On the Contribution of Reproductive and Offspring Investment on Fertility: Human and Animal Societies

Differences in investment into reproduction or offspring rearing are plentiful throughout the world, from the cells inside our bodies to complex sociological interactions among humans. Such differences can lead to profound impacts on species fitness, fertility, and reproductive rates, sometimes in startling ways. In this paper, we create a simple game-theoretical model to qualitatively investigate the effects of such differential investment. We focus on fertility in human societies and show that more wealthy individuals produce more offspring within a a mating group. However, when assortative mating mechanisms are introduced, this effectively leads to a speciation event, and a higher reproduction rate for poorer individuals is noticed, capturing what we call the "wealthy-to-poor switch". We discuss extensions and implications of this work to nupital gifts in ecology and to clonal competition in cancer cell lines under the influence of treatment.