Birth order, conscientiousness, and openness to experience: Tests of the family-niche model of personality using a within-family methodology

We investigated differences between firstborn and secondborn siblings on major dimensions of personality, in the context of the proposal of Sulloway [Sulloway, F. J. (1996). Born to rebel: Birth order, family dynamics and creative lies. New York: Pantheon] that personality is influenced by the specialized niches siblings adopt in the quest for access to parental resources. Using a within-family methodology, we tested two predictions from Sulloway's model: that firstborns are more achieving and conscientious than secondborns and that secondborns are more rebellious and open to new experiences than firstborns. To test an alternative prenatal hypomasculinization theory proposed by Beer and Horn [Beer, J. M., & Horn, J. M. (2000). The influence of rearing order on personality development within two adoption cohorts. Journal of Personality, 68, 769–819], we also examined the size of birth-order effects in sister–sister versus brother–brother pairs. The hypothesized effects of birth order on personality were found in both Study 1 (n=161 sibling pairs) and Study 2 (n=174 sibling pairs) and provided support for Sulloway's family-niche model. No support was found for Beer and Horn's hypomasculinization model.