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We study gender differences in altruism by examining a modi ed dictator game with varying incomes and prices. Our results indicate that the question "which is the fair sex?" has a complicated answer-when altruism is expensive, women are kinder, but when it is cheap, men are more altruistic. That is, we nd that the male and female "demand curves for altruism" cross, and that men are more responsive to price changes. Furthermore, men are more likely to be either perfectly sel sh or perfectly sel ess, whereas women tend to be "equalitarians" who prefer to share evenly.

Experimentally elicited discount rates are frequently higher than what one would infer from market interest rates and seem unreasonable for economic decision-making. Such high rates have often been attributed to present bias and hyperbolic discounting. A commonly recognized bias of standard elicitation techniques is the use of linear preferences for identification. When attempts are made to correct this bias with additional experimental measures, researchers find exceptional degrees of utility function curvature. We present a new methodology for identifying time preferences, both discounting and utility function curvature, from simple allocation decisions. We estimate annual discount rates substantially lower than normally obtained, dynamically consistent discounting, and limited though significant utility function curvature.

Experiments on privately provided public goods generally find that subjects are far more cooperative than predicted, while experiments on oligopolies and the commons almost always obtain the Nash-equilibrium predictions, despite being very similar games. This paper examines whether this difference could be due to the fact that with public goods there is a positive externality, while with the others the externality is negative. The result of the experiments is that subjects are more willing to cooperate when the externality is positive, even though the potential outcomes are the same. This suggests a behavioral asymmetry between the warm-glow of doing something good and cold-prickle of doing something bad. *I would like to thank Paul Brown, Robyn Dawes, and Charles Holt for valuable comments and conversations, and one of the editors and an anonymous referee of this Journal for many helpful suggestions. I am also grateful for financial support from the National Science Foundation and from an Alfred P. Sloan Foundation Research Fellowship. 1. See Davis and Holt [1992] and Ledyard [1994] for summaries of public goods experiments, and see Andreoni [1988a] for a discussion of the puzzle regarding public goods outside the laboratory.

Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Blackwell Publishing and Royal Economic Society are collaborating with JSTOR to digitize, preserve and extend access to The Economic Journal. http://www.jstor.org * We are grateful to Robyn Dawes, Paul Milgrom, John Carter, and two anonymous referees for helpful comments, and to Dan Schneidewend and Soren Hauge for expert programming and research assistance. Andreoni also thanks the National Science Foundation, grant SES 882 I 204, for financial support. Errors are the responsibility of the authors. 1 See also Kreps and Wilson (I982), and Milgrom and Roberts (I982). 2 See, e.g. Palfrey and Rosenthal (i 988), Andreoni (i 989, I 990), and Cooper et al. (I990) for a discussion of this. 3 See Camerer and Weigelt (I988) for a more complete discussion of other experiments that pertain to reputation building. [ 570 ] ' See Roth and Murnigham (I978) and Roth (I988) for reviews of prisoner's dilemma experiments a historical review of prisoner's dilemma experiments in psychology and sociology, see Rapoport Chammah (I965). See Dawes and Thaler (I988) for a recent review and discussion of cooperatic providing public goods. 5 For reviews and discussions of the psychology and sociology literatures on cooperation and altri behaviour, see Dawes (I980) and Piliavin and Charng (I990).

Laboratory researchers in economics assiduously protect the confidentiality of subjects. Why? Presumably because they fear that the social consequences of identifying subjects and their choices would significantly alter the economic incentives of the game. But these may be the same social effects that institutions, like charitable fund-raising, are manipulating to help overcome free riding and to promote economic efficiency. We present an experiment that unmasks subjects in a systematic and controlled way. We show that, as intuition suggests, identifying subjects has significant effects. Surprisingly, we found that two supplemental conditions meant to mimic common fund-raising practices actually had the most dramatic influences on behavior. D