Why do birds of a feather flock together?
Let’s assume that (a) people vary along some invisible attribute – honesty, sex drive, propensity to work hard etc. Furthermore, let’s assume that (b) people project their preferences onto others – honest individuals believe that human beings are “basically honest” and so on. They believe that such people like them are “normal”.
Let’s make all these people randomly interacting with each other, with the option of repeating the interaction if they should so wish. Suppose that (c) in conversation, people end up signaling (consciously or not) what they consider “normal.” Individuals within their range have no problem, but (d) people outside it feel uncomfortable – if you have a high sex drive, you’re not going to comfortable around those who consider you a “pervert” or “slut”. People cease interacting with those who consider them “abnormal”. Thus the only people who interact repeatedly with each others are those who fall in each other’s range of “normal” i.e. people very similar to each other.
As a result of these handful of simple assumptions, we now have a population in which those who repeatedly interact with each other are very similar to each other. Thus, birds of a feather flock together. This model supports the proverb, “tell me who you spend time with, and I’ll tell you who you are.”
A few related points:
(1) One might argue that people with similar tastes tend to “flock” because they have something in common to talk about. This might well be true but the purpose of this model is to explain how people with similar attributes, rather than preferences over different kinds of goods or activities, congregate. People may never talk about such attributes at length. Of course, it’s possible that attributes correlate with tastes.
(2) There are many things that proscribe the kind of friends (or romantic partners) we have – where we work, where we went to school etc. Thus it might seem as if we have no say in what kind of people we can befriend. However, one is always friends with only some of those we interact regularly with. Furthermore, one chooses one’s workplace etc based at least partly on how we like our colleagues, the “atmosphere” or “culture” etc.
(3) There are, I believe, some simple predictions that serve as tests of falsifications – people who are less judgmental should have a wider variety of friends; people who have are well-read or have traveled a lot (and therefore have a broader idea of what is “normal”) should have a wider variety of friends; the larger the organisation a person works/studies at, the more likely that cliques will form.
(4) UPDATE: It is a mistake to extrapolate the experiences of others based on the experiences of oneself and one’s friends, since you may be very different from other people.