Common property externalities: isolation, assurance, and resource depletion in a traditional grazing context.

Institutional alternatives to common property externalities are wider than argued by private exclusive property rights advocates. The 'tragedy of the commons' is not a prisoners' dilemma, characterized by the strict dominance of individual strategies. The nonseparable common property externality is an 'assurance problem'. The assurance problem provides striking perspectives in analytical and policy terms. It redefines the problem of the commons as one of decision making under uncertainty. Institutional rules innovated by the group to reduce uncertainty and coordinate expectations can solve the problem of overexploitation. Rules come in many forms, and private property is only one. -Author