What resource problem is created by positive externalities?

Positive externalities occur when an underallocation of resources leads to underproduction of the product. People who are not directly involved in a market transaction receive benefits from the market transaction without having to pay for them. This failure to account for all benefits, including those received by people not directly involved in the market transaction, shifts market demand curves to the left of where they would be otherwise. In such cases, markets fail to produce all units for which benefits exceed costs. As a result, products featuring positive externalities are underproduced and result in a deadweight efficiency loss.