Social media and the Internet haven't changed our capacity for social interaction any more than the Internet has changed our ability to be in love or our basic propensity to violence, because those are such fundamental human attributes.

It is well to look around at whom, and not just what, surrounds us. Population structure will change everything. Our health, wealth, and peace depend on it.

We are, first of all, not solitary creatures and second of all, we are deeply embedded in the lives of others. It's very easy to forget that and to engage in an atomistic fallacy - where we think that all we have to do is study the individual components of a system in order to understand the system. That's clearly not the case when it comes to social systems.

We discovered that if your friend's friend's friend gained weight, you gained weight. We discovered that if your friend's friend's friend stopped smoking, you stopped smoking. And we discovered that if your friend's friend's friend became happy, you became happy.

It used to be thought that our genes were historically immutable and that it was not possible to imagine a conversation between culture and genetics.

I like to have met someone in real life before being their Facebook friend.

If your friends are obese, your risk of obesity is 45 percent higher. ... If your friend's friends are obese, your risk of obesity is 25 percent higher. ... If your friend's friend's friend, someone you probably don't even know, is obese, your risk of obesity is 10 percent higher. It's only when you get to your friend's friend's friend's friends that there's no longer a relationship between that person's body size and your own body size.