T he weight of evidence clearly suggests that, based on what economics students say and how they play games, those who study economics appear to be less cooperative than those who do not. In a recent issue of this journal, Frank, Gilovich and Regan (1993) compile the evidence for this view. Using the results of others and new studies of their own, they show that economics students are more likely to free ride or defect from coalitions. In addition, they present survey results that indicate lowered cooperation by students after exposure to the principles of economics course and other surveys that find that professional economists report less charitable giving.This evidence is consistent with the proposition that studying economics alters how students play structured games and answer surveys about cooperativeness. However, we disagree with the additional conclusion that, as Frank, Gilovich and Regan (1993, p. 159) put it in their article, "exposure to the self-interest model commonly used in economics alters the extent to which people behave in self-interested ways." In fact, the evidence in this paper implies that even if undergraduate students of economics display uncooperative behavior in specialized games or surveys, their "real-world" behavior is actually substantially more cooperative than that of their counterparts studying other subjects. Analytical IssuesDrawing a connection between the study of economics and changes in cooperative behavior isn't easy. Here, we raise four difficulties in any such analysis.

In this paper we present a model in which individuals act in their own best interest, to explain many behaviors associated with cigarette addiction. There are two key features of the model. First, there is an explicit representation of the withdrawal effects experienced when smokers attempt to quit smoking. Second, there is explicit recognition that the negative effects of smoking generally appear late in an individual's life. Among the things Ž . we use the model to explain are: 1 how individuals can become trapped in their decision Ž . to smoke; 2 the conditions under which cold-turkey quitting and gradual quitting may Ž . occur; and 3 a reason for the existence of quit-smoking treatments. q 1999 Elsevier Science B.V. All rights reserved.JEL classification: I10; D11; D91

Vaccination provides indirect benefits to the unvaccinated. Despite its important policy implications, there is little analytical or empirical work to quantify this externality, nor is it incorporated in a number of cost-benefit studies of vaccine programs. We use a standard epidemiological model to analyze how the magnitude of this externality varies with the number of vaccinations, vaccine efficacy, and disease infectiousness. We also provide empirical estimates using parameters for influenza and mumps epidemics. The pattern of the externality is complex and striking, unlike that suggested in standard treatments. The size of the externality is not necessarily monotonic in the number vaccinated, vaccine efficacy, nor disease infectiousness. Moreover, its magnitude can be remarkably large. In particular, the marginal externality of a vaccination can be greater than one case of illness prevented among the nonvaccinated, so its omission from policy analyses implies serious biases.

Widespread and continuing discussions of nursing shortages frequently involve divergent concepts of shortage that can have differing policy implications. This article explains the shortage concepts used by economists, hospital administrators, and government policy makers. It discusses measurement problems and suggests possible improvements. It then sets forth the divergent policy implications of competing shortage concepts. The article's aim is to promote greater clarity in analyses of nursing shortages and more fruitful conversations among participants who use different notions of shortages.

Economists often try to make plausible inferences from a sizable empirical literature addressing a particular measurement, direction-of-effect, or testing issue. There are serious methodological problems associated with drawing such inferences. This article sets out some of these problems in order to make a case for their importance. After discussing these problems, the paper presents three case study examples of inference difficulties in specific literatures. It then proposes a new hypothesis about the time pattern of publication bias in empirical economics literatures. As support for this hypothesis, it presents evidence that 'reversals in findings' in empirical literatures in economics are not uncommon. Similarities are pointed out between the focus on inference problems in this paper, and the meta-analysis literatures in psychology and medical clinical trials.economic methodology, inference, empirical testing, meta-analysis, publication bias, data mining,