Anyone who's ever watched little girls at play knows that three is a very bad number. Two girls will happily interact, for hours on end. But, once you introduce a third, that's when the trouble starts.
Usually what happens is that two of the girls will form a bond, leaving the third one out. Oftentimes, no malice is intended. The newly formed pair may simply be oblivious to the fact they're excluding someone. But it isn't long before the one who's been sidelined will begin to complain, usually through tears, that "They won't play with me."
Unfortunately, little girls don't always outgrow this tendency to leave someone on the margins. It's not uncommon for grown women to also indulge in this sort of behavior, by forming alliances that may be based on the mutual dislike of another, someone they've chosen as an outcast.
Adult female friendships can be very intense, and full of twists, turns and drama. Social scientists are just beginning to explore the not-so-pretty truth about why women can be so mean to each other.
One psychologist also explains the term "best friends forever" is largely a myth, although it's very painful when a "breakup occurs. People change, grow and drift apart.
Flickr photo by roto kirby