Most of my research focuses on voluntary groups and the social networks that draw people into and out of them. But my research agenda is more ambitious than that. I think that the general ecological theory of affiliation that I have developed shows how any social entity that spreads through networks behaves. We’ve already applied the theory to occupations, musical tastes, religious congregations, voluntary organizations and other social groups. Now I am working on applying it to fuzzier cultural entities like attitudes, beliefs and social identities. I think that the theory will explain some important things about the behavior of virtually any social characteristic.