The principles of the American legal tradition are thought to place limits on the members of the U.S. Supreme Court. Fidelity to written law, support for the doctrine of stare decisis, and deference to elected majorities are among the standard elements of the judicial canon. It is clear, however, that the justices make decisions with an eye toward achieving their policy goals and that they vary considerably in the extent to which they support judicial norms in their resolution of cases. How do the justices form their ideas about what constitutes good judicial policy and how judges should achieve it?One plausible explanation, drawn from the field of evolutionary psychology, suggests that a process of socialization determines one's attitudes toward norms, rules, and authority (Buss 1997). This socialization takes place long before assuming adult roles, such as judging. Indeed, it occurs during childhood, governed by an individual's relative position among siblings. Birth order has long been a subject of intensive study, and in recent years researchers have given close attention to its linkage to a person's receptivity to change. Specifically, evolutionary psychologists posit that, early in life, individuals engage in adaptive behaviors that are conditioned by the presence of siblings and seek certain niches that maximize parental attention (Sulloway 1996). Older siblings-firstborns in particular-are thought to identify with and emulate their parents and are thereby rewarded for their conscientiousness and respect for authority. As a consequence of these behavioral adaptations, firstborns develop not only ideologically conservative preferences but also a tendency to reject intellectual innovation, owing to their strong inclination to support the status quo. Laterborns, by contrast, must be creative and adaptable as a means of distinguishing themselves from their older siblings. Because of their openness to novel ideas, laterborns acquire more liberal political preferences as well as a predisposition to take risks and to rebel against convention.Applying the birth order thesis to the U.S. Supreme Court, I argue that the justices' microenvironments during childhood should affect their approach to legal decisionmaking on the bench. First, birth order should explain the justices' ideological orientations; older siblings on the Court should have more conservative preferences; younger siblings, more liberal attitudes. Second, the impact of birth order should also be revealedinthejustices'roleorientations. Owing to their deference to authority, firstborn justices should support existing regimes, defer to popular decision makers, and exercise restraint. Laterborn justices, by contrast, should evince activism; being less tethered to the status quo, such justices should be more open to questioning the judgments of elected officials. Testing this theory, I find clear support for birth order effects. The evidence reveals that birth order is directly linked to the justices' preferences. Likewise, birth order helps to account for their decisions about how to realize their policy goals; although ideology governs how the justices view the constitutionality of challenged legislation, birth order conditions the justices' willingness to strike down the actions of popular decision makers.In the following sections, I sketch the psychological theory about the importance of birth order, and I derive hypotheses about its relevance to the justices' policy attitudes and their approaches to judging. I then subject those hypotheses to statistical test.Birth Order and the Niche-Seeking HypothesisAnalyses of judicial behavior and theories of psychology have increasingly important intersections (Klein and Mitchell 2010), and studies of the Supreme Court in particular have recently invoked psychological explanations to account for the choices the justices make (see, e.g., Baum and Devins 2010; Wrightsman 2006). Scholars have often assumed that the members of the Court pursue various goals without exploring why they pursue them; thus, attention to the psychological foundations of judging is especially valuable, because it enables researchers to uncover the forces that underlie judicial decisionmaking (Baum 2010: 9). …