The first comes from Elizabeth Bott, an influential anthropologist who published a book in 1957 called Family and Social Networks. In this book, she hypothesized that the degree of clustering in an individual’s network could draw the person away from a tie with somebody else. In other words, if you are part of a group of close friends or relations, you are less able to make strong links outside this group.

The team found that the number of friends that pairs of individual have in common is strongly correlated with the strength of the tie between them, as measured in other ways. That’s regardless of whether people are linked by mobile-phone records or by social ties in rural Indian villages.