[Paleopsych] Bouchard et al: Human Psychological Differences
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Sources of human psychological differences: the Minnesota study of twins reared
Thomas J. Bouchard Jr.; David T. Lykken; Matthew McGue; Nancy L. Segal;
and Auke Tellegen.
Science, Oct 12, 1990 v250 n4978 p223(6)
[What have been the followups on this famous article? Too bad I can't
display the tables!]
Since 1979, a continuing study of monozygotic and dizygotic twins, separated in
infancy and reared apart, has subjected more than 100 sets of reared-apart
twins or triplets to a week of intensive psychological and physiological
assessment. Like the prior, smaller studies of monozygotic twins reared apart,
about 70% of the variance in IQ was found to be associated with genetic
variation. On multiple measures of personality and temperament, occupational
and leisure-time interests, and social attitudes, monozygotic twins reared
apart are about as similar as are monozygotic twins reared together. These
findings extend and support those from numerous other twin, family, and
adoption studies. It is a plausible hypothesis that genetic differences affect
psychological differences largely indirectly, by influencing the effective
environment of the developing child. This evidence for the strong heritability
of most psychological traits, sensibly construed, does not detract from the
value or importance of parenting, education, and other propaedeutic
Monozygotic and dizygotic twins who were separated early in life and reared
apart (MZA and DZA twin pairs) are a fascinating experiment of nature. They
also provide the simplest and most powerful method for disentangling the
influence of environmental and genetic factors on human characteristics. The
rarity of twins reared apart explains why only three previous studies of modest
scope are available in the literature [1-4].
More than 100 sets of reared-apart twins or triplets from across the United
States and the United Kingdom have participated in the Minnesota Study of Twins
Reared Apart since it began in 1979. Participants have also come from
Australia, Canada, China, New Zealand, Sweden, and West Germany. The study of
these reared-apart twins has led to two general and seemingly remarkable
conclusions concerning the sources of the psychological differences -
behavioral variation - between people: (i) generic factors exert a pronounced
and pervasive influence on behavioral variability, and (ii) the effect of being
reared in the same home is negligible for many psychological traits. These
conclusions will not come as revelations to the many behavioral geneticists who
have observed similar results and drawn similar conclusions . This study and
the broader behavioral genetic literature, nevertheless, challenge prevailing
psychological theories on the origins of individual differences in ability,
personality, interests, and social attitudes . Here we summarize our
procedures and review our results and interpretations of them.
Participants complete approximately 50 hours of medical and psychological
assessment. Two or more test instruments are used in each major domain of
psychological assessment to ensure adequate coverage (for example, four
personality trait inventories, three occupational interest inventories, and two
mental ability batteries). A systematic assessment of aspects of the twin's
rearing environments that might have had causal roles in their psychological
development is also carried out. Separate examiners administer the IQ test,
life history interview, psychiatric interview, and sexual life history
interview. A comprehensive mental ability battery is administered as a group
test. The twins also complete questionnaires independently, under the constant
supervision of a staff member.
Reared-apart twins have been ascertained in several ways, such as: (i) friends,
relatives, or the reunited twins themselves, having learned of the project,
contact the Minnesota Center for Twin and Adoption Research (MICTAR); (ii)
members of the adoption movement, social workers, and other professionals who
encounter reared-apart twins serve as intermediaries; (iii) twins who are, or
become aware of, a separated co-twin solicit assistance from the MICTAR staff
in locating this individual. Selection on the basis of similarity is minimized
by vigorously recruiting all reared-apart twins, regardless of known or
presumed zygosity and similarity. We have been unable to recruit to the study
six pairs of twins reared apart whom we believe to be monozygotic.
Zygosity diagnosis is based on extensive serological comparisons, fingerprint
ridge count, and anthropometric measurements. The probability of
misclassification is less than 0.001 . Where appropriate, our data are
corrected for age and sex effects . Due to space limitations and the smaller
size of the DZA sample (30 sets), in this article we focus on the MZA data (56
sets). The results reported here are, for the most part, based on previously
reported findings, so that the sample sizes do not include the most recently
assessed pairs and vary depending on when in the course of this ongoing study
the analyses were conducted.
As shown in Table 1, the sample consists of adult twins, separated very early
in life, reared apart during their formative years, and reunited as adults.
Circumstances of adoption were sometimes informal, and the adoptive parents, in
comparison to parents who volunteer to participate in most adoption studies,
have a lower level of education (mean equals 2 years of high school), and are
quite heterogeneous in educational attainment and socioeconomic status (SES).
Because our sample includes no subjects with IQs in the retardate range ([is
less than or equal to] 70), the mean IQ is higher and the standard deviation
lower than for the general population.
[Tabular Data Omitted]
Components of Phenotypic Variance
If genetic and environmental factors are uncorrelated and combine additively
(points we return to later), the total observed variance, [V.sub.t], of a trait
within a population can be expressed as
[V.sub.t] = [[V.sub.g] + [V.sub.e] + [V.sub.m]
where [V.sub.g] is variance due to genetic differences among people, [V.sub.e]
is variance due to environmental or experiential factors, and [V.sub.m] is
variance due to measurement error and unsystematic temporal fluctuations. For
measures of psychological traits, [V.sub.m] ranges from approximately 10% (of
[V.sub.t]) for the most reliably measured and stable of traits (for example,
IQ) to as high as 50 to 60% for traits that are less reliable or that show
considerable secular instability (for example, some social attitudes). The
environmental component, [V.sub.e], can be divided into variance due to
experiences that are shared, [V.sub.es], and experiences that are unshared,
[V.sub.eu]. Shared events may be experienced differently by two siblings (for
example, a roller coaster ride or a family vacation), in which case they
contribute to the [V.sub.eu] component. If the total variance, [V.sub.t], is
set at unity, the correlation between MZ twins, [R.sub.mz], equals [V.sub.g] +
[V.sub.es]. The heritability of a trait equals [V.sub.g]; the heritability of
the stable component of a trait (for example, the mean value around which one's
aggressiveness varies) equals [V.sub.g]/([V.sub.t] - [V.sub.m]). [V.sub.t] and
[V.sub.m] can be estimated from studies singletons, but [V.sub.g] is more
elusive: for monozygotic twins reared together (MZT), some of the within-pair
correlation might be due to effects of shared experience, [V.sub.es]. The power
of the MZA design is that for twins reared apart from early infancy and
randomly placed for adoption, [V.sub.es] is negligible, so that [V.sub.g] can
be directly estimated from the MZA correlation.
Similarity in the IQ of MZA Twins
The study of IQ is paradigmatic of human behavior genetic research. There are
more than 100 relevant twin, adoptee, and family studies of IQ, and IQ has been
at the center of the nature-nurture debate . The analysis of IQ is also
paradigmatic of the approach taken by this study. It illustrates our use of
replicated measures, evaluation of rearing environmental effects, and analysis
of environmental similarity. We obtain three independent measures of IQ: (i)
the Wechsler Adult Intelligence Scale (WAIS); (ii) a Raven, Mill-Hill
composite; and (iii) the first principal component (PC) of two multiple
The WAIS consists of a set of six verbal and five performance subtests that are
individually administered, requiring about 1.5 hours, and that yield an
age-corrected estimate of IQ . To avoid examiner bias, we administer the
WAIS simultaneously to the twins in different rooms by professional
psychometrists. The Raven Progressive Matrices (Standard Set) is a widely used
nonverbal measure of problem-solving ability often paired with the Mill-Hill
Vocabulary Test, a multiple-choice word knowledge test . In this study, the
Raven and Mill-Hill are both administered and scored by computer. The two age-
and sex-corrected scores are transformed to have a mean equal to 50 and a
standard deviation of 10. The sum of these transformed scores (which
intercorrelate about 0.57) provides a separate estimate of IQ. The first major
ability battery included in our assessment is an expanded version of the
battery used in the Hawaii Family Study of Cognition . The second major
ability battery is the Comprehensive Ability Battery . Detailed results
from analysis of both tests are reported elsewhere .
In each of the three prior studies of MZA twins, two independent estimates of
intelligence were obtained. The sample sizes and intraclass correlations for
all four studies are compared in Table 2. The table illustrates the remarkable
consistency of the MZA correlations on IQ across measurement instrument,
country of origin, and time period. These correlations vary within a narrow
range (0.64 to 0.74) and suggest, under the assumption of no environmental
similarity, that genetic factors account for approximately 70% of the variance
This estimate of the broad heritability of IQ is higher than the recent
estimates (0.47 to 0.58) based on a review of the literature that includes all
kinship pairings [9, 15]. Virtually the entire literature on IQ similarity in
twins and siblings is limited, however, to studies of children and adolescents.
It has been demonstrated  that heritability of cognitive ability increases
with age. A heritability estimate of approximately 70% from these four studies
of mainly middle-aged adults is not inconsistent with the previous literature.
Do Environmental Similarities in Rearing
Environments Explain MZA IQ Similarity?
Such marked behavioral similarities between reared-apart MZ twins raise the
question of correlated placement: were the twins' adoptive homes selected to be
similar in trait-relevant features which, in turn, induced psychological
similarity? If so, given that the total variance equals 1.0, then [V.sub.es]
will equal at least [R.sub.ff] X [r.sub.ft.sup.2], where [R.sub.ff] is the
within-pair correlation for a given feature, f, of the adoptive homes (the
placement coefficient), and [r.sub.ft] is the product-moment correlation
between the feature and the trait in question, t.
A checklist of available household facilities (for example, power tools,
sailboat, telescope, unabridged dictionary, and original artwork) provides an
index of the cultural and intellectual resources in the adoptive home .
Each twin completes the Moos Family Environment Scale (FES), a widely used
instrument with scales describing the individual's retrospective impression of
treatment and rearing provided by the adoptive parents during childhood and
adolescence . The age- and sex-corrected placement coefficients for these
and other measures are shown in Table 3, together with the correlations between
twins' IQ and the environmental measure ([r.sub.ft]) and the total estimated
contribution to MZA twin similarity. The maximum contribution to MZA trait
correlations that could be explained by measured similarity of the adoptive
rearing environments on a single variable is about 0.03(19). The absence of any
significant effect due to SES or other environmental measures on the IQ scores
of these adult adopted twins is consistent with the findings of other
investigators . Rearing SES effects on IQ in adoption studies have been
found for young children but not in adult samples , suggesting that
although parents may be able to affect their children's rate of cognitive skill
acquisition, they may have relatively little influence on the ultimate level
[Tabular Data Omitted]
Has Pre- and Post-Reunion Contact
Contributed to MZA Twin Similarity in IQ?
MZA twins share prenatal and perinatal environments, but except for effects of
actual trauma, such as fetal alcohol syndrome, there is little evidence that
early shared environment significantly contributes to the variance of
psychological traits. Twins are especially vulnerable to prenatal and perinatal
trauma, but these effects are most likely to decrease, rather than increase,
within-pair similarity . There is evidence that twins who maintain closer
contact with each other later in life tend to be more similar in some respects
than twins who engage in infrequent contact . It appears, however, that it
is the similarity that leads to increased contact, rather than the other way
around . MZA twins in this study vary widely in the amount of contact they
have had prior to assessment. All twin pairs spend their formative years apart.
Some had their first adult reunion at the time of assessment, whereas others
met as much as 20 years earlier and had experienced varying degrees of contact.
A small number of the pair actually met at intervals during childhood. As shown
in Table 1, total contact time for the MZA twins ranges from 1 to 1233 weeks.
In the one case of 1223 weeks of contact, the twins met as teenagers and lived
near each other until assessment when they were adults. Since they met on a
regular basis, most of this time was coded as contact time. Degree of social
contact between two members of a reared-apart twin pair accounts for virtually
none of their similarity. The correlations with the within-pair absolute WAIS
IQ difference are 0.06 [+ or -] 0.15 for time together prior to separation,
0.08 [+ or -] 0.15 for time apart to first reunion, -0.14 [+ or -] 0.15 for
total contact time, and 0.17 [+ or -] 0.15 for percentage of lifetime spent
The absolute within-pair difference in WAIS IQ of co-twins as a function of
degree of contact are plotted in Fig. 1. Also shown are the expected absolute
IQ differences between randomly paired individuals and between two testings of
the same individual(26). Although the MZA average difference approximates the
absolute difference expected between two testings of a single individual, we do
observe a wide range of differences. It is not that we have found no evidence
of environmental influence; in individual cases environmental factors have been
highly significant (for example, the 29 IQ point difference in Fig. 1). Rather,
we find little support for the types of environmental influences on which
psychologists have traditionally focused .
Similarity of MZA Twins on a Variety of
Table 4  gives the MZA correlations, most previously published, on
variables ranging from anthropometry and psychophysiology, to aptitudes,
personality and temperament, leisure-time and vocational interests, to social
attitudes. Correlations for MZT twins and retest stability coefficients are
also provided for comparison Stable, reliably measured variables like
fingerprint ridge count and stature show the highest correlations. Brain wave
spectra are highly reproducible  and are strongly correlated in both MZA
and MZT twins. Most other psychophysiological variables (for example, blood
pressure and electrodermal response) vary considerably across time so that the
retest correlations between repeated measurements on the same persons range
from 0.5 to 0.8(30). These retest correlations set the upper limit of
similarity that might be found between MZ co-twins. The retest stability of
aptitude measures, such as IQ, is rather better, ranging from 0.8 to 0.9 ,
whereas stability of personality and interest measures ranges from 0.6 to 0.7.
[Tabular Data Omitted]
With these upper limits in mind, the findings in Table 4 demonstrate remarkable
similarity between MZA twins. In terms of standardized tests and measures, the
MZA twin similarities are often nearly equal to those for MZT twins (last
column) and constitute a substantial portion of the reliable variance (column
5) of each trait.
The Minimal Effect of Being Reared Together
Some of the MZA twins have had considerable contact as adults, but all of them
were reared apart throughout the formative periods of childhood and
adolescence. If being reared together enhances similarity in twins, within-pair
correlations for MZA twins are expected to be smaller than those for MZT twins.
For example, the mean MZT correlation for IQ, based on 34 studies of primarily
children or adolescents, is 0.86  as compared to 0.72 for all, primarily
adult, MZA twins. If the mean MZT correlation were maintained into adulthood,
its difference from the MZA correlation would suggest that common rearing
increases the similarity of IQ in twins (and siblings). However, the MZT
correlation apparently declines with age (for example, as a result of the
accumulation of nonshared environmental effects) , in which even the small
MZT-MZA correlation difference would suggest little influence of common rearing
on adult IQ. In any case, a significant contribution of shared environment is
found for the personality trait of social closeness(31), and possibly religious
interests and values (32).
As illustrated in Table 4, however, adult MZ twins are about equally similar on
most physiological and psychological traits, regardless of rearing status. This
finding and the failure to find significant [r.sub.ft] effects for cognitive
abilities  or personality (31), together with findings from numerous
studies of MZT and DZT twins, sibs, and foster sibs, implies that common
rearing enhances familial resemblance during adulthood only slightly and on
relatively few behavioral dimensions. This conclusion is given detail
discussion by Plomin and Daniels .
[Tabular Data Omitted]
Why Are MZA Twins So Similar?
It is well known to naturalists and to animal breeders that there are wide and
heritable differences in behavior within other species, but there is a curious
reluctance among some scientists  to acknowledge the contribution of
genetic variation to psychological differences within the human species. Our
findings support and extend those from many family, twin, and adoption studies
, a broad consilience of findings leading to the following generalization:
For almost every behavioral trait so far investigated, from reaction time to
religiosity, an important fraction of the variation among people turns out to
be associated with genetic variation. This fact need no longer be subject to
debate ; rather, it is time instead to consider its implications. We
suggest the following:
1) General intelligence or IQ is strongly affected by genetic factors. The IQs
of the adult MZA twins assessed with various instruments in four independent
studies correlate about 0.70, indicating that about 70% of the observed
variation in IQ in this population can be attributed to genetic variation.
Since only a few of these MZA twins were reared in real poverty or by
illiterate parents and none were retarded, this heritability estimate should
not be extrapolated to the extremes of environmental disadvantage still
encountered in society. Moreover, these findings do not imply that traits like
IQ cannot be enhanced. Flynn , in a survey covering 14 countries, has shown
that the average IQ test score has significantly increased in recent years.
This increase may be limited to that part of the population with low IQs .
The present findings, therefore, do not define or limit what might be
conceivably achieved in an optimal environment. They do indicate that, in the
current environments of the broad middle-class, in industrialized societies,
two-thirds of the observed variance of IQ can be traced to genetic variation.
2) The institutions and practices of modern Western society do not greatly
constrain the development of individual differences in psychological traits.
The heritability of a psychological trait reveals as much about the culture as
it does about human nature. Heritability must increase as [V.sub.e], the
variance affected by the environment, decreases. Where the culture's influence
is relatively homogeneous and efficacious, [V.sub.e] will decrease and
heritability will increase; most American boys, for example, have similar
opportunities to play baseball, so that one expects heritability of baseball
skill in American young men to be high. Where culture is efficacious, but
heterogeneous, [V.sub.e] (and total phenotypic variance) will increase; thus,
one would expect the heritability of specific linguistic o religious behaviors
in the United States or in the Soviet Union to be low. Individuals in Western
societies are heterogeneous with respect to personality traits, interests, and
attitudes, yet the heritabilities of these traits are relatively high. We infer
that the diverse cultural agents of our society, in particular most parents,
are less effective in imprinting their distinctive stamp on the children
developing within their spheres of influence - or are less inclined to do so -
than has been supposed.
Psychologists have been surprised by the evidence that being reared by the same
parents in the same physical environment does not, on average, make siblings
more alike as adults than they would have been if reared separately in adoptive
homes. It is obvious that parents can produce shared effects if they grossly
deprive or mistreat all their children. It seems reasonable that charismatic,
dedicated parents, determined to make all their children share certain personal
qualities, interests, or values, may sometimes succeed. Our findings, and those
of others , do not imply that parenting is without lasting effects. The
remarkable similarity in MZA twins in social attitudes (for example,
traditionalism and religiosity) does not show that parents cannot influence
those traits, but simply that this does not tend to happen in most families.
3) MZA twins are so similar in psychological traits because their identical
genomes make it probable that their effective environments are similar.
Specific mechanisms by which genetic differences in human behavior are
expressed in phenotypic differences are largely unknown. It is a plausible
conjecture that a key mechanism by which the genes affect the mind is indirect,
and that genetic differences have an important role in determining the
effective psychological environment of the developing child .
Infants with different temperaments elicit different parenting responses.
Toddlers who are active and adventurous undergo different experiences than
their more sedentary or timid siblings. In addition, children and adolescents
seek out environments that they find congenial. These are forms of
gene-environment covariance, [C.sub.ge]. Moreover, different individuals pay
different attention to or respond differently to the same objective experience,
or both. These are forms of gene-environment interaction, [V.sub.ge]. From
infancy onwards, genetic individually helps to steer the developing organism
through the multitude of possible experiences and choices. That is, Eq. 1 must
be elaborated to include these indirect and modifiable ways in which the genome
exerts its influence
[V.sub.t] = [V.sub.g] + [V.sub.e] + [C.sub.ge] + [V.sub.ge] + [V.sub.m]
The proximal cause of most psychological variance probably involves learning
through experience, just as radical environmentalists have always believed. The
effective experience, however, to an important extent are self-selected, and
that selection is guided by the steady pressure of the genome (a more distal
cause). We agree with Martin et al.  who see "humans as exploring organisms
whose innate abilities and predispositions help them select what is relevant
and adaptive from the range of opportunities and stimuli presented in the
environment. The effects of mobility and learning, therefore, augment rather
than eradicate the effects of the genotype on behavior" (p. 4368).
In this view is correct, the development experiences MZ twins are more similar
than those of DZ twins, again and environmentalist critics of twin research
have contended. However, even MZA twins tend to elicit, select, seek out or
create very similar effective environments and, to that extent, the impact of
these experiences is counted as a genetic influence. Finally, if the genome
impresses itself on the psyche largely by influencing the character, selection,
and impact of experiences during development - if the correct formula is nature
via nurture - then intervention is not precluded even for highly heritable
traits, but should be the more effective when tailored to each specific child's
talents and inclinations.
Relevance to Evolutionary Psychology and
This research focuses on individual differences, but like other animals we
share certain species-specific tendencies by virtue of our being human. Whereas
behavioral geneticists study variatins within a species, evolutionary
psychologists or sociobiologists attempt to delineate species-typical
proclivities or instincts and to understand the relevant evolutionary
developments that took place in the Pleistocene epoch and were adaptive in the
lives of tribal hunter-gatherers. The genes sing a prehistoric song that today
should sometimes be resisted but which it would be foolish to ignore.'
At the interface of behavioral genetics and sociobiology is the question of the
origin and function, if any, of the within-species variability we have been
discussing. One view is that it represents evolutionary debris ,
unimportant to fitness and perhaps not expressed in prehistoric environments.
Another view is that variability has an adaptive function and has been selected
for. Whether sociobiologists can make evolutionary sense of the varieties of
human genetic variation we have discussed here remains to be seen .
Whatever the ancient origins and functions of genetic variability, its
repercussions in contemporary society are pervasive and important. A human
species whose members did not vary genetically with respect to significant
cognitive and motivational attributes, and who were uniformly average by
current standards, would have created a very different society than the one we
know. Modern society not only augments the influence of genotype on behavioral
variability as we have suggested, but permits this variability to reciprocally
contribute to the rapid pace of cultural change. If genetic variation was
evolutionary debris at the end of the Pleistocene, it is now a salient and
essential feature of the human condition.
REFERENCES AND NOTES
[1.] H.H. Newman F. N. Freeman, K. J. Holzinger, Twins: A Study of Heredity and
Environment (Univ. of Chicago Press, Chicago, 1937); N. Juel-Nielson, Acta
Psychiatr. Neurol. Scand. Suppl. 183 (1965); J. Shields, Monozygotic Twins:
Brought up Apart and Brought up Together (Oxford Univ. Press, London, 1962).
There are two other ongoing studies of twins reared apart, one in Sweden (2)
and one in Finland (3). The questionable study by Burt (4) has been omitted.
[2.] N. Pedersen, G. E. McClearn, R. Plomin, L. Friberg, Behav. Genet. 15, 407
(1985); R. Plomin, P. Lichtenstein, N. L. Pederson, G. E. McClean, J. R.
Nesselroade, Psychol. Aging 5, 25 (1990). [3.] H. Langainvainio, J. Kaprio, M.
Koskenvuo, J. Lonnqvist, Acta Gene t. Med. Gemellol. 33, 259 (1984). [4.] L.
Hearnsahw, Cyrill Burt: Psychologist (Hodder & Stoughten, Londo n, 1979); but
see R. B. Joynson, The Burt Affair (Routledge, London, 1990). [5.] R. Plomin
and D. Daniels, Behav. Brain Sci. 10, 1 (1987); L. J. Ea ves, H. J. Eysenck, N.
G. Martin, Genes Culture and Personality: An Empirical Approach (Academic
Press, New York, 1989). [6.] T. J. Bouchard, Jr., in The Chemical and
Biological Bases of Indiv iduality, S. Fox, Ed. (Plenum, New York, 1984), p.
147; N. L. Segal, W. M. Grove, T. J. Bouchard, Jr., in Genetic Issues in
Psychosocial Epidemiology, M. Tsuang, K. Kendler, M. Lyons, Eds. (Rutgers Univ.
Press, New Brunswick, NJ, in press). [7.] D. T. Lykken, Behav. Genet. 8, 437
(1978). [8.] M. McGue and T. J. Bouchard, Jr., ibid. 14, 325 (1984). [9.] T. J.
Bouchard, Jr., and M. McGue, Science 212, 1055 (1981). [10.] J. D. Matarazzo,
Wechsler's Measurement and Appraisal of Adult Intelligence (Williams and
Wilkins, Baltimore, ed. 5, 1972). [11.] J. Raven, Manual for Raven's
Progressive Matrices and Vocabulary Scales (Lewis, London, 1986). [12.] J. C.
DeFries et al., Behav. Genet. 9, 23 (1979). [13.] A. R. Hakstian and R. B.
Cattell, J. Educ. Psychol. 70, 657 (1978). [14.] T. J. Bouchard, Jr., N. L.
Segal, D.T. Lykken, Ada Genet. Med. Gemellol. 39, 193 (1990). [15.] J. C.
Loehlin, Am. Psychol. 44, 1285 (1989); R. Plomin and J. C. Loehlin, Behav.
Genet. 19, 331 (1989). [16.] K. McCartney, M. J. Harris, F. Bernieri, Psychol.
Bull. 107, 26 (1990). [17.] M. McGue and T. J. Bouchard, Jr., in Advances in
the Psychology of Human Intelligence, R. J. Sternberg, Ed. (Erlbaum, New York,
1989), vol. 5, p. 7. This checklist yields four relatively independent scales:
scientific or technical, cultural, mechanical, and material possessions. [18.]
R. H. Moos and B. S. Moos, Manual: Family Environment Scale (Consulting
Psychologists Press, Palo Alto, CA, 1986). (19.) Formally, this is the maximum
linear contribution; nonlinear effects are, of course, possible. For these
data, however, investigation of higher-ordered relationships (quadratic and
cubic) showed no associations that did not exist at the linear level, and there
was no discernible nonlinearity detected in visual inspection of the
scatterplots. [20.] T. J. Bouchard, Jr., Intelligence 7, 175 (1983). [21.] C.
Capron and M. Duyme [Nature 340, 552 (1989)] have shown an SES effect in an
adoption study of young children; S. Scarr and R. Weinberg [Amer. Sociol. Rev.
43, 674 (1978)] did not find an SES effect in a study of young adult adoptees.
[22.] B. Price, Am. J. Hum. Genet, 2, 293 (1950). [23.] R. J. Rose and J.
Kaprio, Behav. Genet. 18, 309 (1988). [24.] D. T. Lykken, T. J. Bouchard, Jr.,
M. McGue, A. Tellegen, Behav. Genet., in press. (25.) As in our earlier
analysis, nonlinear relationships were tested for and found not to exist.
Additionally, deletion of a single outlier (IQ difference of 29 points) did not
appreciably change the correlation estimates. (26.) Expected difference (D) can
be expressed as a function of the correlation (r) and standard deviation as
[Mathematical Expression Omitted] [R. Plomin and J. C. DeFries, Intelligence 4,
15 (1980)]. [27.] K. R. White, Psychol. Bull. 86, 461 (1982). [28.] D. T.
Lykken, T. J. Bouchard, Jr., M. McGue, A Tellegen, Acta Genet. Med. Gemellol.
39, 35 (1990); and (6). [29.] H. H. Stassen, D. T. Lykken, G. Bomben, Eur.
Arch. Psychiatry Neurol. Sci. 237, 244 (1988). (30.) Systolic blood pressure
from Minnesota twin studies. Heart rate from B. Hanson et al., Am. J. Cardiol.
63, 606 (1989). Electrodermal and habituation data from D. T. Lykken, W. G.
Iacono, K. Haroian, M. McGue, T. J. Bouchard, Jr., Psychophysicology 25, 4
(1988). Reliability data from K. Matthews, C. Rakczky, C. Stoney, S. Manuck,
ibid. 24, 464 (1978); M. Llabre et al., ibid, 25, 97 (1988). (31.) MPQ data
from A. Tellegen et al., J. Pers. Soc. Psychol. 54, 1031 (1988); CPI data from
T. J. Bouchard, Jr., and M. McGue, J. Pers. 58, 263 (1990). Reliability data
from test manuals. (32.) MZA and MZT Religiosity data from N. G. Waller, B. A.
Kojetin, T. J. Bouchard, Jr., D. T. Lykken, A. Tellegen, Psychol. Sci. 1, 138
(1990). Reliability of religious leisure time interests and religious
occupational interests and mean of 14 nonreligious social attitude items from
Minnesota twin study data base (28). Reliability of other scales from test
manuals. For a general discussion of the reliability of traits such as those
measured in this study, see K. C. H. Parker, R. K. Hanson, J. Hunsley [Psychol.
Bull. 103, 367 (1988)] and J. J. Conley [Pers. Individ. Differ. 5, 11 (1984)].
[33.] R. C. Lewontin, S. Rose, L. J. Kamin, Not in Our Genes; Biology, Ideology
and Human Nature (Pantheon, New York, 1984). [34.] S. Scarr, Behav. Genet. 17,
219 (1987). [35.] J. R. Flynn, Psychol. Bull. 101, 171 (1987). [36.] R. Lynn,
Pers. Individ. Differ. 11,273 (1990); T. W. Teasedale and D. R. Owen,
Intelligence 13, 255 (1989). [37.] R. Wilson, Child Dev. 54, 298 (1983). [38.]
K. J. Hayes, Psychol. Rep. 10, 299 (1962); C. J. Lumsden and E. O. Wilson,
Genes, Mind and Culture (Harvard Univ. Press, Cambridge, MA, 1981); S. Scarr
and K. McCartney, Child Dev. 54, 424 (1983). [39.] N. G. Martin et al., Proc.
Nat. Acad. Sci. U.S.A. 83, 4364 (1986). [40.] M. W. Feldman and R. C. Lewontin,
Science 190, 1163 (1975); D. Symonds, The Evolution of Human Sexuality (Oxford
Univ. Press, New York, 1979). [41.] D. M. Buss, J. Pers. 58, 1 (1990). [42.] T.
J. Bouchard, Jr., D. T. Lykken, M. McGue, N. L. Segal, A. Tellegen, this
article. (43.) The MZA correlation of 0.771 reported by the late Sir Cyrill
Burt and questioned for its authenticity after his death (4) falls within the
range of findings reviewed here. (44.) WAIS data for MZTs from K. Tambs, J. M.
Sundet, P. Magnus, Intelligence 8,283 (1984). Reliabilities from (10). Raven,
Mill-Hill, and composite data from Minnesota twin studies (6, 42). (45.) MZA
data on SCII and JVIS from D. Moloney, unpublished thesis (University of
Minnesota, Minneapolis, 1990). Minnesota Occupational Interest Scale data from
N. Waller, D. T. Lykken, A. Tellegen, in Wise Counsel: Essays in Honor of Lloyd
Lofquist, R. Dawis and D. Lubinski, Eds. (Univ. of Minnesota Press,
Minneapolis, in press). SCII MZT data from Nichols [Homo 29, 158 (1978)].
Reliability data from test manuals. (46.) We thank our colleagues E. D. Eckert,
L. L. Heston, and I. I. Gottesman for their help on the medical and psychiatric
portions of the study and H. Polesky, director, for the blood testing. This
research has been supported by grants from The Pioneer Fund, The Seaver
Institute, The University of Minnesota Graduate School, The Koch Charitable
Foundation, The Spencer Foundation, The National Science Foundation
(BNS-7926654), The National Institute of Mental Health (MH37860), The National
Institute on Aging (AG06886), and the Harcourt Brace Jovanovich Publishing
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