You know those people you call your "friends"? Yeah, they may not actually deserve that label. Because chances are, a good chunk of them don't consider you a friend.

A recent study published in the scientific journal PLOS ONE analyzed surveys from 600 students in Israel, Europe, and the U.S. to try and determine how many of their friends were actually their friends—that is, how many of their relationships were mutual. The findings of the study will definitely depress you.

As it turns out, approximately half of your friends consider you a friend. That's right: If you believe you have about 20 friends, you probably only have 10. My condolences.

"It turns out that we're very bad at judging who our friends are," Dr. Erez Shmueli, one of the study's authors, said after the study was done. "If you think someone is your friend, you expect him to feel the same way. But in fact, that's not the case."

Oh, and it gets worse. One-sided friendships are, as you would expect, not as valuable as mutual ones, Shmueli explained. The study found that friends in one-sided relationships don't have as much influence over one other. One experiment, for example, found that only reciprocal friendships helped motivate people to exercise.

Who are your real friends, then?

It turns out, there are a few easy ways to tell. The study found that the number of mutual friends you have and the similarity of your social statuses can determine whether you're actually friends or not. Specifically, "the difference in the number of friends of the two individuals" is a good predictor, Shmueli told Complex over email. "The higher this difference is, the lower the likelihood of the friendship to be reciprocal."

So that guy who hangs out with you and all your other friends at the same bar every Friday night? Probably your friend. That girl who's insanely popular and invites you to a party once in a while? Probably not your friend. Sorry.

UPDATED 9:14 a.m. ET: This story was updated to include comment from Erez Shmueli.