This paper sets out a formal framework for the biological-evolutionary study of human economic behaviour. Femina economica is a hypothetical parthenogenetic species with a simple economy. Individuals make decisions about labour supply and fertility subject to time and resource constraints. Labour is of differing types and a parent determines the type of her offspring's labour. Wages are determined as marginal productivities from an economy-wide production function. Three propositions are proved, of which the first shows that under very general conditions there exists a population genetic equilibrium, in which individuals' decisions are assumed to be under genetic control. The second shows that at a population genetic equilibrium, individuals have the same behaviour as they would at an economic equilibrium, in which individuals are assumed to maximise a common utility function. The third proposition shows that if the common utility function fulfils certain conditions, the attainment of an economic equilibrium brings about the same behaviour as a population genetic equilibrium. This suggests a way in which evolutionarily stable behaviour can be brought about without the necessity for changes in gene frequencies. Demographic implications include the possibility of interpreting in Darwinian terms the reductions in offspring number that occur in fertility transitions, and the weak or even negative correlation in economically developed societies between control of resources and offspring number. There are implications for economics of deriving utility maximisation from population genetics.