You’ve probably heard of “Dunbar’s number”. It is the number of stable relationships people are cognitively able to maintain at once. And, in case you’ve forgotten or didn’t know, that number is 150.

But it’s not just about that number 150. In reality, that 150 consists of a series of concentric circles, each representing different kinds or qualities of relationships.

Interviewed about his new book, Friends: Understanding the Power of Our Most Important Relationships, in The Atlantic, Robin Dunbar – of said “Dunbar’s number” – had this to say:

The innermost layer of 1.5 is [the most intimate]; clearly that has to do with your romantic relationships. The next layer of five is your shoulders-to-cry-on friendships. They are the ones who will drop everything to support us when our world falls apart. The 15 layer includes the previous five, and your core social partners. They are our main social companions, so they provide the context for having fun times. They also provide the main circle for exchange of child care. We trust them enough to leave our children with them. The next layer up, at 50, is your big-weekend-barbecue people. And the 150 layer is your weddings and funerals group who would come to your once-in-a-lifetime event.

The layers come about primarily because the time we have for social interaction is not infinite. You have to decide how to invest that time, bearing in mind that the strength of relationships is directly correlated with how much time and effort we give them.

Introverts seem to be risk averse. They prefer to have fewer friends so they can invest more time in each. Extroverts are more socially confident, so they prefer to have more friends at the expense of investing less time in each. They probably feel they can wing it with someone else if one friend says no [to something]. These are just two equally good ways of solving the same problem.