Social capital is the concept that interaction between social groups has value. While physical capital (material goods) and human capital (education) both have value, social capital depends on collective cooperation and individuals working and interacting as groups; the upshot is that all three types of capital affect productivity (Wikipedia).
In the United States, social capital is measured by the interaction of the public and individual groups (the private sector) with government (public sector). Because of the necessary interaction by government and citizenry required by a "government by the people, for the people, of the people," social capital resides in trust, actual participation, and recognition of civic or public responsibilities. This could be termed Social Capital Trust, since the level of trust the public has with the government tends to coincide with the amount of participation; when the public distrusts the government, they tend to interact with it less. According to studies, the level of social capital in the U.S. is decreasing, leading to less interest in public service, civic duties, and overall welfare of the nation as a whole. This is compounded by the public desiring more control with fewer responsibilities; the government reacts by offering less control but more benefits, and the levels of trust decrease accordingly.