Crime as Strategy: Testing an Evolutionary Ecological Theory of Expropriative crime ' Bryan J. Vila University of Caliifornia, Zrvine Lawrence E. Cohen University of California, Davis This article presents a refined specification of Cohen and Macha- lek's general evolutionary ecological theory of expropriative crime and results of tests employing two complementary approaches: (1) the development of a game-theoretic model that mathematically tests the logical adequacy of the theory's fundamental assumptions and (2) the conduct of computer simulation experiments to analyze the model's behavior and test its consistency with novel hypotheses suggested by the theory. Mathematical analysis indicates that the assumptions are logically consistent. Experiments generally confirm the theory's hypotheses, but indicate that several modifications are necessary. Additional theoretical insights obtained from the simula- tion experiments are also discussed. In a recent article, Cohen and Machalek (1988) propose a general theory of expropriative crime that offers a comprehensive synthetic explanation for a wide range of empirical findings generated by the competing disci- plines that have studied crime. Drawing from the theoretical framework of evolutionary ecology, they emphasize that the key to understanding the nature and distribution of illegal behaviors by which offenders expro- priate resources (objects of symbolic or material value) from other lies in understanding the relationship between productive and expropriative strategies and their interdependence within populations of individuals. Unlike many theories that characterize criminal behavior as pathological ' The research reported in this paper was supported in part by University of Califor- nia, Irvine, faculty research grant IFR 90191-23. We would like to thank Diane H. Felmlee, Mary Jackman, Richard McCleary, Peter J. Richerson, Joan Scoll, Sherlyn Pang, and Joan Huber for their incisive comments on earlier versions of this paper. Several anonymous reviewers also contributed critical comments for which we are grateful. Correspondence may be addressed to Bryan J. Vila, School of Social Ecology, University of California, Irvine, California 92717. O 1993 by The University of Chicago. All rights reserved. 0002-9602/93/9804-0005$01.50 A J S Volume 98 Number 4 (January 1993): 873-912 873 American Journal of Sociology or abnormal, this theory argues that expropriative crimes are best viewed as behavioral strategies typically used by normal individuals in unexcep- tional social systems to meet their needs.' This approach is unique among criminological theories because it fo- cuses on a neglected sociological element of crime-the evolution of be- havioral strategies that arise and spread through populations by means of culturally mediated processes. These cultural processes are akin to natural selection in the sense that the more successful strategies are more likely to be imitated and then proliferate within a population. Most extant theories of crime concentrate on identifying the different structural char- acteristics of societies or the differences in individual traits or experiences that increase the probability that people will engage in crime. By con- trast, while evolutionary ecological theory accounts for the possible crimi- nogenic influence of individual and social factors, it argues that expropri- ative criminal strategies should not be viewed solely as the properties of individuals. Instead, these strategies should be viewed as behavioral options, the expression of which often varies independently of the charac- teristics of the persons who adopt them (Cohen and Machalek 1988, p. 467). Thus, the authors identify behavioral strategies, rather than individuals and their personal and social traits, as the focus of analysis for the study of crime. Furthermore, they treat both criminal and non- criminal behavioral strategies as competing entities in their own right. This approach allows them to explain how strategies can influence their own proliferation. In addition, this perspective permits the generation of novel hypotheses that cannot be derived from extant criminological theo- ries. While Cohen and Machalek deduce nine such hypotheses, they offer no new evidence with which one could evaluate their theory's adequacy. In fact, the authors concede that such tests may be difficult to produce, given the complexity of their theory. Here we present the results of a test of the theory that employs two complementary approaches: (1) development of an evolutionary game- theoretic model that tests the logical adequacy of the theory's fundamen- tal assumptions mathematically and (2) computer simulation experiments that employ the model to assess whether or not the novel hypotheses can be supported throughout a wide range of conditions. As we will show, these approaches were quite productive. A number of new theoretical insights were obtained from the experiments by specifying the model so that it accounted for important issues suggested by the literature on cul- tural evolution and systematics (e. g., cultural biases for or against certain This explanation of crime as a normal characteristic of naturally functioning societies is consistent with Durkheim's (1895) dicta but invokes substantially different causal processes. A Theory of Crime types of strategies, lags in their transmission, and transmission reliabil- ity). These approaches also helped refine the theory. As described by Cohen and Machalek, the theory made extensive use of general concepts and examples from evolutionary ecology that are foreign to many social scientists. One, of the most important and useful steps in the modeling process we employed here was to tease out the implications of these concepts and specify them explicitly. As one might expect with such a broad and synthetic theory, several issues neglected in the initial exposi- tion of the theory were identified during this process. The following re- fined specification addresses these issues and describes explicitly how the mechanisms and processes invoked by the theory might function. A REFINED EVOLUTIONARY ECOLOGICAL THEORY OF CRIME As its name implies, the theory uses both ecological and evolutionary concepts to explain crimes by which people illegally expropriate resources from others. These two fields of study have quite different, but comple- mentary, foci. Whereas ecology focuses upon the reciprocal interrelation- ships between living organisms and their environment, the focus of evolu- tion is the processes by which these relationships lead to cumulative change in the characteristics of organisms or populations over time. Ear- lier in this century, the synthesis of ecology with evolution vitalized the biological sciences by uniting explanations of how individuals and groups of organisms function with explanations about why they change over time. Similarly, Cohen and Machalek complement their ecological expla- nation for the differences between expropriative crime rates in popula- tions at a particular point in time with a description of the evolutionary dynamics that produce significant change in these rates. I t is this synthe- sis that makes the theory "general." Ecological Aspects of Criminal Behavior The ecological components of this theory include such factors as routine patterns of activity, the availability and distribution of resources, modes of production, and competition. Cohen and Machalek argue that the social organization of production within human populations creates an opportunity structure that invites invasion by various strategies of expro- priation. Individuals acquire valued resources from a particular resource niche by employing behavioral strategies. If a strategy involves produc- tion of goods or services through one's own efforts, it is characterized as "productive." If it involves taking material or symbolic resources from others through force, fraud, or other illegal actions, it is characterized as American Journal of Sociology "e~pro~riative.T"h~e re is often a great deal of variation in both the ability of individuals to acquire resources within different niche dimen- sions and in their valuation of, or motivation to acquire, a particular resource. Much of this variation arises from sociocultural, psychological, physiological, and developmental factors that are related, either directly or indirectly, to the ability of individuals to compete legitimately (and illegitimately) for valued resources within different social environments. This means that asymmetries between persons that are associated with competitive advantage for acquiring resources (e.g., social characteristics and individual traits as well as differences in motivation) significantly impact strategy selection and evolution within populations. One of the most important and unique features of the theory, however, is the contention that the selection of productive and expropriative strate- gies need not always derive from individual differences. According to the original specification of the theory, the likelihood that an individual will adopt a particular strategy is related to its potential for success-what we will refer to as "payoffs." Strategy payoff is, in turn, strongly depen- dent on the kinds of strategies adopted by others in the population and the frequency with which they are employed. This "frequency depen- dence" (Fisher 1930) means that rarer nonconformist strategies (such as illegal expropriation) will often enjoy an advantage over more common legal