under Mansfield adopted the law merchant and this is the source of the efficient contract and commercial law rules that many have observed in the common law. It is important to note that in many circumstances(and particularly in business or contractual disputes)the parties would pick the forum ex ante, so that both would have an incentive to choose efficient courts. Thus, in this large class of disputes, there would be no pro-plaintiff bias ht exist when plaintiffs choose courts after a dispute has arisen In this view, an important source of efficiency is the existence of competing courts and bodies of law. where such competition exists, rules will tend towards efficiency; where competition is lacking, there will be weak or non-existent tendencies for efficiency. This explains the actual path of the common law. In its formative stages, here was competition, and the law became efficient. More recently, there has been less competition, and the law has moved towards some inefficiencies. For example in the U.S., federal and state common law originally competed. This competition ended in 1938, as a result of the case Erie Railroad v. Tompkins(304 U.S. 64). After that period there was less competition as the federal courts were required to use the relevant state law. Now, arbitrators do compete with the courts for commercial business What can we say about the evolutionary models? By focusing on the role of litigants and, with the addition of the important Zywicki analysis on the role of competing courts, the models have a good deal of explanatory power. They do not, as irst thought, indicate that the common law will always be efficient. However, they do explain both why common law was efficient in the past and why it is now less efficient They also provide some guidance for methods of channeling the law towards efficiency 8under Mansfield adopted the law merchant and this is the source of the efficient contract and commercial law rules that many have observed in the common law. It is important to note that in many circumstances (and particularly in business or contractual disputes) the parties would pick the forum ex ante, so that both would have an incentive to choose efficient courts. Thus, in this large class of disputes, there would be no pro-plaintiff bias as might exist when plaintiffs choose courts after a dispute has arisen. In this view, an important source of efficiency is the existence of competing courts and bodies of law. Where such competition exists, rules will tend towards efficiency; where competition is lacking, there will be weak or non-existent tendencies for efficiency. This explains the actual path of the common law. In its formative stages, there was competition, and the law became efficient. More recently, there has been less competition, and the law has moved towards some inefficiencies. For example in the U.S., federal and state common law originally competed. This competition ended in 1938, as a result of the case Erie Railroad v. Tompkins (304 U.S. 64). After that period there was less competition as the federal courts were required to use the relevant state law. Now, arbitrators do compete with the courts for commercial business. What can we say about the evolutionary models? By focusing on the role of litigants and, with the addition of the important Zywicki analysis on the role of competing courts, the models have a good deal of explanatory power. They do not, as first thought, indicate that the common law will always be efficient. However, they do explain both why common law was efficient in the past and why it is now less efficient. They also provide some guidance for methods of channeling the law towards efficiency 8