Many of your friends probably don't think you're friends, study says

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.

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