Yesterday Science published a study on social networks and job mobility. It suggests that there’s a causal, “inverted U-shaped” relationship between
- the number of mutual friends you share with someone and
- the probability that befriending them leads you to change jobs.
The authors call for a theory to explain this relationship. In fact it has a simple explanation: homophily.
People tend to have friends with similar interests. Those interests influence the jobs we want and hear about. If you and I have lots of mutual friends, then I probably hear about lots of job opportunities that interest you. But you probably hear about those opportunities too because you talk to the same people and follow the same news sources. So befriending me is unlikely to impact your job mobility because the information I could give you is redundant.
The opposite is true if we have few mutual friends. Then we probably hear about different job opportunities because we talk to different people and follow different news sources. But few of the opportunities I hear about will interest you. So befriending me is unlikely to impact your job mobility because the information I could give you is irrelevant.
Thus, homophily creates a trade-off between relevance and redundancy. Befriending “strong ties” (i.e., people with lots of mutual connections) provides information that is relevant but redundant. Befriending “weak ties” (i.e., people with few or no mutual connections) provides information that is irrelevant but novel. Befriending “moderate” ties balances relevance and redundancy. It lets you hear about opportunities you find interesting and wouldn’t hear otherwise.
If everyone acts on those opportunities, then we should see the relationship suggested by the Science study.