He is best known for his claim that birth order exerts large effects on personality, and the subsequent debates about this issue. In his 1996 book Born to Rebel, Frank Sulloway suggested that birth order had powerful effects on the Big Five personality traits. He argued that firstborns were much more conscientious and socially dominant, less agreeable, and less open to new ideas compared to laterborns.
Frank Sulloway's work on birth order and personality traits has sparked debates about the impact of family dynamics on individual characteristics. He argued that firstborns exhibit distinct personality traits compared to later-born children, particularly in terms of conscientiousness, social dominance, agreeableness, and openness to new ideas.