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Such divisions are one key feature of human networks, among several, that figure prominently in what follows. Why we form networks that have such features has some obvious explanations as well as some subtle ones, as we shall see. Ultimately, we care about our networks and their features because of their impact, and so by the end of this book you should know, for instance, why having divisions such as that in the high school network above profoundly impacts decisions to go to college, but yet has almost no impact on contagion of a flu.

Part of what makes the science of networks such fun, beyond the fact that it is so immediately important in all of our lives, is that it cuts across fields: making sense of human networks draws on core concepts and studies from sociology, economics, math, physics, computer science, and anthropology.6 For instance, our discussion will make heavy use of the concept of externalities from economics—the fact that people’s behavior impacts those around them—coupled with various forms of feedback that amplify that impact. These are features of many complex systems: settings that are simple to describe and understand and yet rich in their features and behaviors.
Our discussion will also take us well beyond networks of personal friendships and acquaintances, to include relationships such as treaties between countries as well as contracts between banks. The full set of social and economic networks that we consider are all “human networks,” as they all involve human interaction at some level.7
Our starting point is how your position in a network determines your power and influence, as this matters in almost all of what follows. We will make sense of the many different ways in which you can be influential and see how each depends on your network.