We’re often taught to choose our friends wisely. As youngsters your parents probably warned you about falling in with a “bad crowd” and on the flip side we often hear that we need to run with lions if we aspire to do great things.

This stuff is instinctive but one long term study tracking health behaviours across groups of friends for 32 years (!) has demonstrated that the effect may be far more pervasive than we realise.

Network effect

The study – the first to examine this phenomenon – found that if a person you consider a friend becomes obese, your own chances of becoming obese rise 57%. Among mutual friends, the effect is even stronger, with chances increasing 171%.

Luckily, there are similar effects with regards to positive health behaviours.

This is not say that we need to cut ourselves off from anyone whom we think is overweight – rather the opposite. The power of your example within your social network can positively affect your friends and family. However it does pose an interesting and perhaps uncomfortable question with regards to our social norms: in light of this information about the power of social networks both positive and negative, should we, in the name of political correctness, continue to leave people who care less about their health to their own devices, or should we collectively as individuals each do our part to help our friends and family look after themselves better?

The study:
The Spread of Obesity in a Large Social Network over 32 Years; Nicholas A. Christakis, M.D., Ph.D., M.P.H., and James H. Fowler, Ph.D.; N Eng J Med 2007; 357:370-379; 2007