Spillovers Mean There Is Too Little Investment in R&D The profit-maximizing private investment in R&D is found where the private marginal benefits just equal the (private) marginal costs (point a). Some of the benefits of R&D spill over to people other than the producer. The social benefits of R&D are higher than the private benefits by the amount of the spillover. The optimal social investment in R&D is found where the social marginal benefits just equal the marginal cost (point b). Thus, when the private benefits of R&D are less than the social benefits, private investment in R&D will be less than the optimal social investment in R&D.