Friendship quality and socialising with friends were associated with wellbeing and, in the longitudinal studies, predicted it from six months to twelve years later; in one study friendship quality at 30 predicted wellbeing at 50. Number of friends, how friends react when you share good news, their support for your autonomy, and efforts to maintain the friendship were all positively correlated with wellbeing. Efforts to maintain the friendship, perceived mattering and friendship quality were among six variables that sat in the chain between friendship and wellbeing.
How it was done
A systematic review of 38 studies of adult friendship and wellbeing published between 2000 and 2019, read through the PERMA framework (positive emotion, engagement, relationships, meaning, accomplishment). Nine were longitudinal, two qualitative, two mixed, the rest cross-sectional; 28 used samples of young adults, mainly university students.
What it means
The ordering is the useful part. The quantities people reach for first, how many friends and how often, are in the picture, but the evidence that reaches across years is for quality and for time actually spent together. Number of friends correlated with wellbeing in cross-sectional data, and large, well-integrated friendship groups came out as a source of wellbeing, but no study followed the count forward.
Two of the correlates are behaviours rather than states: how a friend responds when you tell them good news, and whether either of you makes an effort to keep the friendship going. Both are things one person can change on a Tuesday. The review also found that the effort to maintain a friendship mediated the link between friendship and wellbeing, which is as close as this literature gets to saying the upkeep is where the value is made.
Perceived mattering, the sense that you count to a specific other person, was another mediator. It is one of the few places where men and women differed: in one study it tracked positive emotion only among women.