Birth-order effects on facets of extraversion

The present study investigated the reasons for inconsistent findings concerning birth-order effects and extraversion. According to Sulloway, 1995 and Sulloway, 1996, one would expect first-born siblings to rate higher on one of the facets, dominance, and later-born siblings to rate higher on the other facet, sociability. In a within-family design, 96 undergraduate and graduate students rated themselves and their siblings on a 12-item extraversion scale taken from the NEO Five-Factor Inventory. One-sample t-tests revealed, as predicted, that first-borns rated significantly higher than later-borns on the facet of dominance and later-borns rated significantly higher on the facet of sociability. Various alternative explanations of the results were ruled out and the contribution of the within-family method to the study of birth-order effects in personality was discussed.