The influence of rearing order on personality development within two adoption cohorts

There is an extensive literature on the relationship between birth order an d psychological traits, but no previous study has investigated the influenc e of ordinal position on personality development within adoptive siblings. Such a design is important because it effectively separates the effects of biological birth order and rearing order. Here we report data from two adop tion cohorts in which subjects were biological first-borns reared in variou s ordinal positions. Data were analyzed with reference to Sulloway's (1996) evolutionarily based sibling rivalry theory of birth order effects. Betwee n- and within-family analyses indicated that rearing order's influence on p ersonality was very weak. The only clear difference was for conscientiousne ss, on which first-reared siblings scored higher. We draw possible implicat ions for Sulloway's theory and speculate upon an alternative, prenatal biol ogical process that may produce birth order differences.