In Chapter 3 we considered some of the typical structures that characterize social networks, and some of the typical processes that affect the formation of links in the network. Our discussion in Chapter 3 focused primarily on the network as an object of study in itself, relatively independent of the broader world in which it exists.

However, the contexts in which a social network is embedded will generally have significant effects on its structure. Each individual in a social network has a distinctive set of personal characteristics, and similarities and compatibilities between two people's characteristics can strongly influence whether a link forms between them. Each individual also engages in a set of behaviors and activities that can shape the formation of links within the network. These considerations suggest what we mean by a network's surrounding contexts: factors that exist outside the nodes and edges of a networks, but which nonetheless affect how the network's structure evolves.

In this chapter we consider how such effects operate and what they imply about the structure of social networks. Among other observations, we will find that the surrounding contexts affecting a network's formation can, to some extent, be viewed in network terms. By expanding the network to represent the contexts together with the individuals, we will see in fact that several different processes of network formation can be described in a common framework.

Homophily

One of the most basic notions governing the structure of social networks is homophily – the principle that we tend to be similar to our friends. Typically, your friends don't look like a random sample of the underlying population.