Abstract Sulloway (1996) proposed that personality traits developed in childhood mediate the association of birth order with scientific radicalism. Birth-order effects on traits within the five-factor model of personality were examined in three studies. Self-reports on brief measures of Neuroticism, Extraversion, and Openness in a national sample ( N = 9664) were unrelated to birth order. Self-reports on the 30 facet scales of the Revised NEO Personality Inventory (NEO-PI-R) in an adult sample ( N = 612) showed only small effects for Altruism and Tender-Mindedness. Peer ratings ( N = 166) supported the hypotheses that laterborn children would be higher in facets of Openness and Agreeableness, but spouse ratings ( N = 88) did not replicate those findings. Birth order may have subtle effects on perceived personality, but it is unlikely that this effect mediates associations with scientific radicalism.