Birth order has no effect on intelligence: a reply and extension of previous findings

We address points raised by Zajonc and Sulloway, who reject findings showing that birth order has no effect on intelligence. Many objections to findings of null birth-order results seem to stem from a misunderstanding of the difference between study designs where birth order is confounded with true causal influences on intelligence across families and designs that control for some of these influences. We discuss some of the consequences of not appreciating the nature of this difference. When between-family confounds are controlled using appropriate study designs and techniques such as multilevel modeling, birth order is shown not to influence intelligence. We conclude with an empirical investigation of the replicability and generalizability of this approach.     

Birth order and introversion-extraversion

The relationship between birth order and introversion-extraversion was investigated. Subjects were 70 male and 77 female undergraduates, all from three-child families. The measure of introversion employed was the Social Introversion Scale (Scale 0) of the Minnesota Multiphasic Personality Inventory. A two-way analysis of variance indicated that first borns were more introverted than either middle-horns or last-horns, with no difference between the latter two groups. It was suggested that inconsistent results in this area may be due to the use of different measures of introversion.     

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.