Evolution and the trolley problem: People save five over one unless the one is young, genetically related, or a romantic partner.

Abstract

We investigated men's and women's responses to variations of an ethical thought experiment known as the Trolley Problem. In the original Trolley Problem, readers must decide whether they will save the lives of five people tied to a track by pulling a lever to sacrifice the life of one person tied to an alternate track. According to W. D. Hamilton's (1964) formulation of inclusive fitness, people's moral decisions should favor the well-being of those who are reproductively viable, share genes, and provide reproductive opportunity. In two studies (Ns = 652 and 956), we manipulated the sex, age (2, 20, 45, and 70 years old), genetic relatedness (0, .125, .25, and .50), and potential reproductive opportunity of the one person tied to the alternate track. As expected, men and women were less likely to sacrifice one life for five lives if the one hypothetical life was young, a genetic relative, or a current mate.