While doing some research recently on relationships, I came across an old but classic scholarly paper entitled "The Rules of Friendship" by Michael Argyle and Monika Henderson (Journal of Social and Personal Relationships, Vol. 1 (1984), pp. 211-237). Using populations from Britain, Italy, Hong Kong, and Japan, the authors studied several related topics:
Their findings were consistent with common sense but quite interesting.
On the first topic, they identified three general rules that apply to all friendships (in addition to basic things that Aristotle and others identified 2000+ years ago, such as "spend time with your friends"):
On the second topic, they identified six rules that determine whether the friendship advances from ordinary to close:
On the third topic, they identified four rules that, if violated, tend to break up the friendship:
There's more in the paper itself, including the results for 11 distinct hypotheses proposed by the authors. I'm also curious to see if this study has been replicated or extended in more recent psychological literature. However, this brief summary is already long enough, so I'll need to post about friendship again some other time.
(Cross-posted at philosopher.coach.)