Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives
By Nicholas Christakis and James Fowler
Little, Brown $25.99, 320 pages

Why are you reading this review in the Financial Times? You would like to think that it’s because you chose to.

But after reading Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives, you might feel differently. Nicholas Christakis and James Fowler’s book aims to show how we are influenced, shaped and defined by our social networks – the overlapping random groupings of people we know (and those we don’t) from all our different areas of life. Their theory is that understanding human behaviour only makes sense through how we are connected to one another. Rational, individual action is out, it seems. The network is here.

In fact, it always has been but we just didn’t realise it – we thought we were all economic agents, as the economists told us. How foolish we were. Instead, everything from how rich we are to what we wear to how we feel can be explained by our position within our social networks.

Christakis, a professor of sociology at Harvard, and Fowler, a professor of political science at the University of California, make a convincing case. We have all heard of the six degrees of separation – the idea that we are linked by every other person on the planet by only six steps. The authors have a theory of three levels of influence – our friends, their friends and their friends. These three levels can, we are told, explain pretty much everything we do. It’s an unsettling idea.

Perhaps the most controversial theory is that obesity is contagious. People we may have never met have more influence over how fat (or not) we become by changing socially acceptable levels of consumption. By analysing friendship groups, they show how if one person becomes obese, it triples the chances of a mutual friend also becoming obese. If you don’t want to get fat, the best thing to do is to stop hanging out with people who eat cake all day.

Some theories tread a familiar path – how sexually transmitted diseases spread; the clusters of suicides among young adults; why people don’t vote. Much has already been written about how marriage increases life expectancy in men, and the “broken heart” effect of couples dying in close succession.

Connected doesn’t lack ambition but what is lacking is any in-depth observation of online networks. The phenomenon of Facebook and Twitter, for example, deserves more detail. The authors hint at the sheer scale of it all but never quite explore the impact they have had. Equally, there is no mention of the potential political and economic ramifications of China embracing the internet – Chinese web users now outnumber those in the US.

If the power of the network seems a scary proposition, there is still some hope. You might not be in control, but your actions matter. Go and be nice to someone, lose some weight and vote – it will change the world.

Rob Minto is the FT’s interactive editor