Association Between Sibling Composition in the Family of Origin and Moral Foundations in Adulthood

Considerable research has been performed on the association between sibling composition and various characteristics, including personality, intelligence, and social attitudes. These studies suggest that birth order among siblings could weakly but significantly affect the development of personality traits and intelligence. However, few studies on the association between sibling composition and moral foundations have been conducted. Therefore, the present study investigated whether sibling composition (only child, firstborn, laterborn) is related to moral values with a between-family design. A sample of 500 Japanese adults (250 women) completed the self-administered online questionnaire. The mean age was 45.42 years (SD = 8.53, range = 20–60, median = 46). The general linear model was used to test the association between sibling composition and moral values. The results showed no differences in five moral foundation and two higher-order moral dimension scores between firstborns and laterborns. However, only children scored lower on care/harm, fairness/cheating, and individualizing foundation than laterborns. This finding might indicate that growing up with their siblings positively affects the development of care/harm and fairness/cheating (individualizing foundation) regardless of one's position among those siblings.