The Human Network: How Your Social Position Determines Your Power, Beliefs, and Behaviors

The book on Amazon.com.

Oren Cass writes in the WSJ:

Ever have the sneaking suspicion that your friends are more popular than you? Turns out it’s probably true—and not just because you may be insufferable at cocktail parties. Why it’s true is fairly complicated, but in his book “The Human Network” Stanford economist Matthew O. Jackson entertainingly analyzes this and other mysteries. Drawing on the academic discipline known as network theory, Mr. Jackson aims to introduce and popularize a powerful way of understanding some of modern society’s central challenges.

Start with the “friendship paradox”: Imagine drawing a network of everyone from your high school—a circle representing each person and a line between circles representing a friendship. First, assign each circle a “popularity score” equal to the number of other circles connected to it (i.e., that person’s number of friends). Then, for each circle, calculate the average of the popularity scores of the circles connected to it; now each circle has a “popularity score” and a “popularity-of-friends score.”

Here’s the paradox: A typical person will have a “popularity-of-friends” score higher than his or her own “popularity score.” Most people’s friends really are more popular than they are. Precisely because the most popular people have more friends, they show up on the most lists of others’ friends. Thus insecure high-school students comparing their popularity to those in their social circle are not looking at a random sample, but one that overweights the most popular kids in school.

This effect of networks overrepresenting the already-popular helps explain a lot of adolescent behavior. More socially active teenagers tend toward more extreme behaviors, leaving everyone else with a mistaken impression about those behaviors’ frequency. College students overestimate how much a typical student drinks because the average level of drinking in an individual’s own network likely exceeds the actual average…

The fun and games end with the introduction of “homophily,” which the author defines as “the general tendency of people to interact with others who are similar to themselves.” Importantly, he emphasizes, the phenomenon is common to almost all societies and “occurs along many dimensions including gender, ethnicity, religion, age, profession, [and] education level.” Racism and sexism are unnecessary to explain even highly segregated networks—in Africa’s Great Rift Valley, nomadic hunter-gatherers exhibit homophily on dimensions that include height, weight and strength.