The discount rate that equalizes the net present social benefits of the future real gains from a private activity to the real social costs of this activity. Social benefits and social costs include the value of any possible positive or negative externality of the private activity, as well as the costs borne by the society. For example, the social benefits of education may include the value of safer communities, better parenting, and more informed and active citizens, whereas the social costs may include government subsidies to education. See also private internal rate of return.