The study of sex allocation-that is, the investment of resources into male versus female reproductive effort-yields among the best quantitative evidence for Darwinian adaptation, and has long enjoyed a tight and productive interplay of theoretical and empirical research. The fitness consequences of an individual's sex allocation decisions depend crucially upon the sex allocation behaviour of others and, accordingly, sex allocation is readily conceptualized in terms of an evolutionary game. Here, I investigate the historical development of understanding of a fundamental driver of the evolution of sex allocation-the rarer-sex effect-from its inception in the writing of Charles Darwin in 1871 through to its explicit framing in terms of consanguinity and reproductive value by William D. Hamilton in 1972. I show that step-wise development of theory proceeded through refinements in the conceptualization of the strategy set, the payoff function and the unbeatable strategy. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.
Keywords: consanguinity; game theory; reproductive value; sex allocation; sex ratio; unbeatable strategy.