That’s kind of obvious, you’d think… But what is striking about what these two researchers have found is that the strongest influence might not come from your friends, but from friends of friends. Even those you haven’t met. Looking at obesity, for instance, Christakis and Fowler claims that a Framingham resident is nearly 20% more likely to put on weight if a friend of a friend does, even if the connecting friends weight remains the same. Even a friend of a friend’s friend’s weight gain (try saying that quickly!) is also putting you at risk, with a 10% increase.

“You may not know him personally, but your friend’s husband’s co-worker can make you fat. And your sister’s friend’s boyfriend can make you thin.”

Scary? It certainly shows how important a person’s social network is. The same theory applies for happiness – spreading good or bad feelings. But before you run off to unfollow certain tweeters and delete friends from Facebook to stay happy & thin and so on – consider this:

“The subconscious nature of emotional mirroring might explain one of the more curious findings in their research: If you want to be happy, what’s most important is to have lots of friends. Historically, we have often thought that having a small cluster of tight, long-term friends is crucial to being happy. But Christakis and Fowler found that the happiest people in Framingham were those who had the most connections, even if the relationships weren’t necessarily deep ones.”

The argument is that different types of behaviour have different levels of contagiousness and spread in different ways. Being happy, for instance, is not about having a small, tight network – it is the sum of the positive small moments you have during the day – regardless of the source. Have you ever seen someone do something nice for someone else and be touched by it? Have you felt like smiling just because a stranger smiled at you? Mirroring other people’s emotional state directly affects your own, and apparently, happiness trumps unhappiness.

While happiness can be contagious regardless of the source – the contagiousness of other behaviours is directly affected by the type of relationship you perceive to have with the source:

Their other finding is more intriguing and arguably more significant: They discovered that behaviors appear to spread differently depending on the type of friendship that exists between two people. In the Framingham study, people were asked to name a close friend. But the friendships weren’t always symmetrical. Though Steven might designate Peter as his friend, Peter might not think of Steven the same way; he might never designate Steven as a friend. Christakis and Fowler found that this “directionality” mattered greatly. According to their data, if Steven becomes obese, it has no effect on Peter at all, because he doesn’t think of Steven as a close friend. In contrast, if Peter gains weight, then Steven’s risk of obesity rises by almost 100 percent.

I’ve always been interested in why some types of behaviour trumps others. Thompson’s article and Christakis and Fowler’s research is so interesting because it shows that the type of friendship/relationship you have with people as well as where they’re placed in your network matters when it comes to the strength of your behaviour influence.

Understanding where the influence happens in a social network wether that be a person or a subset of people is crucial when you design systems that are intended to encourage a certain behaviour from people, like doing favours or taking actions.