The evolution of public goods altruism

Organisms often behave altruistically by investing resources into public goods production. Because the rewards of public goods are freely available to everyone in a group, they are potentially vulnerable to exploitation by cheaters. Classic kin selection models offer solutions to this public goods dilemma in terms of why public goods investment can be favoured, while game theoretical models offer insights into the conditions where exploitative cheaters can persist. However, existing theory does not provide a unified framework for understanding how much individuals should actually invest in altruism or how this investment strategy relates to why cheaters can coexist with altruists. Here we fill this gap by considering how resource allocation trade-offs shape the generosity of altruists, how this investment decision, in turn, sets the conditions for cheater coexistence, and how local competition impacts these outcomes. We find that, whenever altruists can evolve the optimal level of public goods investment, they exclude cheaters, revealing that stable altruists/cheater coexistence requires overly generous altruists--an outcome frequently driven by mechanistic constraints that limit the range of behaviours in biological systems. Importantly, we show that local competition tends to prevent such coexistence. By capturing key biologically relevant factors, our model provides a compelling framework for interpreting patterns of variation in altruistic behaviour in natural populations, and a means of generating testable predictions.