Abstract
Continuing the centuries-long exchange between economics and biology, our model of reproductive skew is an adaptation of the principal-agent relationship between an employer and an employee. Inspired by the case of purple martins (Progne subis) and lazuli buntings (Passerina amoena), we model a dominant male whose fitness can be advanced not only by coercing a subordinate male but, where coercion is impossible or not cost-effective, also by providing positive fitness incentives for the subordinate that induce him to behave in ways that contribute to the dominant's fitness. We model a situation in which a dominant and subordinate contest over a variable amount of joint total fitness, both the level and division of which result from the strategies adopted by both. Thus there is not some given amount of potential fitness (or 'pie') that is to be divided between the two (or wasted in costly contests). The fitness incentives that in evolutionary equilibrium are conceded to the subordinate by the dominant maximize the dominant's own fitness. The reason is that the larger pie resulting from the subordinate's increased helping more than compensates for the dominant's reduced fitness share. But the conflict over fitness shares nonetheless limits the size of the pie. This article is part of the theme issue 'Evolutionary ecology of inequality'.