Reifman: REVISITING THE BELL CURVE
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psyc.00.11.099.bell-curve.1.reifman Sun Oct 22 2000
ISSN 1055-0143 (61 para, 31 refs, 812 lines)
PSYCOLOQUY is sponsored by the American Psychological Association (APA)
Copyright 2000 Alan Reifman
REVISITING THE BELL CURVE
Target Article by Reifman on Bell-Curve
Alan Reifman
Department of Human Development and Family Studies
College of Human Sciences
Texas Tech University
Lubbock TX 79409-1162
USA
http://www.hs.ttu.edu/hdfs/faculty/reifman.htm
[EMAIL PROTECTED]
ABSTRACT: Charles Murray, one of the authors of The Bell Curve
(Herrnstein & Murray, 1994), predicted that, even with further
scholarly inquiry into the issues raised by the book, none of its
conclusions would be overturned. Now, roughly five years after the
publication of The Bell Curve, this target article reviews
pertinent research published during the intervening time to assess
Murray's prediction. Three primary areas are reviewed: the genetic
contribution to intelligence, the relative contributions of
intelligence and social factors to success in life, and the
potential of educational experience to improve cognitive ability.
The issue of genes and racial/ethnic differences in IQ is also
examined. It is concluded that, contrary to Murray's prediction,
many of The Bell Curve's arguments have been weakened.
KEYWORDS: IQ, adoption studies, behavior genetics, bell curve,
crime, education, intelligence, nature/nurture, poverty, twin
studies, uterine environment.
I. INTRODUCTION
1. In an afterword to the paperback edition of The Bell Curve
(Herrnstein & Murray, 1994), one of its authors, Charles Murray, makes
a bold prediction (the other author, Richard Herrnstein, died shortly
before the initial release of the book in 1994). Murray states that:
"When the Sturm und Drang has subsided, nothing important in The
Bell Curve will have been overturned. I say this not because
Herrnstein and I were so brilliant or farsighted but because our
conclusions were so conservatively phrased and anchored so firmly
in the middle of the scientific road." (p. 556).
Furthermore, Murray suggested that anyone who attempted to refute the
book's claims through further examination of relevant data would be
embarrassed when the facts ultimately supported Herrnstein and Murray's
(1994) thesis. Needless to say, a number of scholars were not
deterred.
2. Roughly five years have now elapsed since the publication of The
Bell Curve (TBC). Not only has the furor surrounding this book
apparently subsided, but many new scholarly studies have come out in
the intervening period. Some of these new studies were launched
specifically to investigate claims and suggestions made in TBC. Others
were perhaps underway before the publication of TBC, but could easily
be used to address points raised by it. Further, unlike the initial
wave of reviews of the book that had to rely on logical arguments
and/or existing data, commentators today can make use of this new
knowledge.
3. The goal of this target article is to examine some of the most
important published scientific research of the last five years to see
how well TBC stands up. Whether or not its major ideas deserve to be
overturned is, admittedly, a matter of subjective opinion. However, it
is the opinion of the present author that three major pillars of TBC -
the genetic contribution to IQ, the importance of IQ (and lack of
importance of other factors) for predicting life outcomes, and the
relative fixedness of cognitive ability -- have sustained major damage
during the past five years. The claims of TBC would at least appear to
need some trimming back, if not a more substantial reclamation
project. The topic of racial-ethnic test-score differences, given its
prominence in scholarly and public reactions to TBC, is also discussed
toward the end of this article.
4. Chabris (1998) published an article entitled "IQ Since 'The Bell
Curve'" in the August 1998 issue of Commentary, with responses from
nearly 20 individuals and a final rebuttal by Chabris appearing in the
November 1998 issue of this journal. Chabris concentrated primarily on
the several post-TBC books available at the time his article was
published, hence there is not much overlap with the studies the present
article reviews below.
5. Also, Chabris (1998) focused heavily on the concept of a general
intelligence ("g") factor, which he deems "as sound and as meaningful
as ever" (p. 39). The present article accepts, for the sake of
argument, the use of a unitary measure of intelligence or cognitive
ability. Not only does such an assumption simplify things, but also
recent evidence showing correlations between psychometric intelligence
and physiological measures (e.g., Jensen, 1998; Tan, 1999) seems
consistent with the idea of a g factor.
6. One can, however, acknowledge the existence of g (or a g-like
construct) while noting that other types of "intelligence" might still
be able to show incremental predictiveness of real-world outcomes.
Sternberg, Wagner, Williams, and Horvath (1995), for example, showed
that even with IQ included in a regression model, practical
intelligence (tacit knowledge) accounted for significant additional
variance in the criterion measure of managerial performance.
7. For simplicity's sake, the present review accepts the soundness of
the data set used by Herrnstein and Murray (1994), the National
Longitudinal Survey of Youth (NLSY), and assumes that most of the
correlations demonstrated by Herrnstein and Murray reflect actual
causation by intelligence, even though critics have raised objections
to these aspects of TBC. For example, Fischer et al. (1996) develop an
extensive argument for why the measure of intelligence in the NLSY, the
Armed Forces Qualifying Test (AFQT), is really a test of schooling.
Were these other critiques to be developed further and added to the
present review, the case against TBC would be eben stronger.
8. A comprehensive review of even five years' worth of research on
intelligence, genes, and the environment would be a massive
undertaking, well beyond the scope of the present essay. Of necessity,
some selectivity must be exercised. In order to ensure some degree of
balance in this review, however, a few steps were taken. First, a
computerized literature search was done, using "Bell Curve" as a key
term. Although relevant research could exist that does not mention TBC
prominently enough to be detected, this approach does help keep the
task more manageable and should pick up on most of the directly
applicable research. Articles were selected for discussion in the
present review to the extent that they appeared relevant; readers are
welcome to conduct a search with the same (or similar) key words to see
what I have omitted. As a way of tapping into additional research
beyond the computerized search, two major recent books were consulted,
Jensen's (1998) "The g Factor" [see also the ongoing Mutiple Book
Review in psycoloquy, Jensen 1999] and Mackintosh's (1998) "IQ and
Human Intelligence." Further, articles from authors whose views would
appear to be congenial to TBC were also consulted (Chabris, 1998;
Gottfredson, 1998).
9. Lastly, some may question the appropriateness of examining a
particular five-year period of research, rather than seeking a longer-
term perspective, unbounded by arbitrary cut-off dates. In general,
this may be prudent advice. In this case, however, Murray's challenge
does appear to call for an examination of research within a definite
post-TBC period.
II. GENETIC CONTRIBUTION TO IQ
10. Two lines of recent research suggest that the genetic contribution
to intelligence is lower than many previous estimates have suggested.
Before delving further into this area, a brief clarification is in
order. It must be remembered that heritability is a population-based
statistic, referring to the proportion of phenotypic IQ variance among
individuals attributable to genes (Plomin & Petrill, 1997). Although
some may object to the term "genetic contribution," it is used as a
simplifying shorthand.
11. Herrnstein and Murray (1994) worked from the assumption that "the
genetic component of IQ is unlikely to be smaller than 40 percent or
higher than 80 percent" (p. 105). Some research emerging over the last
five years tends to suggest that the genetic contribution to
intelligence is closer to the low end of Herrnstein and Murray's
range.
12. One important development that has gained increasing attention
within the last five years is the role of environmental womb effects
(EWE) on human development. These would include fetal exposure to
nutrients, toxins, and hormones. One of the ways in which genetic
contributions to intelligence are measured is by looking at the
similarity of IQ in identical twins (who share all their genes) who
have been reared apart. Herrnstein and Murray (1994) recognized that
such twins would also share a womb environment, and Gardner (1995)
discussed development in utero in his review of TBC. However, it was
apparently not until an article by Devlin, Daniels, and Roeder (1997)
that EWE were quantified and presented to a wide scientific audience.
(The topic of EWE has now reached an even broader audience, as the
cover story of the September 27, 1999 Newsweek reports on how one's
risk for cancer and heart disease can be affected in the womb; Begley,
1999.)
13. Using elaborate statistical models to synthesize over 200 previous
studies, Devlin and colleagues (1997) showed that the presence of
substantial EWE results in a downward revision of the genetic
contribution. They concluded that:
"broad-sense heritability, which measures the total effects of
genes on IQ, is perhaps 48%; narrow-sense heritability, the
relevant quantity for evolutionary arguments because it measures
the additive effects of genes, is about 34%" (p. 470).
In an accompanying editorial, McGue (1997) stated that the narrow-sense
percentage was "certainly too low to support the establishment of a
high-IQ caste" (p. 417). Further, he added that,
"by the third or fourth generation, descendants of gifted
individuals are not much more likely to be gifted than are
descendants of ordinary people."
14. As with any research, cautions must apply to the findings of Devlin
et al. (1997). They themselves state that, "Our statistical analyses
cannot, of course, be considered definitive" (p. 470). Among the
possible complicating factors they note are age affecting IQ
heritability (discussed further below), and unmodeled effects of
cultural inheritance and gene-environment interactions. McGue's (1997)
adjoining commentary also notes that direct measures of pre- and
perinatal factors have shown only weak associations with IQ.
(Interestingly, small effect sizes also appear to prevail in studies
attempting to link specific genes to IQ [Plomin & Petrill, 1997] and in
ones attempting to link specific measures of nonshared environment to
nonshared variance in various outcomes [Turkheimer & Waldron, 2000].)
Additional information on hormones and brain development can be
obtained from Tan, Pence, and Tan (1998).
15. The second line of research suggesting lesser genetic effects than
previously thought involves a different paradigm: adoption studies.
Stoolmiller (1999) argues that because the types of homes into which
individuals are adopted represent a relatively narrow stratum of
society (tending to be white, older, and of higher socioeconomic
status), the effects of shared family environment would be biased
downward and the effects of genes biased upward. Stoolmiller suggests
that:
"Corrections for range restriction applied to IQ data from recent
adoption studies indicate that if adoptive families were
representative of the U.S. census, [shared environment] could
account for as much as 50% of the total phenotypic variance" (p.
405).
16. More specifically, what Stoolmiller (1999) did first was to review
the literature to evaluate the range of home environments that exists
in adoptive families, relative to the full population. By looking at
studies' descriptive statistics on home environment measures, he could
compute each study's variance ratio, "the ratio of the variance in the
adoptive sample compared with the normative variance on the measure of
interest" (p. 395). These variance ratios tended to be around
one-third; alternatively, one could say that variance restriction or
bias was 67%.
17. Applying this empirical estimate of home environment variance
restriction, Stoolmiller (1999) yielded a formula (Equation 9) into
which the IQ correlation for unrelated adoptive siblings could be
inserted to yield an estimate of the proportion of variance due to
shared environment.
18. Skeptics of shared environmental influences might counter with the
finding that adopted siblings, by the time they reach adolescence, show
zero correlation in IQ. Gottfredson (1998) and Plomin and Petrill
(1997) cite this finding as one of the most noteworthy developments in
IQ research in recent years.
19. Two responses can be given. First, although most studies do appear
to show zero correlation for this type of design, not all do
(Mackintosh, 1998; see pp. 83-84). Specifically, Scarr, Weinberg, and
Waldman (1993) note that in their research, "The correlation for
genetically unrelated siblings at adolescence (r = .19, p = .025) was
higher than IQ correlations of zero among genetically unrelated
adolescent siblings previously reported in the literature" (p. 550).
20. Second, as Stoolmiller (1999) notes, "The observed correlations may
be zero or even slightly negative precisely because of range
restriction" (p. 406). Further, Stoolmiller points out that in two
longitudinal sibling adoption studies testing the idea that the
importance of shared environment for IQ declines from childhood to
adolescence, the amount of change in shared environmental influence was
insignificant, though in the predicted direction.
21. Stoolmiller (1999) does acknowledge that his findings depend on a
number of assumptions, including one that there is no restriction to
genetic variance in adoption samples. Ultimately, Stoolmiller notes, "I
recommend treating my corrected estimates of [shared environment] as
upper limits until more definitive data are available" (p. 406).
22. As noted above, an important potential phenomenon is age affecting
IQ heritability. Gottfredson (1998) and Plomin and Petrill (1997) both
cite as one of the most noteworthy findings of recent years the
apparent discovery that IQ heritability rises with age. However, for
whatever reason, commentators have offered a wide variety of
characterizations of this finding.
23. Gottfredson (1998) refers to it as "In hindsight, perhaps... no
surprise. Young children have the circumstances of their lives imposed
on them by parents, schools and other agents of society, but as people
get older they become more independent and tend to seek out the life
niches that are most congenial to their genetic proclivities" (p. 27).
24. Plomin and Petrill (1997), in contrast, deem the finding of greater
genetic influence on IQ later in life "counterintuitive", as "We would
expect that environmental factors rather than genetic factors become
increasingly important, for example, as accidents and illnesses
accumulate" (p. 60); however these authors also acknowledge the
possibility of a scenario similar to the one suggested by Gottfredson.
25. Devlin et al. (1997) and McGue (1997) appear more skeptical of
whether age differences in IQ heritability truly exist. Devlin et al.
write that "the large direct estimates derived from studies of adult
twins raised apart have been explained by the conjecture that, as twins
get older, IQ becomes more heritable. Evidence has been provided to
support this 'age hypothesis', although the results are inconclusive
and relationship unclear" (p. 470). Further, Devlin et al. estimated
one of their models to include age effects, and found that "age-effects
models fail to fit the data better than a simpler model that invokes
maternal effects" (p. 470). McGue (1997) elaborates on some similar
issues.
26. Clearly, much remains to be resolved in discovering the
determinants of human behavior and its development. Turkheimer and
Waldron (2000), who, as noted above, focused on nonshared environment,
closed their article on the following note:
"some aspects of the development of complex human behavior may
remain outside the domain of systematic scientific investigation
for a very long time. Although developmentalists may be
disappointed that a substantial portion of human development
remains too complex, too interactive, and too resistant to
controlled investigation and straightforward statistical methods to
yield to systematic scientific analysis as we currently understand
it, it must be remembered that the alternative -- a world in which
human behavior could be understood all the way down in terms of
correlations between difference scores -- would present its own
gloomy prospects in the ethical evaluation of human agency" (p.
93).
27. Acknowledging all the aforementioned complexities, a recent major
review (Plomin and Petrill, 1997) nonetheless concludes that about half
of the IQ variance is due to genes, thus corroborating the studies
reviewed above. Plomin and Petrill write that, "The reason for hoping
that the pendulum is coming to rest at a point in between nature and
nurture is not merely that we want everyone to be happy. It is what
genetic research on intelligence tells us" (p. 55).
28. Mackintosh's (1998, Chapter 3) review of the heritability of IQ
makes several points clear about this literature: many of the
calculation models rest on assumptions that are at least questionable;
often there are only five or fewer studies addressing a particular
issue; and estimates of heritability from different approaches do not
necessarily converge well. Mackintosh notes that many scholars have
accepted the .50 estimate of heritability, although some believe it is
higher. His own judgment is that: "The more reasonable conclusion... is
that the broad heritability of IQ in modern industrialized societies is
probably somewhere between 0.30 and 0.75, and that neither the data nor
the models justify much greater precision" (p. 93).
29. Thus, with many of the above scholars converging on a heritability
estimate of .50, it appears that the genetic contribution to IQ is on
the low end of the range given by Herrnstein and Murray (1994), with
the kinds of social consequences they suggest also being less likely.
30. Many Bell Curve critics may be pleased to see that some recent
estimates of heritability are toward the low end of Herrnstein and
Murray's (1994) range. Mackintosh (1998) cautions against having such
an automatic reaction, when he writes:
"Other things equal, in a society where the heritability of IQ is
low, this must be because that society permits significant
differences in those environmental circumstances that affect IQ...
In a society where many children are severely malnourished and live
in grossly overcrowded slums, ravaged by infectious diseases; where
relatively few children receive any formal education at all...
there can be little doubt that differences in IQ scores will be
partly a result of these environmental differences... A low
heritability for IQ could be regarded as a mark of an unjust
society" (pp. 66-67).
III. IMPORTANCE OF IQ AND OTHER FACTORS IN DETERMINING LIFE OUTCOMES
31. Comparing TBC's contentions about the importance of IQ in
determining life outcomes to what newer analyses reveal is made
somewhat difficult by the fact that observers exhibit a fair amount of
disagreement over exactly what Herrnstein and Murray (1994) were
saying. For example, Mackintosh (1998) writes that, "In their less
guarded moments, Herrnstein and Murray (1994) give the impression that
they believe the social consequences of differences in IQ to be of
almost boundless significance" (pp. 377-378). At the other extreme,
Gottfredson (1997a) characterizes Herrnstein and Murray's conclusion as
simply being: "that (a) phenotypic intelligence has a greater impact
than (b) parental social status (education and income) on whether (c)
adults are poor" (p. 743). Fortunately, because some of the newer
studies reanalyzed the same data set as was used in TBC, statistical
results from the different research teams can be compared directly.
32. Even by the time TBC first came out, commentators had seized upon
the fact that the statistical relationships reported by Herrnstein and
Murray (1994) between IQ and outcomes such as poverty and crime were
small or modest in magnitude (e.g., Gould, 1994). Further, two flaws
were cited by critics at the time. The first was that a very weak
measure of SES was used. The other was that in testing the role of IQ
as a potential determinant of poverty, crime, and other outcomes (using
multiple regression), Herrnstein and Murray omitted many other
potential determinants. By commiting the latter faux pas (known as
"specification error"), they were likely to have given a misleadingly
inflated picture of the potency of IQ.
33. Two teams of scholars -- Claude Fischer and colleagues (1996) from
Berkeley who focused on predicting poverty, and Francis Cullen and
colleagues from various universities who focused on crime outcomes
(Cullen, Gendreau, Jarjoura, & Wright, 1997) -- set out to rectify
matters in their respective areas using the same data source as was
used by Herrnstein and Murray, the NLSY.
34. Fischer et al. (1996), in their book Inequality By Design,
recalculated TBC's equation predicting the probability of being poor.
They found that when a fuller set of potential determinants was
included (whether someone was reared on a farm, came from a two-parent
family, had been in an academic track in school, etc.), the social and
environmental factors were more powerful predictors than IQ.
35. Cullen and colleagues (1997) noted that:
"In a normal scientific approach, Herrnstein and Murray would have
first identified the known predictors of crime and then sought to
demonstrate that IQ could explain variation above and beyond these
criminogenic risk factors. These factors are identified in the
readily available literature including Herrnstein's own [prior]
work... By limiting their analysis primarily to three factors --
IQ, SES, and age -- they risk misspecifying their model and
inflating the effects of IQ" (p. 393).
Reanalyses including additional predictors such as religious
involvement and academic aspirations indeed showed IQ to have less of
an impact on criminal behavior than did the original TBC analyses.
36. A related assertion by Herrnstein and Murray (1994) was that not
only is IQ an important determinant of life outcomes now, but that
during the course of the 20th century, it has been playing an
increasing role in distinguishing the successful from the unsuccessful.
Hauser and Huang (1997) put this idea to the test. They first
scrutinized the bases of TBC's historical analyses and found many of
Herrnstein and Murray's conclusions to be questionable.
37. Hauser and Huang (1997) then conducted their own statistical
analyses, using existing data from the General Social Survey, a more-
or-less annual study conducted by the University of Chicago's National
Opinion Research Center (apparently for the purpose of increasing the
sample sizes, the data sets were combined to form three period
groupings: 1974-1982, 1984-1989, and 1990-1994) . The GSS protocol for
these years contained a short verbal ability test, as well as extensive
socioeconomic measures. This allowed Hauser and Huang to test whether
the impact of cognitive ability on earnings would show an increase from
the earliest time period to the latest, as would be predicted by TBC.
No such trends were found. In other words, the value given to cognitive
ability in the marketplace was no greater in the 1990s than in the
1970s.
38. Although multiple-regression analyses of the relative predictive
potency of IQ and social-environmental factors -- the focal point of
many of the above studies -- can appear dry and esoteric, Ceci and
Williams (1997) illustrate the practical implications behind such
statistics. These authors addressed the relative contribution of
schooling and "raw" intelligence to future earnings, an area made
complicated by the reciprocal nature of schooling and intelligence
(i.e., schooling can increase one's IQ, but higher IQ can lead a person
to stay in school longer).
39. Herrnstein and Murray (1994) pondered this issue in the following
quote: "If somehow the government can cajole or entice youths to stay
in school for a few extra years, will their economic disadvantage in
the new labor market go away? We doubt it. Their disadvantage might be
diminished, but only modestly. There is reason to think that the job
market has been rewarding not just education but intelligence" (p.
96).
40. While findings in the Ceci and Williams (1997) article do show
cognitive ability to predict earnings even when education is
statistically held constant, education also predicts earnings when
cognitive ability is held constant. One of the more interesting
findings was that individuals in the next-to-last quintile of cognitive
ability who still managed to complete a four-year college degree earned
more on average ($466 a week) than those in the top quintile of
cognitive ability who completed only high school ($447).
41. Whereas the above paragraphs focus on apparent limitations of IQ in
predicting real-world outcomes, there is also research that supports
its role. Gottfredson (1997b), for example, has argued for the
importance of measured intelligence to performance in real-world tasks.
She reviews a great deal of validation evidence for IQ, focusing
heavily on work-related outcomes, and covering some of the same
territory as Herrnstein and Murray (1994). One interesting section of
her review summarizes findings from the National Adult Literacy Survey
(NALS). The NALS measured a number of "everyday" skills, such as
locating an intersection on a street map and calculating postage and
fees for certified mail. Gottfredson reports that a general literacy
factor correlates .8 with the "academic G" factor from the GED exam (a
test in the United States for a high school equivalency credential).
IV. MALLEABILITY OF COGNITIVE ABILITY
42. Before discussing recent research on the degree to which cognitive
abilities can be raised through educational intervention, it is useful
to review a distinction from Jensen (1998) between skills, which are
conceived of as specific responses in specific situations developed
through practice, and intelligence (g), which is conceived of as a more
pervasive ability that permits "far transfer" of skills obtained in one
setting to other, relatively different settings.
43. Jensen (1998) asks and answers a question about attempts to boost
cognitive abilities:
"Aside from whether or not [participants'] level of g has been
altered by the treatment, was any knowledge or set of skills
inculcated that has practical utility for the treated persons in
'real life'? Skill training, the acquisition of a useful or
employable skill, or of beneficial habits in the conduct of one's
life, is valuable in its own right, regardless of any general
carry-over to g" (p. 334).
44. Accordingly, the present review examines the impact of educational
intervention both on what might be referred to as "skills" and on what
might be referred to as "g". Although some researchers might not
accept such a distinction, it is clear that, in some instances at least,
education can boost cognitive abilities of some sort.
45. There appears to be a broad consensus that preschool Head Start-
type programs -- as have been implemented in the past -- do not produce
lasting improvements in cognitive ability, unless they are extremely
intensive. One such intensive program, however, is the Abecedarian
Project, the results of which showed a five-point IQ difference in
favor of the treatment group at age 15. Writes Jensen (1998):
"Judging from a comparable gain in scholastic achievement, the
effect had broad transfer, suggesting that it probably raised the
level of g to some extent" (p. 344).
Nonetheless, typical early interventions appear to have more modest
effects. The present review, therefore, focuses on how education may
affect the cognitive abilities of older children (late elementary
school and beyond) and young adults.
46. Herrnstein and Murray (1994) presented a picture of cognitive
ability as largely unchangeable through education. Among their
conclusions, they state the following: "Formal schooling offers little
hope of narrowing cognitive inequality on a large scale in developed
countries, because so much of its potential contribution has already
been realized with the advent of universal twelve-year systems" (p.
389).
47. Later, when talking about affirmative action in higher education,
Herrnstein and Murray raise a series of questions that again implies a
static model of intelligence: "How much harm is done to minority self-
esteem, to white perceptions of minorities, and ultimately to ethnic
relations by a system that puts academically less able minority
students side by side with students who are more able?" (p. 470). That
minority students might gain in abilities during college and achieve
positive outcomes in the areas mentioned above was apparently not
considered by Herrnstein and Murray.
48. Like some of the other researchers cited above, Myerson, Rank,
Raines, and Schnitzler (1998) went back to the same NLSY data set used
by Herrnstein and Murray to address an aspect of the malleability
question. These investigators found results suggesting that, among
individuals in the NLSY who ultimately graduated college, black
students' scores on the Armed Forces Qualification Test (the same
cognitive ability test used in TBC, and which Herrnstein and Murray
described as "highly loaded on g", p. 73) rose sharply over the four
years of college, in comparison to a relatively flat line for whites,
resulting in a substantial narrowing of the race difference upon
college graduation. This shows the apparent error in Herrnstein and
Murray's characterization of cognitive ability as a relatively static
entity.
49. Perkins and Grotzer (1997) review a variety of programs designed to
improve thinking skills (which they acknowledge is not a comprehensive
list of such programs), giving proper attention to both the persistence
and transferability of the taught skills. In some cases, Perkins and
Grotzer note that data on persistence are lacking. Other programs do
appear, however, to have both persistence and transferability. One
example is Adey and Shayer's (1993) Cognitive Acceleration through
Science Education (CASE) program, conducted with 11-12 year-olds. This
program taught scientific/methodological reasoning during a two-year
intervention. Some program effects were present at a two-year
follow-up, with transfer even to subjects such as English.
50. A more extensive detailing of evidence to the effect that schooling
increases intelligence is available in the aforementioned Ceci and
Williams (1997) article. Again, the evidence appears to contradict the
thesis of TBC. As we have seen throughout this article, environmental
exposures from the womb to college appear to affect individuals'
cognitive abilities. Further, although cognitive ability does not
appear to have the all-dominant impact on individuals' life success
suggested by TBC, it certainly does play a role, arguing that efforts
to improve cognitive abilities should be undertaken.
V. RACIAL-ETHNIC TEST SCORE DIFFERENCES
51. An area of TBC that generated as much or more reaction and debate
than other parts of the book is that of racial-ethnic test score
differences. A particularly contentious aspect of this debate is the
degree (if any) to which black-white test score differences are
genetically based.
52. Perhaps the most significant piece of work to come out in the past
five years on the general topic of group differences is The Black-
White Test Score Gap, an edited volume by Jencks and Phillips (1998a).
In the introductory chapter, Jencks and Phillips (1998b) characterize
the evidence on genes and the test score gap in the following
quotations:
"despite endless speculation, no one has found genetic evidence
indicating that blacks have less innate intellectual ability than
whites" (p. 2).
"we find it hard to see how anyone reading these studies with an
open mind could conclude that innate ability played a large role in
the black-white gap" (p. 20).
53. Although a review of all chapters of the Jencks-Phillips book is
well beyond the scope of the present article, a couple that cover
recent theories of the test score gap will be mentioned. In one, Steele
and Aronson (1998) present evidence in support of their "stereotype
threat" model. In another, Cook and Ludwig (1998) present evidence that
tends to go against the "acting white" theory. The latter authors write
that: "While ethnographers observe that black adolescents sometimes
taunt high-achieving black students for acting white, it appears that
either these taunts do not inflict especially grievous social damage or
high achievement has offsetting social benefits" (p. 391).
54. Fischer et al. (1996), in their own book, propound a social caste
theory of test score differences. Building upon the work of other
social scientists, such as John Ogbu, Fischer and colleagues focus on
findings that, across the world, "ethnic groups that are inferior in
status and caste position score worse on achievement and 'intelligence'
tests" (p. 191; see especially their Table 8.1). Fischer et al.
continue that, "A reading of the table shows that ethnicity or race,
understood biologically, cannot be the cause of the test-score
differences. Particularly striking are the substantial gaps in test
scores between groups of the same ethnicity or race in countries like
Israel, Japan, and South Africa" (p. 191). (In South Africa, the
comparison is between high status individuals of English origin and
their low status counterparts of Dutch origin.)
55. Fischer and colleagues (1996) propose that low ethnic caste or
status position leads to low test scores through three mediators:
socioeconomic deprivation, group segregation, and stigma of
inferiority. Among the processes at work under the rubric of "stigma of
inferiority", according to Fischer et al., are those of Steele's
stereotype threat model.
56. Jensen (1998) has offered critiques of both stereotype threat and
social caste theories. Regarding the former, Jensen claims that
stereotype threat findings can be explained in terms of more basic
test-anxiety principles. Regarding the latter, Jensen raises a number
of objections to social caste theory, including the fact that the
amount of psychometric evidence in some countries is relatively small
(Fischer et al., 1996, themselves acknowledge similar limitations).
57. Jensen's (1998) preferred position is what he refers to as the
"default hypothesis". This states that "human individual differences
and population differences in heritable behavioral capacities, as
products of the evolutionary process in the distant past, are
essentially composed of the same stuff, so to speak, controlled by
differences in allele frequencies, and that differences in allele
frequencies between populations exist for all heritable
characteristics, physical or behavioral, in which we find individual
differences within populations" (p. 444).
58. Jensen (1998, p. 447) provides a theoretical equation for
calculating between-group heritability from within-group heritability
and other parameters, but notes that it is "not empirically applicable,
because a single equation containing two unknowns... cannot be solved"
(p. 448). Jensen then goes on to review numerous calculations and
studies, which lead him to the following conclusion:
"[The default] hypothesis is consistent with a preponderance of
psychometric, behavior-genetic, and evolutionary lines of
evidence. And like true scientific hypotheses generally, it
continually invites empirical refutation" (p. 515).
VI. CONCLUSIONS
59. Herrnstein and Murray (1994) essentially challenged social
scientists who disagreed with TBC's conclusions to produce scientific
results showing otherwise. Over the last five years, various scholars
have done just that, as reviewed above. Just as an inoculation at the
doctor's office leads the body to build resistance, TBC's claims that
intelligence is predominantly due to genetic factors and largely
unmodifiable appear to have stimulated a number of researchers to build
up a body of scholarship reaching opposite conclusions.
60. Importantly, one could argue that the basis for supporting
environmental enrichments in areas such as education and nutrition
appears to be stronger now than when The Bell Curve was published.
Although the new research studies on cognitive benefits of schooling
and on environmental womb effects have their own limitations (as with
any research), they do present an optimistic picture about
environmental enrichment. At the same time, as argued above, some of
TBC's claims appear to need at least some trimming back.
61. Chabris (1998), reflecting upon the writings of some of TBC's
critics and upon Murray's responses, wrote the following:
"The conclusion one may reasonably draw... is that while
intelligence may matter more or less than family background, it
certainly matters, and that if it is not entirely heritable, it is
heritable in some degree" (p. 35).
Such a statement may have the potential to allow (at least some) Bell
Curve supporters and detractors to find common ground.
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