Few decisions in American politics are more important than those reached by the justices of the United States Supreme Court. These nine judges routinely make decisions involving the most significant social and political issues in the country. They decide, among other things, whether government can mandate health insurance, detain suspected terrorists without a trial, ban abortion, or execute criminals who are mentally impaired.
How do justices make these momentous decisions? Many political scientists claim justices have essentially no constraints and, as political creatures, use their positions on the bench to pursue their ideological predilections. In this view, law does not matter. Likewise, according to this view the preferences of elected officials do not matter. Instead, justices themselves are like elected officials who use the power of their office to advance their policy goals.
This explanation has much to recommend for it. It moves past naive views of justices as apolitical legal sages. And it is consistent with empirical evidence that suggests that the votes cast by the justices frequently split along predictable ideological lines. For this reason, it has become a part of the political science canon.
But is it right? Do justices simply base their decisions on the policy preferences they bring to the bench? Many lawyers, students of American politics, judges, and political scientists believe that there is more to justices’ decision-making. Elaborate opinions, cites to precedent, and even the robes suggest something beyond a purely political Court. Without denying the role of politics, many scholars and lawyers argue that law really does matter. According to this view, legal doctrine, not just policy preferences, constrains the decisions justices reach. Others argue that the legislative and executive branches also can constrain justices. In this view, the Supreme Court and its justices act interdependently with the other branches that make up the American political system. As such, the actions of justices depend in part upon the preferences of elected political actors.
While such explanations are intuitive, they are hard to prove empirically. Throughout this book, we provide evidence that justices are not simply pursuing their policy preferences. Instead, they are subject to constraints, whether they come from their internal value systems or from external political forces. We have wrestled with this task for eight years, and the result is a model that allows us statistically to identify legal and institutional influences on justices. To test this model, we have created a