The members of the U.S. Supreme Court have different ideas about what constitutes good judicial policy as well as how best to achieve that policy. From where do these ideas originate? Evolutionary psychology suggests that an answer may lie in early life experiences in which siblings assume roles that affect an adult's likely acceptance of changes in the established order. According to this view, older siblings take on responsibilities that make them more conservative and rule‐bound, while younger ones adopt roles that promote liberalism and greater rebelliousness. Applying this theory to the Court, I show that these childhood roles manifest themselves in later life in the decisions of the justices. Birth order explains not only the justices’ policy preferences but also their acceptance of one important norm of judicial decisionmaking, specifically their willingness to exercise judicial review.