The Biology of Sibling Competition

Sibling competition is widespread among bird and animal species and sometimes leads to siblicide. By influencing the strategies that siblings employ in their struggles for dominance, birth order affects the outcomes of such contests. In our own species, birth order is a proxy for disparities in age, physical size, and status, all of which contribute to personality. In addition, birth order is related to the roles and niches available to offspring within the family system. On average, firstborns—who tend to act as surrogate parents—are more conscientious than laterborns, whereas laterborns are more agreeable, extraverted, and nonconforming. As strategies for dealing with rivals in a dominance hierarchy, as well as for optimizing parental investment, these sibling differences are consistent with a Darwinian perspective on family life. So are other links between personality and family dynamics, particularly those associated with parental investment and parent-offspring conflict. In adulthood, human behavior continues to reflect these formative influences, although such behavioral dispositions generally need to be catalyzed by appropriate situations in order to be fully expressed. 1. THE BIOLOGY OF SIBLING COMPETITION A wide variety of animal species exhibit birth-order differences in behavior, usually in competition for parental investment. These behavioral effects are influenced by two distinct kinds of biological causes: ultimate and proximate. Ultimate causes include adaptive tendencies that have evolved by natural selection. Proximate causes comprise influences operating during the lifetime of the organism and encom-