Birth order and personality: A within-family test using independent self-reports from both firstborn and laterborn siblings

• We utilize a within-family design to study birth order effects on personality.
• We examine personality ratings from sibling pairs, their parents and their peers.
• We find no within-family or between-family effects of birth order on personality.

Assumptions about the effects of birth order on personality abound in popular culture and self-help books. Indeed, when one sibling is asked to compare themselves to others in their family, birth order shows weak-to-moderate effects on personality (e.g., Healey and Ellis, 2007 and Paulhus et al., 1999). No study to date, however, has utilized a complete within-family design that includes independent self-reports from both firstborn and laterborn siblings in the same family. To fill this gap, we collected Big Five personality data on 69 young adult firstborn–laterborn sibling pairs. We also obtained data from parents of the sibling pairs and peer ratings of original participants’ personality traits. Within-family analyses revealed that neither siblings’ independent self-reported personality traits, nor parents’ reports of their children’s personality traits, differed systematically as a function of birth order. Our findings are consistent with results from between-family designs and they provide further evidence, employing a within-family design that utilizes data from multiple family members, that birth order does not have enduring effects on personality.