[Paleopsych] Bouchard et al: Human Psychological Differences

[Premise Checker]

Sources of human psychological differences: the Minnesota study of twins reared 
apart.
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 
interventions.

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 [5]. 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 [6]. 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 [7]. Where appropriate, our data are 
corrected for age and sex effects [8]. 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 [9]. 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 
abilities batteries.

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 [10]. 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 [11]. 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 [12]. The second major 
ability battery is the Comprehensive Ability Battery [13]. Detailed results 
from analysis of both tests are reported elsewhere [14].

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 
in IQ.

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 [16] 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 [17]. 
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 [18]. 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 [20]. Rearing SES effects on IQ in adoption studies have been 
found for young children but not in adult samples [21], 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 
attained.

[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 [22]. 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 [23]. It appears, however, that it 
is the similarity that leads to increased contact, rather than the other way 
around [24]. 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 
apart(25).

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 [27].

Similarity of MZA Twins on a Variety of

Dimensions

Table 4 [28] 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 [29] 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 [10], 
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 [9] 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) [16], 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 [17] 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 [5].

[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 [33] 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 
[15], 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 [34]; 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 [35], 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 [36]. 
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 [37], 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 [38].

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. [39] 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

Sociobiology

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 [40], 
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 [41].

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

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(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 
Company.