Marginal charity is the idea that individuals can have the most social gain by unit of private loss by shifting their choices marginally in a prosocial direction. A person's choices are by default close to the private optimum, which sometimes diverges significantly from the social optimum. Thus, slight deviations away from the former and toward the latter should achieve outsized social gains. The expression "marginal charity" was introduced by Robin Hanson,^[1] though, as the author notes, the idea is a relatively straightforward implication of optimization theory.

Possible examples of marginal charity include divesting from the most harmful companies or industries, reducing consumption of animal products, and being generally nicer to others.^[2][3]

Kevin Simler and Robin Hanson speculate that marginal charity, despite its efficiency, is not very popular because acts of marginal charity tend to be indistinguishable from ordinary self-interested behavior. As a consequence, such acts are ill-suited to play the role of moral signaling, which requires behavior to be visibly costly to the agent.^[4][5] For example, a moderately altruistic developer who estimates a profit-maximizing building should be 12 stories high may decide to build one with 13 stories instead. From the outside, however, this decision does not look any more altruistic than the purely self-interested alternative.^[2]

Marginal charity need not involve changes in a person's own behavior—it can apply to cases where others are paid to be more marginally prosocial. For example, longtermists can pay self-interested or short-termist demographers to slightly expand the time horizon of their projections.^[3]