CONNECTED Nicholas Christakis and James Fowler

(Harper Press, £12.99)

Humans have evolved to be social beings and our networks are vitally important to us. These two social scientist authors recount their exploration of how these networks help our health and happiness and share some surprising conclusions, saying that to truly know who we are, we must understand how we are connected. They unpick some of the rules of social networks. For example, in a study of sex in a US high school they found a rule: “Don’t date your old partner’s current partner’s old partner.” There’s a neat little plan (looking a bit like a blob-and-stick molecule model) of the relationship links between the 288 students. Germs spread and so do behaviours. They mapped out more than 50,000 ties in an investigation of obesity, claiming from this work, as other studies confirm, that obesity is contagious. Similar results are cited for reductions in smoking or increases in happiness. They claim almost unbelievably that if your friend’s friend’s friend gains weight, stops smoking or becomes happy, then there’s a good chance that you will gain weight, stop smoking or become happier, too.

A section on bank runs shows how social networks can make a problem worse, when individual decisions that appear sensible can lead to collective madness.

Barack Obama was praised for his ability to connect with voters, but the book shows how his team’s ability to connect voters with each other through a social networking Internet site made the crucial difference. Our networks are becoming faster and larger as we email, text, Twitter, and Facebook each other in increasing volumes, but networking became fundamental to our natures much earlier, as we evolved. This notion is explored in a chapter on cooperation and altruism (people who help others should be less likely to survive than those who are selfish); on friendship groups; and on language development. This is a most intriguing and entertaining book. I wonder if David Cameron read a preview copy of it before devising his “Big Society”.