Two linked studies, by Fowler and Christakis (doi:10.1136/bmj.a2338) and Cohen-Cole and Fletcher (doi:10.1136/bmj.a2533), relate to the transmission of health related factors through social networks.1 2 The concept underlying this new field of research is that behaviours may spread over time from one person to another through their immediate and more distant social contacts. Social epidemiology has established the relevance of social connectedness for health, and social network transmission may be one mechanism through which both beneficial and adverse effects are mediated.3

The article by Fowler and Christakis investigated the social transmission of happiness. Happiness is related to several aspects of wellbeing, including better work performance, greater job satisfaction, good family relationships, and a more satisfying social life,4 but what has it got to do with health? It is no surprise that happiness is reduced when people are ill, and that negative emotional states such as depression and anxiety may influence the prognosis of several physical illnesses. But over recent years it has been suggested that happiness influences future ill health.

A recent meta-analysis of longitudinal observational studies found that measures of happiness, cheerfulness, and related constructs were associated prospectively with reduced mortality, both in initially healthy people and in …