Comment by bitexploder

I don't think it is complex. The theme of a social group is just there as a filter. If you like rock climbing and meet someone at a rock climbing gym that person is far more likely to be interested in things you are interested in: physical fitness, the particular mental challenges of rock climbing, etc. It was just an example. I won't analyze the sexism or male only nature of the fraternity, but I think Freemasonry anecdotally reinforces the idea that men want/need/form these kind of clubs more than women on average.

When we study this we notice very small actual bias at an individual level on socialization preference. The differences are modest and more like slight preferences. There is more overlap than not at a local individual level. What gets missed is that even though the differences are relatively small, the network effect greatly amplifies these small variances resulting in non-linear outcomes. Even small biases at an individual level essentially produce significant effect in socialization behavior.