Book excerpt: Is there a physics of society? Ranging from Hobbes and Adam Smith to modern work on traffic flow and market trading, and across economics, sociology and psychology, this study shows how much we can understand of human behaviour when we cease to try to predict and analyse the behaviour of individuals and look to the impact of hundreds, thousands or millions of individual human decisions. The book encourages us to examine our own behaviour, whether in buying the new Harry Potter book, voting for a particular party or responding to the lures of advertisers.