The second problem has to do with what one might call the positive externalities of group membership. While all groups require some degree of social capital to operate, some build bonds of trust and hence social capital outside of their own memberships. As Weber indicated, Puritanism mandated honesty not simply towards other members of one's religious community, but towards all human beings. On the other hand, norms of reciprocity can be shared among only a small subset of a group's members. The final problem concerns negative externalities.
Some groups actively promote intolerance, hatred and even violence toward non-members.
While the Klu Klux Klan, Nation of Islam, and Michigan Militia possess social capital, a society made up of such groups would not be particularly appealing, and might even cease to be a democracy. Not only do such groups have problems cooperating with each other, the bonds of community uniting them are likely to make them less adaptive by sealing them off from influences in the surrounding environment. It should be clear that coming up with a believable number expressing the stock of social capital for a large and complex society like the United States is next to impossible. We have empirical data, of varying reliability, on only a certain subset of the groups that actually exist, and no consensus means of judging their qualitative differences. It is clear that much more work needs to be done on the measurement of social capital, which is the starting point for understanding how to increase our stock of it.