Evolutionary theory and birth order effects on Big Five personality traits among the Shuar of Amazonian Ecuador: The first cross-cultural test

This dissertation reports on the effects that birth order, the ordinal position occupied in a sibship, has on the development of human personalities. Robust birth order effects on personality have been found among people of Western, industrialized societies. Here I report the first cross-cultural test of birth order effects on Big Five personality traits among a non-Western, non-industrialized cultural group, the Shuar of Amazonian Ecuador. The thesis of this dissertation is that differing birth order in the family means that first born and later born children effectively face different familial social ecologies, and therefore develop different behavioral and personality strategies to deal with those differing social niches. For humans, birth order directly relates to disparities in size, status, and inclusive reproductive value: critical factors in the inevitable competition over parental investment. Since the environment of development differs for humans based on their birth order, it follows that the developing behavioral characteristics (including personality) will differ within a sibship. This is not a theory of a genetic difference underlying personality differences---it is a developmental one, and yet firmly Darwinian. On this view, differences in personality reflect a species-typical behavioral and emotional plasticity that responds during development to, among other things, the social conditions associated with the ordinal position a child occupies in the family. If so, these birth order effects should be apparent across human societies, and cross-cultural tests are crucial to show these effects are not simply the product or byproduct of Western industrialized culture. This dissertation begins by explaining the value of an anthropological, biocultural investigation of birth order effects, and then moves on to review the neo-Darwinist approach to understanding biological phenomenon with a focus on life history theory. A review of the pertinent personality literature follows, focusing on birth order effects on personality and the debates surrounding this subject. As predicted, this study shows robust birth order differences in Big Five personality factors among the Shuar that parallel the patterns found among Western subjects.