(Focus Volume 17:2, Fall/Winter 1995) The Bell Curve (1994), a long, complex, four-part book by the psychologist Richard Herrnstein and the political scientist Charles Murray, argues that intelligence is a highly heritable trait, that it plays a critical role in socioeconomic achievement and social pathology, and that it is increasingly unequally distributed. Furthermore, the authors assert, compensatory interventions will not, indeed cannot halt the consequent trend toward an increasingly stratified society. The controversy that this book immediately generated has tended to greater emotional than analytic content. But as the decibel level begins to drop, it is important that a steady and objective gaze be turned upon the logical and empirical support for the claims made in The Bell Curve. Two recent articles by Institute affiliates address these claims. The Bell Curve: A perspective from economics In a review article in the Journal of Economic Literature,[1] economists Arthur Goldberger and Charles F. Manski turn their attention to the evidence presented in The Bell Curve (see box below)--estimates of models relating behavior to measured IQ, socioeconomic status, and other attributes, based upon data from the National Longitudinal Survey of Youth. As an example of their careful and closely reasoned analysis, Focus reprints below their conclusion regarding the flaws in the empirical analyses presented in The Bell Curve. Visions and empirical analysis Herrnstein and Murray do not write in the standard language of economic analysis, but we may recast their vision of the American future in these terms. Here is a simple model that seems to us to capture their main assertions: Assume that each individual is endowed with an IQ and with other attributes X. At time t, individual output Y is determined by (IQ, X) through a production function Y = f[t](IQ, X). Assume that individual output and earnings are identical. Then the distribution of earnings at time t is determined by the distribution of (IQ, X) and by the form of ft(*,*). Assume that, holding X fixed, f[t](*, X) is an increasing function of IQ. Also assume that endowments of IQ and X are statistically independent within the population. Then it follows that the economy at time t is characterized by a positive association between IQ and earnings. Now consider the economy at time t + 1. Assume that technological change makes f[t+1](*, X) increase more with IQ than f[t](*, X). Assume that the distribution of X remains unchanged between t and t + 1 but that increased stratification in mating patterns makes the distribution of IQ more dispersed at t + 1. Then the economy at time t + 1 is characterized by a stronger positive association between IQ and earnings than the economy at t. Moreover, the economy at t + 1 is characterized by greater earnings inequality than the economy at t. Finally, assume that there is no feasible intervention that can alter the distribution of IQ at time t + 1 or change the form of the production function f[t+1](*,*). Also assume that it is not possible to use taxes or other means to substantially alter the identity of output and earnings. Then increasing cognitive stratification is inevitable. While we find it relatively easy to interpret Herrnstein and Murray's vision in standard economic terms, we cannot similarly interpret the empirical analysis that Herrnstein and Murray use to support their vision. In Part I, they offer only scattered anecdotes, hypothetical vignettes, and selective citations of serious empirical studies to justify their assertions of increasing demand for intelligence, and increasing assortative mating by intelligence. In Part II they are obsessed with using the National Longitudinal Survey of Youth (NLSY) data to show that IQ is "more important" than socioeconomic status (SES) in determining social behavior. The gist of the empirical analysis in Part III is that, in comparing outcomes across ethnic groups, some findings depend on whether one controls for IQ. Whatever one makes of the NLSY regressions, these regressions offer no meaningful empirical evidence on the dynamic of American society. In Part IV, Herrnstein and Murray revert to the loose style of Part I to make their case that no practical policy instrument can raise cognitive ability. Where they do cite evaluations of policy interventions, they systematically slant their interpretation of the findings in two ways. First, whenever evaluations yield ambiguous results, Herrnstein and Murray take the position that, in the absence of firm empirical evidence for a treatment effect, one should maintain the null hypothesis of no effect. For example, on the effects of nutrition on IQ, their assessment is: "Many studies that seem to be well-conducted variations of the successful ones have failed to demonstrate any effect on IQ at all [p. 393]." "Failed to demonstrate any effect on IQ at all" must mean that, using classical statistical significance tests, these studies do not reject the null hypothesis of no effect. But every empirical researcher should know that "failure to reject" the no-effect hypothesis does not establish its truth. It simply implies that the empirical findings are consistent with the no-effect hypothesis; it may well be that the findings are also consistent with alternative hypotheses that assert substantial treatment effects. Second, Herrnstein and Murray repeatedly discount evaluations that do yield clearcut findings of treatment effects. Consider, for example, the Perry Preschool Project undertaken in Ypsilanti, Michigan, in the early 1960s. Here is what Herrnstein and Murray have to say: Although this intensive attempt to raise intelligence failed to produce lasting IQ gains, the Ypsilanti group believes it has found evidence for a higher likelihood of high school graduation and some post-high school education, higher employment rates and literacy scores, lower arrest rates and fewer years spent in special education classes as a result of the year or two in preschool. The effects are small and some of them fall short of statistical significance. They hardly justify investing billions of dollars in run-of-the-mill Head Start programs. (p. 405) The rhetoric of this assessment is revealing. Herrnstein and Murray begin by portraying the Perry Preschool Project as a failed attempt to raise IQ. In fact, the investigators have long been concerned with the effect of the intervention on a range of social behaviors, such as those that occupied Herrnstein and Murray in Parts I and II. They conclude by stating that the findings of the Perry Preschool Project do not justify "investing billions of dollars in run-of-the-mill Head Start programs," when they should instead ask what the findings reveal about the benefits of intensive interventions. En route, they belittle the Perry Preschool findings by saying that the group of investigators "believes it has found evidence" for various outcomes and by saying that "the effects are small and some of them fall short of statistical significance." We find it difficult to reconcile these statements with the well-known findings of the project. For example, the Ypsilanti investigators have reported that 67 percent of the treatment group and 49 percent of the control group were high school graduates by age 19.[2] This effect is neither small nor statistically insignificant by conventional criteria. We conclude that The Bell Curve is driven by advocacy for Herrnstein and Murray's vision, not by serious empirical analysis. America may or may not be on the way toward a custodial state. Policy interventions may or may not be effective. We know no more after studying The Bell Curve than we did before. # --------------------------------------------------------------- 1. Arthur S. Goldberger and Charles Manski, "Review Article: The Bell Curve by Herrnstein and Murray," Journal of Economic Literature 33 (June 1995):762-76. (Available as IRP Reprint no. 725.) IRP is grateful to the American Economic Association for permission to reproduce pp. 774-75 here and to place the entire article on the IRP World Wide Web site (for information on accessing the site, see p. 22). 2. John Berrueta Clement et al., Changed Lives: The Effects of the Perry Preschool Program on Youths through Age 19 (Ypsilanti, Mich.: High/Scope Press, 1984).