The researchers looked at the Big Five personality traits—openness, agreeableness, neuroticism, conscientiousness, and extraversion—and predicted that “firstborns (versus laterborns) should be higher in Conscientiousness, Neuroticism, and the dominance aspect of Extraversion, whereas laterborns should be higher in Agreeableness and the sociability aspect of Extraversion.”
Over this entire huge population, they found that firstborns did tend to be a little more conscientious and dominant, and less sociable, but also that they tended to be more agreeable and less neurotic (which goes against the stereotype). But the effect size of this finding was tiny—so tiny that if they hadn’t looked at hundreds of thousands of people, it wouldn’t have been significant at all. When it came to intelligence, firstborns did have an advantage—of one IQ point. As a comparison, another study found that some adolescents’ IQs changed by as much as 20 points within four years.