking each one to rate every other person in the study on a scale of zero ("I do not know this person") to five ("One of my best friends"), with three as the minimum score needed to qualify for friendship. The participants also wrote down their guesses for how each person would rate them.

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