conomists have increasingly turned to the experimental model of the physical sciences as a method to understand human behavior. Peerreviewed articles using the methodology of experimental economics were almost nonexistent until the mid-1960s and surpassed 50 annually for the first time in 1982; and by 1998, the number of experimental papers published per year exceeded 200 (Holt, 2006). Lab experiments allow the investigator to influence the set of prices, budget sets, information sets, and actions available to actors, and thus measure the impact of these factors on behavior within the context of the laboratory. The allure of the laboratory experimental method in economics is that, in principle, it provides ceteris paribus observations of individual economic agents, which are otherwise difficult to obtain. A critical assumption underlying the interpretation of data from many laboratory experiments is that the insights gained in the lab can be extrapolated to the world beyond, a principle we denote as generalizability. For physical laws and processes like gravity, photosynthesis, and mitosis, the evidence supports the idea that what happens in the lab is equally valid in the broader world. The American astronomer Harlow Shapley (1964, p. 43), for instance, noted that "as far as we can tell, the same physical laws prevail everywhere." In this manner, astronomers are able to infer the quantity of certain gases in the Sunflower galaxy, for example, from observations of signature wavelengths of light emitted from that galaxy.

Crime dropped sharply and unexpectedly in the United States in the 1990s. I conclude that four factors collectively explain the entire drop in crime: increases in the number of police, increases in the size of the prison population, the waning of the crack epidemic, and the legalization of abortion in the 1970s. Other common explanations for declining crime appear far less important. The factors identified are much less successful in explaining fluctuations in crime in the preceding two decades. The real puzzle is not why crime fell in the 1990s, but rather, why crime did not begin falling earlier.

A critical question facing experimental economists is whether behavior inside the laboratory is a good indicator of behavior outside the laboratory. To address that question, we build a model in which the choices that individuals make depend not just on financial implications, but also on the nature and extent of scrutiny by others, the particular context in which a decision is embedded, and the manner in which participants and tasks are selected. We present empirical evidence demonstrating the importance of these various factors. To the extent that lab and naturally occurring environments systematically differ on any of these dimensions, the results obtained inside and outside the lab need not correspond. Focusing on experiments designed to measure social preferences, we discuss the extent to which the existing laboratory results generalize to naturally-occurring markets. We summarize cases where the lab may understate the importance of social preferences as well as instances in which the lab might exaggerate their importance. We conclude by emphasizing the importance of interpreting laboratory and field data through the lens of theory.

In previous research, a substantial gap in test scores between white and black students persists, even after controlling for a wide range of observable characteristics. Using a newly available data set (the Early Childhood Longitudinal Study), we demonstrate that in stark contrast to earlier studies, the black-white test score gap among incoming kindergartners disappears when we control for a small number of covariates. Real gains by black children in recent cohorts appear to play an important role in explaining the differences between our findings and earlier research. The availability of better covariates also contributes. Over the first two years of school, however, blacks lose substantial ground relative to other races. There is suggestive evidence that differences in school quality may be an important part of the explanation. None of the other hypotheses we test to explain why blacks are losing ground receive any empirical backing. © 2004 President and Fellows of Harvard College and the Massachusetts Institute of Technology.