(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).