Distinguishing private action from government action is the first question of constitutional law. The distinction blurs most when the government and private actors jointly cause harm. Not surprisingly, then, the Supreme Court's cases in this gray area have been inconsistent. For example, state court enforcement of a private racially restrictive covenant is government action, but agency placement of a child in a home where the child is abused is private action. The ad hoc nature of these decisions reflects a reluctance to fully embrace joint government-private causation of constitutional harm: Without a limiting principle, doing so would threaten to convert all private action into government action. The concept of 'social capital' provides a needed limiting principle. Social capital is the value that lies in one's network of relationships. Specifically, social capital creates value through enforceable trust: That is, members of a community trust that other members will follow certain norms of behavior, and that trust is enforced by a threat of informal punishment (e.g., ostracism) for violating the community's norms. Constitutional law ought to recognize that government action can interact with private norms (created by enforceable trust) and, in doing so, burden constitutional values. This Article argues that the government violates the Constitution when a private group improperly uses government power or property to enforce the group's social capital. Through enforceable trust, social capital leads to predictable behavior - adherence to group norms. When government regulates in an area with strong social capital, that government action might also have predictable effects. The Supreme Court already holds the government responsible for the predictable effects of mandatory disclosure laws on unpopular groups, that is, groups whose beliefs or behavior violate prevailing norms. When the government requires an unpopular group to disclose its members, and disclosure will likely lead to threats and harassment against the members, the disclosure law violates the First Amendment. This Article extends the lessons of social capital to the case of private prayer at public occasions. Consider a private prayer recited at a public high school football game. If all in attendance are at liberty to object to or abstain from the prayer, the prayer would not violate current Establishment Clause doctrine. Yet, objecting to the prayer may violate local norms of religious belief or practice, opening a person to harassment and retaliation, just as the disclosure law did to members of unpopular groups. The religious objector falls into a gap in Establishment Clause protection, and this Article proposes a legal test to close the gap.