Seventy-five years ago, most of the problems modern societies faced seemed best met by extending services to a needy public. Diseases could be prevented through inoculation and new treatments, ignorance through extending education, poor living conditions through building new houses and unemployment through macro-economic interventions to stimulate demand. But as the current crop of party manifestos all point out, today's challenges are not just about "delivering" better services but about building capacity in people to look after themselves and others and make a contribution to larger social goals. We can't tackle obesity, child neglect, long-term unemployment, loneliness among older people or climate change simply by providing better services.

It is not surprising, then, that social scientists and policy-makers are increasingly attentive to the contribution that supportive, "pro-social" family and community ties make to our well-being, and are increasingly interested in how to create and sustain them.

The authors of this book  two American professors who work across the fields of medicine, public health and political science  have risen to academic fame in the past five years with some brilliant research casting new light on the role and importance of social networks in shaping behaviour and wellbeing. Last year they featured on the front cover of the New York Times magazine. In the UK, Labour and the Tories compete in claiming how far their thinking has been influenced by Christakis and Fowler  Ed Miliband, for instance, claims the Labour manifesto commitment to invest in helping people give up smoking together comes from them.

Christakis and Fowler's greatest achievement lies in the use they made of the Framingham Heart Study, a longitudinal medical investigation, stretching from 1948 to the present, of the small community of Framingham in Massachusetts. It was designed to shed light on the causes of heart disease, but Christakis and Fowler used the very detailed data it collected  including information on people's family and friendship ties, their diet, lifestyle and sense of wellbeing  to demonstrate that behaviours and moods tend to spread through a population on the model of a contagion. They showed in particular (using computer programmes that allow them to construct "network maps") that different behaviours and moods, much like different viruses, spread according to different patterns. So obesity tends to be transmitted not through spouses but through same-sex friends. Starting and stopping smoking are more contagious than obesity and are more likely to be transmitted across the sexes. While eating and smoking habits can be transmitted across long-distance networks, the transmission of happiness tends to require face-to-face contact. Christakis and Fowler were also able to demonstrate with more rigour than ever before that our position in a social network has a deep effect on how we fare  while occupying a marginal place in a social network can have advantages (it makes you less susceptible to harmful contagions), as a general rule people with more friends and connections are happier, healthier and better off.

Connected aims to offer an accessible and lively overview of social networks, how they work and why they matter. It shows how network-thinking can help us to understand phenomena as different as teenage sexual practices, suicide, crime, voting, financial markets and the behaviour of political leaders. It also suggests some of the implications for public policy of taking networks seriously.

Overall, however, the result is a somewhat unfocused and ill-disciplined book. Christakis and Fowler oscillate uneasily between arguing for the explanatory power of social network analysis and beating the drum for social networks. They acknowledge the way in which people's life chances and quality of life are shaped by a complex interplay of factors, including parenting, material wealth, education and skills, as well as networks, but then focus on networks to the exclusion of almost everything else. They scarcely stop to consider how their concept of a network relates to older and more familiar (and richer?) concepts of "social capital" and "community", or the ethics of government intervention in people's "private" networks.

That said, this book is extraordinarily rich in insights. For one thing, its analysis strongly suggests that if governments and public services want to effect change in an individual's behaviour, they need to start by understanding the nature of relevant networks and how they in turn shape behaviours and attitudes. If people start or stop smoking in groups, or mugging spreads like a virus through a city, then group-based approaches to tackling these problems are likely to be more effective than ones focused on individuals  or on the "community" as a whole.

The parlous state of our public finances means that whatever the political colour of the next government, it will have to introduce the most far-reaching cuts the modern welfare state has ever seen. As this books helps to make clear, minimising the damage to social networks will be essential.

Ben Rogers is an associate fellow of Demos and the Institute for Public Policy Research.