A former colleague recently emailed me to say that a woman we’d once worked with had been murdered. Here’s what I learned: she’d been homeless for a number of years and had waged a losing battle with alcoholism, only to finally be killed by an unknown assailant. After the shock of the news, the next thing that sprang to my mind was, “But where were her people – her family and friends?” In other words, how could the people around her have let this happen – where was her safety net? And while surely every tragic end is more complicated than that, my response reflects something we all know – namely, that a strong social network is crucial to our well being.

It’s an idea that’s explored in depth by Nicholas A. Christakis and James H. Fowler in Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives. Of course, social networks are all the rage – with Goldman Sachs valuing Facebook at $50 billion and the film The Social Network receiving eight Oscar nominations. But during a recent conversation with Christakis, he was quick to point out that social networks are not a passing fad.

“It’s important to stress that humans have always been connected,” Christakis says. “And one of the arguments that we make in the book is that it’s very deeply rooted in our heritage – not just to befriend one another, but to assemble ourselves into networks of a very particular kind, face-to-face networks with very baroque, very particular structures. And these structures obey certain mathematical rules. They obey certain biological rules. They follow certain psychological principles. So we’ve been making networks for hundreds of thousands of years.”

One of the structures vis-a-vis human social networks was the discovery of a type of group mind – “the fact that groups of people can exhibit complicated, shared behaviours without explicit coordination or awareness.” The authors of Connected cite, by way of example, the spontaneous human waves that form among spectators at sporting events. These waves can be understood using the same mathematical models that scientists use when they examine excitable media like the spread of fire through a forest or electrical signals firing through cardiac muscle. In other words, there is no central control – no individual will, no leader – but rather a collective intelligence, just as we see when flocks of birds or schools of fish or swarms of insects all move in unison.

Even more intriguing are the authors' findings that point to how influence spreads through a social network, with the most famous examples being that if your friends’ friends' friends are obese, then there are higher chances that you’ll gain weight as well. The same holds true for happiness, wealth, suicide, and other seemingly intangible and difficult to transmit outcomes.

Christakis helps explain the idea through the analogy of how a germ might travel a great distance through an interconnected group and eventually make its way, jumping from person to person, until the last person in the sequence is infected.

“Our claim is that other type of phenomena that aren’t as obvious – for instance ideas or norms or behaviours – behave similarly,” Christakis says. “And furthermore, if you grant this what you come to realize is that you then have little pockets within the network, little niches and groups of interconnected people, and so just as you have a germ that spreads you can have little outbreaks of smoking cessation or outbreaks of buying ipads or fashions in clothes.”

Connected is a stimulating read. Human social networks, it turns out, may be the reason humans have such overdeveloped brains. Social networks also link us, potentially at least, by six degrees of separation to every person on the planet. That said, our brains – while proportionately massive compared to all other species – have their limits too: we can only realistically maintain about 150 true friends. This number comes from the research of Robin Dunbar, a primatologist; he found that humans, or any animal, simply cannot maintain the cohesion and integrity of social groups larger than a size governed by the information processing capacity of their brains. And, for humans, 150 is the max – something to keep in mind if your roster of Facebook friends is in the thousands.