In Majority Rule or Minority Will, Harold Spaeth and Jeffrey Segal provide much needed empirical examination of the influence of precedent upon the decisions of Supreme Court Justices. The central question addressed by the authors is not whether justices are influenced by the decisions that were reached in previously decided cases, but whether this influence exists at a systematic and substantively meaningful level in the Supreme Court. There are three answers to this question. First, it could be found that justices are precedental by nature and follow the doctrine of stare decisis in all or nearly all of the cases in which it could be applied. Second, justices could be "legal moderates" in that precedent is an occasional influence upon their decisions, but other factors are often present. The final possible answer is that justices are preferential by nature and are rarely influenced by precedent.

In an effort to answer their question, the authors sought to examine the decisions of the justices in the progeny of established precedents. To complete this task in an appropriate manner, the authors used a sampling of major and minor decisions in which precedent was set. The author's sampled 100% of the cases with dissent from Elder Witt's list of "Major Decisions" from his Guide to the US Supreme Court (1990). The justify their use of these cases as they see Witt's list as more broad than others and the list has been employed by the authors in earlier research. So as to inquire if their findings would be generalizable, the authors also included a sampling of minor decisions of the Court. Sampling became necessary for the authors due to the amount of cases that the Court has decided that include dissent. While a pure random sample would prove unbalanced in that dissent rates vary greatly throughout the history of the court, the authors used a stratified random sample to find their data. Using the data of Epstein et al., the authors found three distinct periods within which to sample: 1793-1867, where dissents were issued less than 10 times per term; 1868-1936, where levels of dissent began to rise; and 1937 through the present, where high levels of dissent are common. In all, approximately 100 cases and their progeny were examined for each of the defined eras. Progeny was related to its particular precedent by the authors using two different sources. First, the authors examined the case syllabi for the cases that they used. Appearance in case syllabi was not the only determinant of precedent; the authors also relied upon Shepard's Citations for this data. In the end, the authors analyzed 2,425 votes and opinions cast by 77 justices in 1,206 progeny of 341 cases spanning from 1793-1995.

After analyzing their data, Spaeth and Segal found that justices do not heed precedent unless they are in previous agreement with it. In other words, justices who dissent from original opinions will rarely reverse their votes in subsequent progeny. The findings of the authors show that roughly 12% of justices will follow precedent, while 88% of justices will continue to decide cases according to their preferences. Therefore, the influence of precedent upon the justices of the Court is neither systematic, nor is it meaningful. These findings run counter to conventional wisdom and the legal model, which have often assumed that Supreme Court justices are inclined to follow previously established legal rules (precedent) even when they disagree with the precedents. The research here is well done and I recommend this book for anyone interested in precedent in the legal system or in political science.