"It is customary to think about fashions in things like clothes or music as spreading in a social network. But it turns out that all kinds of things, many of them quite unexpected, can flow through social networks, and this process obeys certain rules we are seeking to discover. We've been investigating the spread of obesity through a network, the spread of smoking cessation through a network, the spread of happiness through a network, the spread of loneliness through a network, the spread of altruism through a network. And we have been thinking about these kinds of things while also keeping an eye on the fact that networks do not just arise from nothing or for nothing. Very interesting rules determine their structure."