In chapter 3 I focused on how the actions of individuals can be explained by reference to their beliefs, desires and opportunities, and how these mental states and action opportunities are, in turn, influenced by the actions and behaviours of others. This chapter builds upon and extends this foundation. The explanatory focus is no longer on individual actions, however, but on the macro-level or social phenomena that these actions bring about.

As mentioned in chapter 1, the types of social phenomena I focus on are collective properties that are not definable for a single member of the collectivity. Examples of different types of social phenomena and some associated why-questions include:

• Typical actions, beliefs or desires Why have some racial prejudices changed over time? Why are some communities more conformist than others?

• Distributions and aggregate patterns Why are some cities more ethnically segregated than others? Why are some societies more unequal than others?

• Topologies of networks Why are social networks more tightly knit in some communities than in others? Why are some networks highly clustered while others are not?

• Informal rules or social norms Why are norms of reciprocity common in some groups but not in others? Why are work norms stronger in some societies than in others?

In all of these cases, the entity to be explained concerns a social phenomenon that characterizes a collectivity of actors.