3 . Efficiency in the presence of externalities Parks confer many external benefits on society: open space, trees that reduce pollution, and so on. Therefore, the market equilibrium quantity of parks is not equal to the socially optimal quantity. The following graph shows the demand for parks (their private value), the supply of parks (the private cost of producing them), and the social value of parks (the private value and external benefits). Use the black point (plus symbol) to indicate the market equilibrium quantity. Next, use the purple point (diamond symbol) to indicate the socially optimal quantity. Supply (Private Cost) Market Equilibrium Socially Optimal Level PRICE OF PARKS Social Value Demand (Private Value) QUANTITY OF PARKS