The Yale professor visiting Sydney this week studies how you, me and everyone interacts online and in face-to-face networks.

He and his colleagues at Yale's Human Nature Lab have worked out the four ways online networks are different to our real-life interactions.

And they've found that despite technology changing our lives in amazing ways, the core of our networking has remained remarkably consistent.

"The six degrees of Kevin Bacon rule tells us something about the structure of human social networks. On average every person on the planet is six handshakes away from any other person.

But Professor Christakis has uncovered what might be called a "half a Kevin Bacon" rule. 

"We found a three-degrees-of-influence rule, also. When you take an action it doesn't just affect the people to whom you're directly connected."

His research has explored how things such as obesity, smoking, or even altruism can act as a social contagion.

"Your behaviour also affects your friend's friends at two degrees of separation and your friend's friend's friends at three degrees of separation. And then it peters out."

Professor Christakis mapped networks online as well as face-to-face networks in modern cities and in small-scale societies, such as the Hadza hunter-gatherers in Tanzania.

"We find that across all of these places the mathematics of social networks are extremely similar," he said. 

Professor Christakis's grandmother grew up in a small village in southern Greece.

"I asked her how many friends she had growing up, she'd say she had one or two best friends and four or five girls hung together," he says.

"If you ask my daughter the same question, she gives you the same answer, despite the fact she has an iPhone in her pocket.

"So these modern technologies are all grafted onto a very ancient and fundamental part of our humanity."

But there are four things that have changed online, he says.

Professor Christakis says that through social media, the number of relationships you can track and influence are much larger. However, he says this hasn't really changed "Dunbar's number" of about 150 - the suggested number of people with whom we can have stable, ongoing social relations.

"Even 20 years ago if you wanted to find a Norwegian army veterinarian, this was very difficult thing to do. But now you can search online and find such an individual very quickly," he says.

"If you had unusual tastes in food or hobbies - forget about sexual tastes - these would be hard to satisfy. Now you can form online communities very easily. We call this specificity."

Professor Christakis said now we have the ability of large groups of people to come together to work collaboratively, "for example on Wikipedia, or in revolutions for that matter". He said this phenomenon has been abetted by online technologies.

Professor Christakis says: "Online, it's much easier to pretend to be someone you're not. It was always possible for human beings to do this. But it was hard.

"Now, on the internet, as the classic New Yorker cartoon goes, "nobody knows you're a dog". Disabled people can have able-bodied avatars, you can switch genders online ... We call this virtuality, the capacity of humans to reinvent themselves."

Professor Christakis says: "Gathering a deep understanding of human social interactions offers us whole new ways to intervene in the world to make it better.

"If you're able to understand the network structure of a village, you can figure out how best to get people to vaccinate their children, or use clean water.

"Or you can figure out interactions between a group of people with diabetes and help them better manage their disease."

It is this application for positive outcomes that garnered him an invitation to visit Australia from the Black Dog Institute, the Brain and Mind Centre and the Charles Perkins Centre at the University of Sydney.

The academic director of the Charles Perkins Centre, Stephen Simpson, says: "Understanding how networks form and change is key to tackling many of society's chronic health problems, like diabetes, obesity and cardiovascular disease."  

But is it all about doing good? Is there not some potential for wrongdoing? 

He says that social networks are agnostic. "They can magnify anti-vaccination groups, Ebola, fascism or gossip, lies and hatred. But equally they will magnify health, happiness, love, or spread positive ideas."

It can work both ways. "These technologies absolutely can be reverse engineered. This is a generic problem in science."