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CHAPTER 2 BIRTH ORDER, SIBLING COMPETITION, AND HUMAN BEHAVIOR FRANK J. SULLOWAY University of California, Berkeley Abstract : 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 af- fects 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 aver- age, firstborns—who tend to act as surrogate parents—are more con- scientious 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 invest- ment, these sibling differences are consistent with a Darwinian per- spective on family life. So are other links between personality and family dynamics, particularly those associated with parental invest- ment and parent-offspring conflict. In adulthood, human behavior con- tinues 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 be- havioral 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- 39
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SULLOWAY 40 pass biological as well as environmental factors, which almost always interact with one another. For example, some avian species possess an instinct to migrate during the autumn and spring, an adaptation that has its ultimate cause in natural selection. Temperature and day length, along with the various neuropsychological mechanisms they trigger, supply the proximate causes of bird migration (Mayr, 1961). Viewed in these conceptual terms, a biological propensity to en- gage in sibling rivalry is one of the ultimate causes of personality development. Darwin’s theory of natural selection explains this part of the story, which focuses on the biological dispositions that most offspring have to compete for parental favor. As William Hamilton (1964) recognized, natural selection maximizes inclusive fitness. This form of Darwinian fitness can be defined as an organism’s own reproductive success, together with its contribution to the reproduc- tive success of close relatives, discounted according to their coeffi- cient of relatedness. On average, siblings share half of their genes.
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This note was uploaded on 01/16/2012 for the course BI 200 taught by Professor Potter during the Fall '11 term at Montgomery College.

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