Evolutionary stable investment in products that confer both an individual benefit and a public good.

Why should a microbe manufacture extracellular enzymes if its competitors can free-ride on these enzymes? Similarly, why should an animal place seeds into storage when others can exploit this stored resource? A solution to this general class of problems becomes apparent if one assumes that investors directly benefit from a proportion of the investments they make. Thus, when individuals benefit from a proportion p of their investments, but share the rest with other individuals in the system, then an evolutionarily stable level of investment can evolve which is higher the higher the value of p. These evolutionarily stable investment points mark the junction at which several classical games meet, so that changes in investment can move interactions from one game type to another. Non-zero optimal levels of investment also arise under conditions when investments are only shared locally, and even when producers lose more product to competitors than they save for themselves. Overall, this "personal gain" approach offers a simple yet robust explanation for why individuals engage in activities which may concomitantly benefit others.