Everyday interactions we have with other people are definitely contagious, in terms of happiness.

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

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.

It is the spread of the good things that vindicates the whole reason we live our lives in networks. If I was always violent to you or gave you germs, you would cut the ties to me and the network would disintegrate. In a deep and fundamental way, networks are connected to goodness, and goodness is required for networks to emerge and spread.

We will create life from inanimate compounds, and we will find life in space. But the life that should more immediately interest us lies between these extremes, in the middle range we all inhabit between our genes and our stars.

There are very fundamental reasons we live our lives in social networks and if we really understood the role they're playing in our society we would take better care of social networks and find ways to take advantage of their power to improve our society.

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.