Prior moral consensus gives members of the group a basis for mutual trust

From Trust: The Social Virtues and the Creation of Prosperity by Francis Fukuyama. Page 26.

Trust is the expectation that arises within a community of regular, honest, and cooperative behavior, based on commonly shared norms, on the part of other members of that community. Those norms can be about deep "value" questions like the nature of God or justice, but they also encompass secular norms like professional standards and codes of behavior. That is, we trust a doctor not to do us deliberate injury because we expect him or her to live by the Hippocratic oath and the standards of the medical profession.

Social capital is a capability that arises from the prevalence of trust in a society or in certain parts of it. It can be embodied in the smallest and most basic social group, the family, as well as the largest of all groups, the nation, and in all the other groups in between. Social capital differs from other forms of human capital insofar as it is usually created and transmitted through cultural mechanisms like religion, tradition, or historical habit. Economists typically argue that the formation of social groups can be explained as the result of voluntary contract between individuals who have made the rational calculation that cooperation is in their long-term self-interest. By this account, trust is not necessary for cooperation: enlightened self-interest, together with legal mechanisms like contracts, can compensate for an absence of trust and allow strangers jointly to create an organization that will work for a common purpose. Groups can be formed any time based on self-interest, and group formation is not culture-dependent.

But while contact and self-interest are important sources of association, the most effective organizations are based on communities of shared ethical values. These communities do not require extensive contract and legal regulation of their relations because prior moral consensus gives members of the group a basis for mutual trust.

The social capital needed to create this kind of moral community cannot be acquired, as in the case of other forms of human capital, through a rational investment decision. That is, an individual can decide to "invest" in conventional human capital like a college education, or training to become a machinist or computer programmer, simply by going to the appropriate school. Acquisition of social capital, by contrast, requires habituation to the moral norms of the community and, in its context, the acquisition of virtues like loyalty, honesty, and dependability. The group, moreover, has to adopt common norms as a whole before trust can become generalized among its members. In other words, social capital cannot be acquired simply by individuals acting on their own. It is based on the prevalence of social, rather than individual virtues. The proclivity for sociability is much harder to acquire than other forms of human capital, but because it is based on ethical habit, is also harder to modify or destroy.