Most parents of teenagers have spent at least some time worrying about the company their children keep. But it is not just teenagers who are influenced by those with whom they associate. As adults, the people we spend time with can have a tremendous impact upon our health and happiness, as evidenced by two fascinating Harvard studies.

The researchers tracked over 12,000 individuals for 30 years and found that an individual’s likelihood of becoming obese increased by 57 per cent if he or she had a friend who became obese.

Interestingly, if an individual’s spouse became obese, the chance of them becoming obese increased by only 37 per cent. This suggests that individuals of the same sex have a greater influence on each other than those of opposite gender. So clearly, we should think twice before praying, “Lord, if I can’t be thin at least make my friends fat!” There is a better than even chance that we’ll actually gain weight with them!

Intriguingly, the researchers discovered that in social networks, an individual who gains weight will increase the chance (by about 10 per cent) that the friends of the friends of their friends—that is, three degrees of separation—will also put on some kilos. Indeed, these results demonstrate that obesity is a socially contagious condition.

But it isn’t just obesity that’s socially contagious; so is happiness. In a separate study of the same group of people, happy people also infectiously spread their joy up to three degrees of separation. The researchers concluded that happiness is a collective phenomenon.

What is the take-away message? They say that you can’t choose your family but you can choose your friends. Indeed, it helps to seek out positive influences, or better still, be one.