doi:10.1017/S0003055408080283 Does Legal Doctrine Matter? Unpacking Law and Policy Preferences on the U.S. Supreme Court

J udicial scholars often struggle to disentangle the effects of law and policy preferences on U.S.Supreme Court decision making. We employ a new approach to measuring the effect—if any—ofthe law on justices ’ decisions. We use positions taken on Supreme Court cases by members of Congress and presidents to identify policy components of voting. Doing so enables us to isolate the effects of three legal doctrines: adherence to precedent, judicial restraint, and a strict interpretation of the First Amendment’s protection of speech clause. We find considerable evidence that legal factors play an important role in Supreme Court decision making. We also find that the effect of legal factors varies across justices. We don’t turn a matter over to a judge because we want his view about what the best idea is, what the best solution is. It is because we want him or her to apply the law....They are constrained when they do that....They need to be bound down by rules and precedents:... the rules, the laws that you [Congress] pass, the precedents that judges before them have shaped. —John Roberts (U.S. Congress, 2005)...Legal rules governing decision making (e.g