An interesting Dutch study by Gerald Mollenhorst found that just over half of close friends turn over every 7 years. And only 30% of friends were discussion partners and practical helpers some seven years later.
Whether this applies to American friends is up for grabs, although one aspect of the Dutch study clearly doesn’t transfer across the Ocean. Mollenhorst found that the size of close friendship networks remained constant over the last 7 years despite the volatility in the composition of these networks. In the U.S., the best careful study of close friends-see below- found that close friendship networks have collapsed between 1985 and 2004, although there has been no careful work on this subject of trends since 2000. [To be clear, in the U.S., unlike in the Netherlands, the study was not longitudinal; in other words, researchers were not tracking the same individuals over these 16 years, but nevertheless average close friendship networks were collapsing over this period.]
Mollenhorst was also interested in how the social context (whether you met someone through school, work, neighborhood, etc.) affected friendships. He found, surprisingly, that the social context did not affect how similar friends, partners and acquaintances were to each other. In this sense, it was a somewhat deterministic view of the importance of social context on our friendship networks.
The survey interviewed 1007 people ages 18-65 and then reinterviewed 604 of these individuals 7 years later.
The relevant U.S. study on the collapse of our close friendships is as follows: Two prominent sociologists, Lynn Smith-Lovin and Miller McPherson, and former critics of Bowling Alone found confirming evidence of social isolation in the General Social Survey data. From 1985-2004, the percentage of Americans lacking anyone to discuss important matters with has nearly tripled. Almost half the U.S. population now has either no one or only one confidante with whom to discuss important matters. See June 23, 2006 stories in Boston Globe, Washington Post, and an essay in TIME magazine by Robert D. Putnam.
For article on the Mollenhorst Dutch study, see “Half of Your Friends Lost In Seven Years” (Science Daily)
Umbrella Project: “Where Friends are Made: Context, Contact and Consequences (Beate Volker)