William Rawlins, a well-known communications scientist, studied thousands of friendships and set down a series of five stages that each friendship usually progresses through. His is considered to be the definitive list.

  1. Role-Limited Interactions: General public interactions governed by the rules of civility. In English this means just stuff like learning someone’s name and general small talk. You are each still playing a role and not opening yourself up.
  2. Friendly Relations: Moving beyond the rules of public interaction and into conversations that relates to someone else as a person, not just a job or role they are fulfilling. You learn more info about someone else, their likes and dislikes, whether they have any hobbies, etc.
  3. Moves Toward Friendship: This involves spending more time with a person, usually in a group setting. And learning more intimate information.
  4. Nascent Friendship: This is where the first elements of trusting begin to appear. Before this most of the things that were shared were just biographical facts, but now you begin to trust each other with more personal thoughts and ideas.
  5. Stabilized Friendship: One where the participants have developed complete trust in each other. It involves both trusting behavior, which is any behavior that increases someone’s vulnerability to another, and trustworthy behavior, which is a response to trusting behavior that protects the vulnerability of the other person. People weigh the possible costs and rewards for disclosing personal information and use that to decide whether they will open up more or not.

    and if necessary…

  6. Waning Friendship: The people have started to grow independent of each other. There is often less and less contact coupled with less sharing of information. The friendship can either be saved and moved back into a stabilized position or it can die.

Overall his main measurement of the strength of a relationship is the amount of information shared and the amount of trust the participants have in each other. I do take exception to the fact that he overlooks the establishment of inside jokes (which would start taking place in stage 3) in the strengthening of a relationship.