Birth order and a two-dimensional assessment of personality.

Higher order personality dimensions of extraversion-introversion and neuroticism were studied as functions of birth order in two-sib families, using 141 female subjects, with control over sex of sibling and sib age separation. No significant personality dimension variance was attributable to birth order in females. The results were considered briefly in terms of the importance of birth order, heritability questions, and the possible contribution of birth order to lower order traits.