and complexity: Is IQ ready for genetics?
Harvard Medical School, Boston, MA, USA
The study of
genetic contributions to intelligence has been one of the longest-standing
controversies in modern science. Although discussions over this
issue can be traced back in one form or another to earliest times,
the advances in biology in the 19th century, Darwin ’s theory
of evolution and the re discovery of Mendel’s Laws of inheritance
at the dawn of the 20th century add ed weight of apparent scientific
authority to the claims of the IQists.
Capron et al.
show how the types of evidence considered acceptable with in this
field of research have undergone constant revision. The field has
progressively become more sophisticated in areas such as statistics,
in consideration of the role of environment in the development of
intelligence, and in modifying the earlier more deterministic representations
of genetic influences. It would appear that we have come a long
way from Galton’s 19th century simplistic descriptions of
famous successful families to current efforts to detect quantitative
trait loci in the search for intelligence genes.
it is clear that the debate is still far from over. While most of
those who are involved in research on this issue, largely trained
in psycho logy, assert that there is consensus for a large heritability
(40-80%) of IQ (Neisser et al., 1996), the claim is received with
considerable skepticism in other branches of psychology, among many
geneticists and even by the once unquestioning media.
One of the major
reasons for the failure of IQists to convince is what, to many of
us, appear to be the unwarranted simplifying assumptions that underlie
their research. Yet, parsimony is often considered as a virtue and
even essential in the evolution of new scientific fields. As philosophers
of science such as Richard Boyd put it, “one of the features
of the scientific approach is the methodological preference for
simplicity or parsimony [which] scientists often call elegance (or,
perhaps, beauty)…” (Boyd, 1995). Boyd goes on to point
out that logic al positivists have offered a “pragmatic”
explanation for the focus on parsimony: that it is more rational
and efficient to first investigate the less complex theories.
with strong explanatory power appeal to the scientific community
and can even catalyze revolutionary changes in direction of a science.
Simple theories may eventually be found wanting in their Commentary/Misconceptions
of biometrical IQists -2- ability to explain all relevant phenomena,
and are either modified and made more complex or are overthrown
by novel theories (Kuhn, 1970). In much of science, the new or expanded
theories do not vitiate the successes of their predecessors, but
appear necessary in the face of new information or newly perceived
at least at the outset of a field is no vice, it has to be pointed
out that choices of how to simplify are not unbiased. These choices
reflect assumptions about which are the important facts to consider
and explain in building the theory. So, while parsimony in the evolution
of a scientific field is accepted, the simplifying assumptions offered
or implicit in a field may well doom it to failure. I will argue
that simplifying assumptions have created enormous problems for
the field of IQ and genetics research.
these assumptions have forced a continuous thorough rethinking of
the approaches. The problem persists today and raises questions
about whether, for such a complex issue, parsimony really works.
In this commentary,
I will describe some of the simplifications inherent in IQ studies
over the last century and how they have influenced conclusions.
While Capron and coworkers, themselves, give a number of examples,
I will focus on other aspects of the twin studies, which have provided
the core of the data. Some of these examples will be quite familiar
to readers of this article, but putting several of them together
demonstrates a problematic pattern in this scientific field.
twins and their placement
The idea behind
studies on genetically identical twins that have been separated
at an early age is that they provide an ideal experiment for separating
the contributions of genetics and environment to intelligence. So
the argument goes, identical twins raised in their biological family
experience the same environment, whereas such twins placed in two
different families experience different environments. The differences
in correlation of scores on IQ tests between the two classes of
twins should allow an estimation of the relative contributions of
environment and genetics to intelligence. But, importantly, the
success of this analysis depends on situations in which the separated
twins are placed in truly different environments.
Leon Kam in, recognizing this issue in the 1970's, reexamined the
major studies that had been done up until that time and discovered
that three of the four studies were severely flawed (Kam in, 1974).
The twins had often been placed in homes with very similar environments
(e.g., the homes of relatives of the biological parents). Sir Cyril
Burt, the director of the fourth study, clearly recognized this
problem and reported that his twins subjects were mostly placed
in very different environments (e.g., one of a pair in a wealthy
family and the other in a poor family). However, Burt’s reports
are no longer considered reliable, in part, because the data could
not be recovered after his death, and, in part, because of charges
that there were fraudulent aspects to the studies (MacKintosh, 1995).
critique played a large role in forcing a more sophisticated look
at the environments in which twins were placed. Because of Kamin,
behavior geneticists had to sharpen the ir arguments, design new,
more careful studies, obtain fresh evidence” (MacKintosh ,
1995, p. 142). Th us, in recent years, researchers report attempts
to be more quantitative in their evaluation of home environments
into which adopted twins are placed. In some cases, they quantify
the number of books in the home, parental vocabulary, nutrition,
and other such factors (Rowe, 1994). The study of Thomas Bouchard
and his coworkers evaluated the availability of household facilities
such a s “power tools, sailboat, telescope, unabridged dictionary,
and original artwork”. They also took into account retrospective
impressions of family environment by separated twins (Buchard et
While this new
quantitative app roach might b e considered an advance over the
earlier more simplistic analyses, one can certainly ask whether
it is sufficient. Do w e know what combination of factors both in
the home and outside environment may provide the appropriate mix
for the development of the complex capabilities being measured?
Further, by focusing on those features of the environment that are
easily Commentary/Misconceptions of biometrical IQists
-3- quantifiable, the less tangible features o f such environments
are left out. So, while research has moved from the most simplistic
approaches to these more sophisticated studies, I would argue that
we do not understand enough about the development of intelligence
to be able to measure the factors in the home or cultural environment
that nurture it.
monozygotic and dizygotic twins
where increasing recognition of complexity is confounding issues
in IQ research is the study of twins that are raised within their
biological families. Many of the conclusions concerning inheritance
of intelligence come from comparisons between these monozygotic
and dizygotic twins. A fundamental simplifying assumption underlying
these studies is that the environment in which each member of a
twin pair develops and grows up, from conception on, is essentially
identical (Scan & Carter-Saitzman, 1979). The degree to which
twins are considered to experience the same environments is thought
to be the same for either monozygotic or dizygotic pairs. Thus,
according to the argument, the difference in concordance for any
trait between the two classes of twins can be ascribed to genetics.
This “equal environment assumption” is based on the
fact that such twins develop in the same womb and are raised together
by their biological parents a t the same time in the history o f
the family an d in the history of their society. This assumption
did not encounter serious criticism until the early 1960's (Joseph,
One of the major
problems with this assumption is that the physically identical nature
of monozygotic twins could contribute to making their environments
much more similar than that of dizygotic twins (Billings et al.,
1992). This consequence of genetic identity could result from the
ways in which both family and the larger society treat or deal with
individuals who are know n to be identical and physically present
as identical. In addition, the known closer bonds between monozygotic
twins as compared to dizygotic twins can contribute to greater behavioral
similarity. Studies designed to respond to these criticisms have
yielded conflicting results. The research suffers from the same
problems of defining quantitatively those factors that could be
considered to influence behavioral development in a family and social
setting. Furthermore, contrary to the original simplifying assumptions,
the studies have revealed that there are differences in the shared
environments between the two classes of twins (Joseph, 1998). In
a review of these studies, Joseph claims that, in the case of studies
of inheritance of schizophrenia, “the evidence suggests that
the classical twin method ... is [based] on the fallacy of the equal
environment assumption” (Joseph, 1998).
papers cast further doubt on the equal environment assumption as
it relates to early stages of development. Dr. Bernie Devlin and
co workers showed that conditions with in the womb may have substantial
effects on the concordance of subsequent scores on IQ tests for
identical twins (Devlin et al.,1997). They estimated that including
the effects of maternal environment reduces the estimate of the
heritability of IQ from 60% to 48%. Although the approaches used
in this study are criticized by Capron and colleagues, the appearance
of this work still underscores the un settled nature and wildly
varying estimates of IQ heritability that depend on input assumptions.
In addition, Devlin et al. note that their analyses do not “preclude
other, unmodelled factors, such as cultural inheritance and interaction
between genes and the environment, from having important effects
on IQ”. This is a rather remarkable commentary on the state
of the field. What this comment highlights is that the statistical
analyses used by most researchers in the field have excluded the
more complex aspects of human behavior, presumably because they
are more difficult to model or because they have been dismissed
a priori as unimportant factors.
arise in the evaluation of twin studies from recent findings on
environmental and genetic factors that affect the presumed identity
of identical twins. Drs. Elisabeth Spitz and Michêle Carlier
found that identical twins who develop in a single chorionic sac
in the womb (monochorionic MZ twins) show different concordances
for certain aptitudes when compared to identical twins who develop
in separate sacs (dichorionic M Z twins) (Spitz & Carlier, 1996).
When the calculations are corrected for the Commentary/Misconceptions
of biometrical IQists -4- high proportion of monochorionic MZ twins,
genetic differences could no longer account for the differences
in performance on an IQ-type test.
and coworkers observed that the greater incidence of premature births
among twins makes it difficult to extend specific conclusions drawn
from the study of twins to conclusions about the role of genes in
behavior in the overall population (Ainshe et al , 1987) The resultant
lower birth weights and other complications associated with prematurity
could result in enhanced similarities in twin development compared
to ordinary siblings Because of these and other developmental factors
peculiar to twins (Machin 1996), some of which may differentially
affect dizygotic vs monozygotic twins, it may be unwarranted to
conclude anything about the role of genes in behavior from genetic
studies of identical twins These issues have been said by one researcher,
Geoffrey Machiu to “strike at the heart of twin studies...”
One last and
perhaps most important point about the complexity of making arguments
about genetics a trait or behavior can be highly correlated with
genetics, but, in fact turn out to be largely due to environmental
factors As an example, consider a study that examines an essential
aspect of IQ research the tests of cognitive ability What is beyond
debate is the fact that scores on tests of cognitive ability are
on average correlated in this country with genes for skin color.
But a recent
study by Steele and Aaronson (Steele & Aronson , 1995) high
lights the complex interaction between social psychological historical,
and other factors that mix into the achievement of scores on such
a test These researchers administered tests of cognitive ability
to mixed groups of Black and White college students. In some cases
they told the students that the tests would measure their abilities
and in others that these tests were simply problem solving tasks
that were “nondiagnostic” of ability. To quote their
conclusions: “Blacks underperformed in relation to whites
in the ability-diagnostic condition but not in the non-diagnostic
condition.” They attributed this striking finding to what
they called “stereotype vulnerability” the fears that
result from years of being exposed to an environment in which Blacks
are considered intellectually inferior to Whites.
The main reason
for including this example is to highlight the primitiveness of
our real knowledge of the mix of factors that influence how human
behavior and human aptitudes are manifested. There are unexplored
realms influencing the development of human behavior and of human
capabilities, some of which we probably cannot even imagine at this
point. The likely intricate interplay of so many societal and familial
and genetic factors with the ultimate score that is achieved on
a test by an individual will not be usefully reduced to the specification
of the location of a gene on a chromosome. Attempts to quantify
these factors will usually reflect the biases or the limited knowledge
that we bring to such an analysis.
Given the examples
I have provided here, Capron et al.‘s statement that, in such
studies, “... not all environmental factors may be known...”
appears a gentle understatement of the extent of the problem.
genetics and complexity
in genetics of human disease facilitated by the Human Genome Project
has enhanced our appreciation of the complex nature of the development
of what might in the past appeared to have been even relatively
simple genetic traits physical diseases. This is occurring because
the new DNA technologies allow detection of anybody who carries
a mutation known to be associated with various health conditions.
findings are that individuals who carry mutations thought to be
deterministic for many conditions such as Gaueher’ s Disease,
cystic fibrosis and even Huntington’s Disease never exhibit
symptoms of the disease (Beckwith, 1999).
variability could be due to the effects of other genes within each
individual on the expression of the trait or environmental factors,
including prenatal conditions, diet, and even the actions of individuals
themselves in directing their own lives. These findings with relatively
simple human traits may Commentary/Misconceptions of biometrical
IQists -5- also explain the continuing difficulty in identifying
genes for human behavioral traits (Riseh & Botstein, 1996).
simplification to complexity: lessons from the IQ controversy
What the history
of IQ research shows is that fundamental assumptions or mind-sets
that permeate the field reflect attitudes toward the relative importance
of environment in the development of human behaviors and aptitudes.
While the avoidance of dealing with these assumptions may also reflect
the difficulty in measuring environmental factors, a scientific
problem with such difficulties is usually put aside until they can
be dealt with.
is integral to the evolution of scientific fields, its use (or overuse)
in the field of IQ research raises special problems. First, moving
from simplicity to complexity in many realms of science often does
not vitiate all previous conclusions, but rather expands upon them
this field, the results of recognizing oversimplification and the
moves toward greater sophistication in analysis, rather then advancing
knowledge, appear to many of us to have returned the science back
to its starting point.
science which utilizes simple starting assumptions is not used to
promote social agendas. In contrast, IQ research, from the eugenics
movement to “The Bell Curve” has repeatedly been used
to influence social policy. The social implications may be fostered
by scientists themselves, by the media, or by other interest groups
within society. What is most striking about this state of affairs
is that the simplifying assumptions practically dictate the conclusions.
The research field appears to have been imbued with a social perspective
from its outset to its presentation to the public.
is a geneticist at Harvard Medical School. This article originally
appeared in Current Psychology of Cognition (citation enclosed in
article, the pagination is different from the original). This is
the first of many articles from the Genetic Screening Study Group
that we intend to publish here. The study group is an offshoot of
Science for the People an important radical/progressive science
formation from the '60s and '70s.
are of particular importance since confusion over what science really
can and does say about the capabilities of human beings figures
heavily into social policy at all levels of human societies. It
is still being used in a variety of ways to justify class, race,
ethnic and gender stratification. These articles expose not only
the political implications of genetic determinism, they also lay
bare the unscientific nature of these claims that have been with
us every since Spencer coined the phrase "survival of the fittest".
This article is available in PDF format.
Correspondence should be sent to Jon Beckwith, American Cancer
Society Research Professor, Harvard Medical School, Department of
Microbiology and Molecular Genetics, 200 Longwood Ave., Boston,
MA 02115, USA (e-mail: email@example.com).
Cahie rs de Ps ycholo gie Cogn itive
Current Psychology of Cognition
1999, 18 (2), 161-169
COMMENTARY ON “MISCONCEPTIONS OF
BIOMETRICAL IQISTS” (C. CAPRON ET AL.)
I thank Joseph
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