Horizontal federalism, a theory of constitutional interpretation, is one of the methods available to aid state high courts in analyzing questions arising under their state constitution. Horizontal federalism has been defined as "federalism in which states look to each other for guidance."(1) Law from other states is one available basis upon which a state court may rest its opinion.(2) As obvious as that sounds, the decision to look to other states is controversial. Professor Teachout characterized the way in which state courts address horizontal federalism as one of the four most important questions facing state courts in the area of state constitutional adjudication.(3) The challenge is whether one state high court should rely on other state high court interpretations of state constitutional provisions similar to its own and, if so, then how extensively should it rely on those decisions?(4) This study will attempt to answer to what extent a court seeks this sister-state guidance.
In summarizing the importance of horizontal federalism to the development of state constitutional law, Justice Stewart Pollock of New Jersey speculated that it "may be the hallmark of the rest of the century."(5) Despite its recognized importance, there is a lack of studies that take a comprehensive look at the attention given to one state high court by its sister-state courts in an effort to quantitatively and qualitatively measure the effect of that court across the country.(6) Most studies of state constitutional law focus either on one state's experience(7) or on one topic as treated by many states.(8) In contrast, this study approaches the topic from the inside-out. It attempts to gauge the importance of the Supreme Judicial Court of Massachusetts (SJC) to the rest of the nation by measuring the frequency with which the other forty-nine state high courts rely on Bay State opinions. In so doing, it presents a model for future analysis of the depth and breadth of one court's impact on state constitutional law.
Not every question can be answered by a study of this length. Instead, the purpose of this study is to hypothesize about why one state, Massachusetts, influences its sister states. This is an attempt to show the factors that may influence one court to look at state X but not state Y and what issues prompt a state court to look to its sister courts in the first place. In Part I, horizontal federalism is examined to show how it has emerged under the "new judicial federalism."(9) Part II details the methods used in studying the Massachusetts court.(10) Part III explains why Massachusetts' history makes it a logical choice for this study,(11) Part IV discusses multiple models that attempt to explain, when taken as a whole, why state high courts rely on decisions from other state high courts and which state courts are examined when a state engages in horizontal federalism.(12) The cases that cited to Massachusetts cases are listed and the results of the study are contained in the Appendix.(13)
I. HORIZONTAL FEDERALISM
The term horizontal federalism is credited to a work written by Mary Cornelia Porter and George Alan Tarr in 1982.(14) While the term may be new, the use of decisions from sister states by state high courts is not.(15) The importance of horizontal federalism has become more clear recently, as more state courts have relied on sources other than federal precedent to resolve constitutional issues.(16) As Professor Teachout noted, "it]his sort of cross-fertilization has been a distinctive feature of the state constitutional law movement."(17) One strong indicator of its importance is that several state high courts have directed their state bar to look to decisions of other states when advocating constitutional issues before them(18) or have explicitly announced that they will consider sibling-state precedent in ruling on state constitutional issues.(19)
Tarr and Porter have cited several explanations for horizontal federalism. …