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

The social sciences offer equal promise for improving human welfare; our lives can be greatly improved through a deeper understanding of individual and collective behavior. But to realize this promise, the social sciences, like the natural sciences, need to match their institutional structures to today's intellectual challenges.

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.

What constrains or enables the capacity of human beings to work in groups is not so much the technology, but rather the capacity of the human brain to have and monitor social interactions.

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.

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.

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.