about relative magnitudes of costs is incorrect, then doctrines claimed to be efficient may not be so. Nonetheless, Posner's analysis has been the intellectual spark behind the growth of law and economics, and questions about efficiency of the sort he was the first to raise, have dominated the literature. Much of this literature may be considered as detailed attempts to answer the positive efficiency question first posed by Posner, and much of the rest aims at deriving normative conclusions as to what is efficient This method of analysis is fundamentally different from efficiency analysis in other branches of economics. There, a process (market competition) is postulated and it is shown that the process leads to efficient outcomes. Economists do not generally examine consumers to see if they are equating ratios of marginal utilities to prices, or irms to see if they are charging marginal cost. Rather, the process by which outcomes are generated is shown to lead to efficiency. It is for this reason that many economists are uncomfortable with the efficiency arguments of law and economics. Nonetheless because this is the standard method in law and economics, any analysis of the efficiency of any particular body of law can be considered as evidence for or against Posner's hypothesis. If some law is found efficient, then this is evidence for the hypothesis. If some law is found inefficient, or if there are proposals for reform, then this is evidence against the hypothesis. In this sense, much of law and economics is aimed at testing this fundamental hypothesis Evolutionary Models Scholars are of course aware of these difficulties and have sought to identify a process that would lead to efficiency. I first proposed the evolutionary model of legalabout relative magnitudes of costs is incorrect, then doctrines claimed to be efficient may not be so. Nonetheless, Posner’s analysis has been the intellectual spark behind the growth of law and economics, and questions about efficiency of the sort he was the first to raise, have dominated the literature. Much of this literature may be considered as detailed attempts to answer the positive efficiency question first posed by Posner, and much of the rest aims at deriving normative conclusions as to what is efficient. This method of analysis is fundamentally different from efficiency analysis in other branches of economics. There, a process (market competition) is postulated and it is shown that the process leads to efficient outcomes. Economists do not generally examine consumers to see if they are equating ratios of marginal utilities to prices, or firms to see if they are charging marginal cost. Rather, the process by which outcomes are generated is shown to lead to efficiency. It is for this reason that many economists are uncomfortable with the efficiency arguments of law and economics. Nonetheless, because this is the standard method in law and economics, any analysis of the efficiency of any particular body of law can be considered as evidence for or against Posner's hypothesis. If some law is found efficient, then this is evidence for the hypothesis. If some law is found inefficient, or if there are proposals for reform, then this is evidence against the hypothesis. In this sense, much of law and economics is aimed at testing this fundamental hypothesis. Evolutionary Models Scholars are of course aware of these difficulties and have sought to identify a process that would lead to efficiency. I first proposed the evolutionary model of legal 2