Despite the existing vast literature on corporate social responsibility (CSR), there is a lack of research on the determinants of CSR categoriesenvironment, employees, community, and governance. Why do some firms dedicate more effort to community, while others focus on environment or employees? We address these issues with special attention to the roles of market structure and firm size, controlling for industry, geography, regulatory and macroeconomic variables. Answers are relevant for firms' strategic decisions, taking into account what rivals likely choose, in a world where CSR is often a competition tool. Public authorities also benefit on choosing focus: lack of competition; small or large sized firms; which categories are more affected. We find that all CSR categories profit more from a competitive setup, particularly employees and environment, and from larger firms, mainly environment. Size and market concentration thus contribute in different directions to CSR, contradicting their usually presumed positive association.