The winnowing process even has a clinical name: socioemotional selectivity theory, a term coined by Laura L. Carstensen, a psychology professor who is the director of the Stanford Center on Longevity in California. Dr. Carstensen’s data show that the number of interactions with acquaintances starts to decline after age 17 (presumably after the socially aggressive world of high school) and then picks up again between 30 and 40 before starting to decline sharply from 40 to 50…

But not all friends (or ex-friends) will go easily. By the time she was in her mid-30s, Carolyn Miller, an office manager in Norwalk, Conn., found herself unwilling to put up with an old friend’s domineering ways, so eventually she sent her an e-mail listing her grievances and asking for space. The friend called her and begged her to reconsider. Ms. Miller stood her ground.

A few weeks later, when Ms. Miller’s grandfather died, the friend sent her a letter saying, oddly, that he had been a wonderful veteran (he had never been in the service), and not long after that, an invitation to her wedding. When Ms. Miller sent back the enclosed card declining the invitation, the friend called her and asked why.

During that call, Ms. Miller knew it was time to administer the friendship equivalent of the lethal injection. “I wish you love, joy, peace and happiness, but this friendship is over,” Ms. Miller recalled saying. “I said goodbye and hung the phone up. I met another friend for drinks that night and honestly, I was sad. I divorced a friend.”