It's safe to assume that if you consider someone your friend , they think of you the same way, right? Not quite. According to a new study published in the journal PLOS ONE, you might want to think twice before buying friendship bracelets , because fewer friendships than you'd expect are mutual. Warning: this is about to get depressing.Researchers from Tel Aviv University surveyed students in the U.S., Europe, and Israel about their friendships with one another. The research team found that only 50% of the people they called their friends considered them their friends as well. Yikes.But does this really matter as long as you're getting a lot out of the friendship? Well, here's another sad finding: You're probably not. In a separate experiment, the researchers tracked people's physical activity after learning about their friends' exercise habits. It turned out that if a "friend" didn't consider someone a friend in return, they were less likely to influence that person's physical activity.How do you know who your true friends are? The authors found two major signs. First, if you and a friend have a lot of mutual friends, chances are higher that your friendship is reciprocal. Second, if someone has a lot more friends than you, they're less likely to consider you a friend.Really sorry to have to break that one to you. But hey, at least if your unrequited friends don't exercise, that won't cut into your own fitness routine.