  When people prefer to coordinate their behaviors with their friends -- e.g.,
choosing whether to adopt a new technology, to protest against a government, to
attend university -- divisions within a social network can sustain different
behaviors in different parts of the network. We define a society's `behavioral
communities' via its network's `atoms': groups of people who adopt the same
behavior in every equilibrium. We analyze how the atoms change with the
intensity of the peer effects, and characterize the atoms in a prominent class
of network models. We show that using knowledge of atoms to seed the diffusion
of a behavior significantly increases diffusion compared to seeding based on
standard community detection algorithms. We also show how to use observed
behaviors to estimate the intensity of peer effects.
