Psychologists consider it an inevitable life stage, a point where people achieve enough maturity and self-awareness to know who they are and what they want out of their remaining years, and have a degree of clarity about which friends deserve full attention and which are a drain. It is time, in other words, to shed people they collected in their youth, when they were still trying on friends for size.

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.

“When time horizons are long, as they typically are in youth, we’re collectors, we’re explorers, we’re interested in all sorts of things that are novel,” Dr. Carstensen said. “You might go to a party that you don’t want to go to, but know you should — and it’s there you meet your future spouse.”

Hrmm.