Covariance structure of neuroticism and 001

PERSONALITY PROCESSES AND INDIVIDUAL DIFFERENCES

Covariance Structure of Neuroticism and Agreeableness: A Twin and
Molecular Genetic Analysis of the Role of the Serotonin Transporter Gene
Kerry L. Jang

Stella Hu

University of British Columbia

National Cancer Institute, National Institutes of Health

W. John Livesley

Alois Angleitner

University of British Columbia

Universitat Bielefeld

Rainer Riemann

Juko Ando and Yutaka Ono

Friedrich-Schiller Universitat Jena

Keio University

Philip A. Veraon

Dean H. Hamer

University of Western Ontario

National Cancer Institute, National Institutes of Health

The Revised NEO Personality Inventory domains of Neuroticism and Agreeableness are considered
factorially distinct despite several intercorrelations between these domains. The genetic correlation, an
index of the degree to which these intercorrelations are caused by genetic influences, was estimated using
data from 913 monozygotic and 562 dizygotic volunteer twin pairs from Canada, Germany, and Japan.
The serotonin transporter gene, 5-HTTLPR, was assayed in a sample of 388 nontwin sibling pairs from

the United States to determine the contribution of the serotonin transporter locus to the covariation
between the Neuroticism and Agreeableness scales. In all four samples, genetic influences contributed to
the covariance of Neuroticism and Agreeableness, with the serotonin transporter gene accounting for
10% of the relationship between these domains.

all human behavior can be shown to be heritable and "the very
ubiquity of these findings make them a poor basis for reformulating scientists' conceptions of human behavior" (p. 782). What are
more useful are multivariate genetic studies, as demonstrated by a
growing body showing that the observed structure of personality
traits reflects the covariance structure of heritable and experiential
influences (e.g., Carey & DiLalla, 1994; Livesley, Jang, & Vernon,
1998; McCrae, Jang, Livesley, Riemann, & Angleitner, 2001).
Multivariate genetic studies are useful because they enable
researchers to estimate the degree to which the same genetic
factors influence two or more variables, or pleiotropy, as well as
the joint influence environmental factors have on multiple phenotypes. Personality data obtained from monozygotic (MZ) and
dizygotic (DZ) twins provide a straightforward way to estimate the
genetic correlation coefficient, rG, which indexes the degree to
which two variables are influenced by the same genetic factors.
The genetic correlation coefficient can be interpreted in the same

way as the familiar Pearson's r. It is also estimated in the same
way as Pearson's r, except that genetic variances and covariances
are obtained from genetically informative participants, such as MZ
and DZ twins. The heritability of a single variable is estimated by
comparing the similarity of MZ with DZ twins. A higher within-

One of the most replicable results found in the social sciences is
that about half of the total variance in personality trait scores is
directly attributable to genetic differences between individuals and
the other half to experiences that are not shared by children from
the same family, known as nonshared environmental effects (see
Bouchard, 1997, for a review). However, Turkheimer (1998) argued that findings such as these are not of great use because almost

Kerry L. Jang and W. John Livesley, Department of Psychiatry, University of British Columbia, Vancouver, British Columbia, Canada; Stella
Hu and Dean H. Hamer, National Cancer Institute, National Institutes of
Health, Bethesda, Maryland; Alois Angleitner, Department of Psychology,
Universitat Bielefeld, Bielefeld, Germany; Rainer Riemann, Department of
Psychology, Friedrich-Schiller Universitat Jena, Jena, Germany; Juko
Ando, Department of Education, Faculty of Letters, Keio University,
Tokyo, Japan; Yutaka Ono, Department of Psychiatry, School of Medicine,

Keio University, Tokyo, Japan; Philip A. Vemon, Department of Psychology, University of Western Ontario, London, Ontario, Canada.
Correspondence concerning this article should be addressed to Kerry L.
Jang, Division of Behavioural Sciences, Department of Psychiatry, University of British Columbia, 2255 Wesbrook Mall, Vancouver, British
Columbia V6T 2A1, Canada. Electronic mail may be sent to kjang@
interchange.ubc.ca.

Joumal of Personality and Social Psychology, 2001, Vol. 81, No. 2, 295-304
Copyright 2001 by the American Psychological Association, Inc. 0022-3514/01/S5.00
DOI: 10.1037//0022-3514.81.2.295

295

296

JANG ET AL.

pair correlation for the MZ twins than the DZ twins suggests that
genetic influences are present, because the greater similarity is
directly attributable to the twofold increase in genetic similarity in
MZ as compared with DZ twins. In the multivariate case, common

genetic influences are suggested when the MZ cross-correlation
(the correlation between one twin's score on one of the variables
and the other twin's score on the other variable) exceeds the DZ
cross-correlation. Variables may also covary because the same
environmental factors influence their development, and this is
estimated by the environmental correlation coefficient, rE. The
degree to which traits share a common etiological basis can be
used to help resolve some of the persistent controversies in personality research, such as the location of some facet traits within a
domain or the problem with the number of factors.
At the present time, there is general agreement that there are five
basic trait domains variously referred to as Extroversion, Agreeableness, Conscientiousness, Neuroticism, and Openness to Experience (Goldberg, 1993). These trait names actually represent
broad domains or higher order constructs that are arranged hierarchically above a series of lower order traits, or facets, that define
the range of behaviors covered by the domain. Despite this general
consensus, the actual definition or content of these five domains
can vary quite widely across scales, whose differences lie in the
definition of a trait domain.
An excellent example of this problem is highlighted in Depue
and Collins's (1999) review of the Extraversion trait that showed
that all the major models recognized sociability and affiliation, but
only some included agency, activation, impulsivity or sensation

seeking, positive emotions, or optimism. These differences are also
found within the major personality trait measures. For example,
the Neuroticism scale from the Revised NEO Personality Inventory (NEO-PI-R: Costa & McCrae, 1992) contains items assessing impulsive behaviors, whereas these behaviors are lacking
in the Eysenck Personality Questionnaire-Revised Neuroticism
scale (Eysenck & Eysenck, 1992). These differences are surprising
given that extraversion and neuroticism are central concepts in
virtually all models and measures of personality.
A second problem is the debate on the number of domains
required to represent the higher order structure of personality. The
answer to this issue is really a function of how the degree of
overlap between domains is handled. For example, the correlation
of NEO-PI-R Neuroticism and Conscientiousness domains is reported at -.53 and the correlation between NEO-PI-R Extraversion and Openness to Experience at .40 (Costa & McCrae, 1992).
Eysenck (1991, 1992) would argue that this overlap suggests that
five domains are too many.
Etiological criteria may help resolve some issues of personality
structure, but this raises the question of which etiological factors,
genetic, environmental, or both, should be selected for use in these
studies. A recent study by McCrae et al. (2001) suggests that
genetic factors are more important than environmental influences
in shaping the structure of personality. Factorial analysis of three

matrices (phenotypic, genetic, and environmental) between the 30
NEO-PI-R facets yielded five highly congruent factors suggesting
that the phenotypic structure mirrors genetic and environmental
architecture. However, nonshared environmental effects are estimated as a residual term and thus may also include systematic bias
because of implicit personality theory. To test for this bias, the
authors conducted an additional analysis that supplemented the

twin data with cross-observer correlations on the NEO-PI-R to
allow the computation of a nonshared environmental covariance
matrix free of systematic implicit personality theory bias. Analysis
of this matrix did not produce the familiar five-factor solution,
suggesting that genetic factors, such as that indexed by the genetic
correlation, become the focus of study in the interim.
Although pleiotropy is readily indexed by the genetic correlation, it should be noted here the genetic correlation coefficient
provides only an estimate of statistical pleiotropism—in which the
genetic effects of one variable are correlated with the genetic
effects of another variable. Statistical pleiotropy, which can include linkage effects, should not be confused with biological
pleiotropism, for which the same loci are shown to influence the
two variables (Carey, 1988). Unlike statistical pleiotropism, biological pleiotropism unequivocally links actual genes to behavior,
whereas statistical pleiotropism is primarily useful in identifying

areas where biological pleiotropism may be found. To find biological pleiotropy, one needs to turn to molecular genetic methods
and DNA analysis. Virtually all the multivariate genetic studies of
personality to date have been limited to studies of statistical
pleiotropism.
The purpose of the present study is to test for evidence of
biological pleiotropism between personality measures to begin to
resolve some of the persistent problems in personality research.
Our focus is on the factorial structure of the NEO-PI-R, which, by
virtue of its popularity in research and clinical settings, has become
the primary measure of the five-factor model. The facets defining
each of the domains are assumed to form a coherent cluster that is
mutually exclusive of facets defining the other domains (Costa
& McCrae, 1995, 1998), a claim that Eysenck (1991, 1992;
Paunonen & Jackson, 1996) has disputed. A particularly problematic area of the NEO-PI-R is the relationship between the domains
of Neuroticism and Agreeableness. The facet scales that define
the NEO-PI-R Neuroticism domain are: Anxiety, Angry Hostility, Depression, Self-Consciousness, Impulsiveness, and Vulnerability. NEO-PI-R Agreeableness is defined by the facets
Trust, Straightforwardness, Altruism, Compliance, Modesty, and
Tender-Mindedness.
Phenotypically, the factorial structure of the Neuroticism and
Agreeableness domains is distinct, but several significant relationships occur between domains. For example, although Costa and

McCrae (1992) reported the correlation between Neuroticism and
Agreeableness total domain scores at a moderate r = —.25, the
correlation between the Neuroticism facet Angry Hostility and the
Agreeableness facets Trust, Altruism, and Compliance were —.42,
— .34, and —.49, respectively. Similarly, the correlation between
Angry Hostility and the total Agreeableness domain score was
-.47, and the correlation between the total Neuroticism domain
score and the Agreeableness facet Trust was —.37. The determination of what accounts for these correlations will refine the
description of these two domains and provide some criteria to
evaluate their independent status.
Another reason to focus on the relationship between these
domains is several converging lines of molecular genetic evidence.
Recent work on the serotonin transporter that regulates serotonergic neurotransmission suggests a pleiotropic relationship between
these two domains. Studies on humans and primates indicate that
altered serotonin activity is related to negative emotional states
such as depression, anxiety, and hostility and to social behaviors

297

PERSONALITY STRUCTURE AND SEROTONIN

such as dominance, aggression, and affiliation with peers (Graeff,
Guimaraes, De Andrade, & Deakin, 1996; Knutson et al., 1998;
Murphy et al., 1998). Knutson et al. (1998) found that administration of paroxetine, a specific serotonin reuptake inhibitor that
targets the serotonin transporter, both decreased negative affect
and increased scores on a behavioral index of social affiliation in
normal human participants.
In humans, the expression of the serotonin transporter gene is
variable because of the presence of a common length polymorphism, the 5-hydroxytryptamine (5-HTTLPR), in the transcriptional regulatory region of the gene (Heils et al., 1996; Lesch et al.,
1996). The long (5-HTTLPR-L, or I) version of this polymorphism
is a more efficient promoter than the short (5-HTTLPR-S, or s)
version, and cultured cells and brain tissue homozygous for the /
allele produce more serotonin transporter mRNA and protein than
do cells heterozygous or homozygous for the s allele (Greenberg et
al., 1999; Lesch et al., 1996; Little et al., 1998). A number of
studies have demonstrated the importance of the serotonergic
system on personality. For example, Rinne, Westenberg, den Boer,
and Van den Brink (2000), showed that sustained traumatic stress
in childhood affects the responsivity of the postsynaptic serotonergic (5-HT) system of traumatized borderline personality disorder patients, for whom anxiety-related traits are a key feature (e.g.,
Livesley et al., 1998). Similarly, Hansenne and Ansseau (1999)

showed a positive relationship between Tridimensional Personality
Questionnaire Harm Avoidance and serotonergic activity measured by prolactin response to fiesinoxan, a highly potent and
selective 5-HTla agonist.
Lesch et al. (1996), in a study recently replicated by Greenberg
et al. (2000), were the first to report that individuals carrying the
5-HTTLPR-S allele had increased scores on NEO-PI-R Neuroticism domain and increased facet scores on Anxiety, Angry Hostility, Depression, and Impulsiveness and that the allele accounted
for 3% to 4% of the total variance in these scales. Unexpectedly,
the s allele was also associated with a decreased NEO-PI-R Agreeableness score. Similar associations were found using the Tridimensional Character Inventory (TCI; Cloninger, Svrakic, & Przybeck, 1993). Hamer, Greenberg, Sabol, and Murphy (1999)
showed that 5-HTTLPR-S genotypes were significantly associated
with increased scores on the Harm Avoidance scale of this measure (shown to correlate .66 with NEO-PI-R Neuroticism), and
decreased scores on the Self-Directedness (correlated —.64 with
NEO-PI-R Neuroticism), Reward Dependence, and Cooperativeness (shown to correlate .43 and .66 with NEO-PI-R Agreeableness) scales, accounting for 0.80%, 1.98%, 0.97%, and 2.60% of
the total variance in these scores, respectively. Mazzanti et al.
(1998), Peirson et al. (2000), and Benjamin et al. (2000) have
reported replications of these findings. Although it appears the
total proportion of the variance accounted for by 5-HTTLPR-S is
small, this is typical for the effects of a single gene on a complex,
multifactorial phenotype such as personality (Plomin, DeFries,
McClearn, & Rutter, 1997; Lesch et al., 1996).
It is important to note some significant failures to replicate these
associations using a variety of scales, including the NEO-PI-R. For
example, Gelernter, Kranzler, Coccaro, Siever, and New (1998)
found no association between 5-HTTLPR and TCI Harm Avoidance and NEO-PI-R Neuroticism. Herbst, Zonderman, McCrae,
and Costa (2000) reported no association between 5-HTTLPR and
TCI Harm Avoidance, and Gustavsson et al. (1999) also failed to

replicate these findings using measures of the Karolinska Scales of
Personality.
One of the most frequently cited reasons for failed replications
is population stratification. Population stratification occurs when
racial or ethnic subgroups of the study population have different
allele frequencies and, coincidentally, different average phenotype
scores that produce spurious associations (see Gelernter et al.,
1998). Another is that these studies tend to use only total domain
scores that may confound competing genetic influences on the
phenotypes. Should several different genes influence individual
facet traits, the sum of the facet scores could obscure the effects of
any individual loci.
In the present study, we estimated the contribution of the serotonin transporter locus, 5-HTTLPR, to the covariation (biological
pleiotropy) between the scales of Neuroticism and Extraversion
with data obtained from two samples of nontwin sibling pairs from
the United States. We addressed issues surrounding replicability of
results by proposing and using a test for population stratification,
and conducted the analyses at the level of the facet scores. Furthermore, twin data from three independent samples from Canada,
Germany, and Japan were used to estimate the phenotypic and
genetic correlations (statistical pleiotropy) between the Neuroticism and Agreeableness scales to support the genotyping results.
Method
Participants
Participants were 913 MZ and 562 DZ volunteer general-population
twin pairs from Canada, Germany, and Japan and 388 nontwin sibling pairs
from the United States. The Canadian twin sample consisted of 253 MZ
and 207 DZ twin pairs recruited from Vancouver, Canada, by the University of British Columbia Twin Project. The German twin sample consisted
of 526 MZ and 269 DZ pairs recruited from across Germany by the
University of Bielefeld Twin Study (see Riemann, Angleitner, & Strelau,
1997). Results from a subset of these samples were previously published in
Jang et al. (1998); the present study includes additional data from 70 MZ
and 32 DZ newly recruited pairs from Canada and an additional 91 MZ
and 64 DZ pairs from Germany. The Japanese twin sample consisted of
134 MZ and 86 DZ pairs from Keio University Twin Project, Tokyo, Japan
(see Ono, Ando, Onoda, Yoshimura, & Asai, 2000). All three samples of
twins were recruited using media appeals. Zygosity was diagnosed by a
questionnaire that assesses the frequency of confusing the twins by different relations across the life span. The Canadian, German, and Japanese
samples used well-established questionnaires designed by Nichols and
Bilbro (1966), Oniszczenko, Angleitner, Strelau, and Angert (1993), and
Ooki, Yamada, and Asaka (1991), respectively, supplemented by the
examination of recent color photographs by researchers experienced in
working with twins. Table 1 presents the details of each twin sample.
The fourth sample consisted of 388 same-sex, nontwin sibling pairs from
different families, comprising 236 male pairs and 152 female pairs recruited by the National Institutes of Health in Bethesda, Maryland. The
average age of this sample was 29.13 years (SD = 11.13 years). Reliable
ethnicity information was collected on this sample: 79.6% were Caucasian
(non-Hispanic), 6.1% were Asian or Pacific Islander, 5.2% were Hispanic, 4.8% were African American, and 4.3% other or unknown. The
present sample is composed of the 427 siblings described in Lesch et al.
(1996) and an independent sample of 349 siblings described in Greenberg
et al. (2000).

Measures
The Canadian and U.S. samples completed the English self-report version of Costa and McCrae's (1992) NEO-PI-R, the German sample com-

298

JANG ET AL.

Table 1

Twin Sample Characteristics
Zygosity
Canadian twins
MZ male
MZ female
DZ male
DZ female
DZ female-male
German twins
MZ male
MZ female
DZ male
DZ female
DZ female-male
Japanese twins
MZ male
MZ female
DZ male
DZ female
DZ female-male
Note.

Pairs

M

SD

Range

88
165
52
107
48

35.90
34.53
31.46
33.00
30.27

15.62
14.76
12.13
12.92
9.91

16-86
15-76
16-66
16-76
16-49

103
423
38
163
68

32.42
32.29
30.76
32.28
29.34

13.07
13.55
13.77
12.92
9.54

15-67
15-80
15-66
15-65
14-57

38
96
21
40
25

18.82
20.16
19.10
19.90
18.96

3.28
3.08
2.61
3.52
3.25

15-26
15-27
15-23
15-26
15-26

cism and Agreeableness scales using data obtained from the three twin
samples. In the second part, we estimated the degree to which the serotonin
transporter accounted for the covariance between the scales.
The genetic correlations between the Neuroticism and Agreeableness
scales were estimated for each twin sample by subjecting the MZ and DZ
within-pair covariances (PRELIS 2.20; Joreskog & Sbrbom, 1993) to
Cholesky decomposition (Neale & Cardon, 1992) by the method of maximum likelihood. The matrix decomposition and subsequent standardization was conducted using the Mx program (Neale, Boker, Xie, & Maes,
1999). The Cholesky decomposition models specified two effects: additive
genetic effects (i.e., the extent to which genotypes "breed true" from parent
to offspring) and the nonheritable component known as nonshared environmental effects (i.e., events that have differential effects on individual
family members). These two variance components have been shown to
satisfactorily account for all of the variance in these scores (e.g., Jang et al.,
1998; Ono et al., 2000; Riemann et al., 1997). McGue and Bouchard (1984)
showed that age and gender may bias heritability estimates, and we
adjusted all of the present data for these effects by computing the standardized residual of the simultaneous regression of each score on age and
gender prior to any analyses.

Genotyping Analyses

MZ = monozygotic; DZ = dizygotic.

pleted Ostendorf and Angleitner's (1994) German-language self-report
version, and the Japanese sample completed a Japanese-language selfreport version developed by Yoshimura et al. (1998). All participants were
instructed to complete the forms independently of one another in a nondistracting setting.

Statistical Analyses
The analyses were conducted in two parts. In the first part, we estimated
the phenotypic and genetic correlation coefficients between the Neuroti-

The 776 nontwin full-siblings were genotyped for the / and s forms of
the serotonin transporter gene, 5-HTTLPR. DNA was extracted from
peripheral blood and the 5-HTTLPR was analyzed by PCR and gel electrophoresis (Lesch et al., 1996). Allele frequencies were / = 56.7% and
i = 43.3%. On the basis of previous biochemical and phenotypic measurements (Greenberg et al., 1999; Lesch et al, 1996), the genotypes were
grouped as 5-HTTLPR-L = /// (30.9%) and 5-HTTLPR-S = sll (51.5%)
plus sis (17.5%).
The proportion of the total variance on the individual Neuroticism and
Agreeableness scores attributable to 5-HTTLPR (h2) was calculated as the
ratio of the variance that was due to the specific gene to the variance that

Table 2
Varimax-Rotated Principal-Components Analysis of the Revised NEO Personality Inventory
Neuroticism and Agreeableness Facet Scales for the Canadian Twins (n = 460 pairs), German
Twins (n = 795 pairs), Japanese Twins (n = 220 pairs), and U.S. Siblings (n = 388 pairs)
Factor loading
Canadian twins
Domain and scale

1

2

German twins
1

Japanese twins
1

2

2

Neuroticism
Anxiety
Angry Hostility
Depression
Self-Consciousness
Impulsiveness
Vulnerability
Agreeableness
Trust
Straightforwardness
Altruism
Compliance
Modesty
Tender-Mindedness

.82
.67
.89
.83
.48
.82

.07
-.45
.03
.04
-.23
.04

.82
.73
.88
.76
.35
.86

.08
-.37
.05
.12
-.18
.09

.81
.82
.45
.45
.81
.83

.06
-.15
-.65
-.34
-.06

-.49
-.12
-.22
-.10
.22
.08

.56
.76
.72
.77
.65
.64

-.41
-.02
-.21
-.14
.24
.14

.48
.62
.73
.77
.66
.63

Eigenvalue
% variance

4.24
35.4

2.70
22.5

3.81
31.8

2.68
22.3

Note.

U.S. siblings
1

2

-.05

.84
.60
.87
.75
.45
.82

.02
-.53
-.02
.15
-.22
.02

-.15
.11
.60
.08
.19
-.30

.73
.73
.28
.64
.55
.66

-.40
-.05
-.06
-.13
.28
.10

.67
.76
.65
.72
.63
.66

3.87
32.2

2.62
21.9

3.88
32.3

2.82
23.5

Loadings greater than .40 in absolute value are given in boldface.

299

PERSONALITY STRUCTURE AND SEROTONIN

Table 3
Phenotypic Correlations (Pearson's r) Between Neuroticism and Agreeableness Domains and Facets in All Members of the Canadian
Twin, German Twin, Japanese Twin, and U.S. Nontwin Sibling Pair Samples
Domain and scale

Agreeableness

Trust

Straightforwardness

Altruism

Compliance

Modesty

Tender-Mindedness

-.17**
-.06
-.52**
-.03
-.02
-.20**
.01

.09
.12
-.10
.19**
.14*
-.16**
.18**

.01
.06
-.13*
.03
.08
-.03
.04

-.14**
-.04
-.44**
-.05
.03
-.19**
-.02

.15**
.17**
-.06
.20**
.25**
-.17**
.20**

.13**
.12**
-.04
.13**
.10*
.12**
.14**

-.01
.14
-.06
-.24**
-.11
.14
.13

.01
.10
.07
-.21*
.03
.07
.13

.04
-.12
-.30**
_44**
-.12
-.16
-.18*

-.22**
-.08*
-.58**
-.09*
.06
-.23**
-.05

.14**
.17**
-.12**
.23**
.21**
-.07*
.24**

.01
.06
-.19**
.06
.04
.05
.06

Canadian twins (n = 460)
Neuroticism
Anxiety
Angry Hostility
Depression
Self-Consciousness
Impulsiveness
Vulnerability

-.24**
-.07
-.49**
-.13*
-.10
-.25**
-.10

-.47**
-.33**
-.56**
-.40**
-.37**
-.21**
-.30**

Neuroticism
Anxiety
Angry Hostility
Depression
Self-Consciousness
Impulsiveness
Vulnerability

-.13**
-.03
-.38**
-.06
.00
-.15**
-.01

-.33*
-.25**
-.36**
-.32**
-.28**
-.03
-.24**

-.18**
-.03
-.32**
-.12
-.05
-.24**
-.11

-.25**
-.03
-.37*
-.18**
-.19**
-.17**
-.25**

German twins (n = 795)
-.06
-.02
-.18**
-.02
.04
-.19**
.04

-.18**
-.07
-.36**
-.13**
-.11*
-.06
-.13**

Japanese twins (n = 220)
Neuroticism
Anxiety
Angry Hostility
Depression
Self-Consciousness
Impulsiveness
Vulnerability

-.09
.15
-.05
_49**
-.19
.10
.12

-.26**
-.01
-.19*
-.44**
-.20*
-.12
-.15

Neuroticism
Anxiety
Angry Hostility
Depression
Self-Consciousness
Impulsiveness
Vulnerability

-.18**
-.06
-.50**
-.07
.04
-.19**
-.04

-.40**
-.30**
-.53**
-.32**
-.16**
-.18**
-.27**

-.17
.14
-.11
-.54**
-.33**
.07
.05

.36**
.36**
.38**

.03
.03
.36**
.47**

U.S. nontwin sibling pairs (n = 388)

*/7 where N = Neuroticism and A = Agreeableness) was
calculated as the ratio of covariation that was due to the specific gene to
covariation that was due to all causes.' We performed the calculations on
the complete data set without correcting for the nonindependence of
siblings. Equivalent results were obtained when the calculations were
performed on a single, arbitrarily chosen member of each sibling pair or
when a regression method that took into account the residual correlation
between siblings was used (e.g., Lesch et al., 1996).

Test for Potential Population Stratification Effects
Spurious genetic associations (h2, h^A) can occur when subgroups (e.g.,
ethnicity) of the study population have different allele frequencies and
different average phenotype scores. Because full-sibling pairs share identical backgrounds, population stratification effects are suggested when the
sibling cross-correlation of Neuroticism in one sibling with Agreeableness
in the other does not differ between sibling pairs concordant and discordant
for 5-HTTLPR genotypes. This test can be implemented using the multiple

regression approach of DeFries and Fulker (1985; Gayan et al., 1999;
Knopik, Alarcon, & DeFries, 1997):
A 2 = B,N,

B2BG

where Bl is the partial regression of the second sibling's score for Agreeableness on the first sibling's score for Neuroticism, B 2 is the partial
regression of the second sibling's score for Agreeableness on B G , B 3 is the
partial regression of the second sibling's score for Agreeableness on the
product of the first sibling's score for Neuroticism and B G , and k is a
constant. B G indicates sibling sharing of genotype G, with B G = 0 for
discordant pairs (Us) and B G = 1 for concordant pairs (/// and sis). When
the data are fit to this regression model, a significant value of B 3 , the
regression coefficient corresponding to the genotype-phenotype interaction term N ^ Q , rejects the null hypothesis that the results are due to
population stratification. The regression analysis was conducted by double

1
The equations used to estimate h2 and fiJ,A are not provided here for
brevity. The computational equations and their derivation are available by
request.

300

JANG ET AL.

entry of the sibling data with the appropriate downward correction of the
degrees of freedom (DeFries & Fulker, 1985).

Results
We used varimax-rotated principal-components analyses of the
phenotypic correlation matrices to examine the phenotypic structure of the NEO-PI-R Neuroticism and Agreeableness facet scales
in each sample (Table 2). The pattern of factor and cross-factor
loadings are similar across the U.S., Canadian, and German samples and the NEO-PI-R normative sample (Costa & McCrae,
1992), suggesting the domains are factorially distinct but not
mutually exclusive. The Japanese twin sample displayed some
anomalies: The Depression facet loaded on the Agreeableness
factor and the Altruism facet loaded on the Neuroticism factor,
likely because of the relatively small size of this sample. Ono et al.
(2000) obtained a factor structure similar to the NEO-PI-R normative sample using a larger sample. Despite the obvious differences in factor structure from the Keio twins, both Neuroticism
and Agreeableness domains were clearly recognizable.
Table 3 presents the phenotypic correlations (Pearson's r) between the NEO-PI-R Neuroticism and Agreeableness scales in all
four samples. We observed several modest but significant negative
correlations between the Agreeableness facets Trust and Altruism
and most of the Neuroticism facet scales, and between the Angry
Hostility facet from the Neuroticism domain and most of the
Agreeableness facets.
The MZ and DZ intrapair correlations exceed the DZ correlations for all domain and facet scores, suggesting the presence of
heritable effects on all measures (Table 4).
The estimates of ra are presented in Table 5. The largest values
of rG were found between the Agreeableness facets Trust and
Altruism with nearly all of the Neuroticism facets and between the
Neuroticism facet Angry Hostility and the Agreeableness facets
Trust, Straightforwardness, Altruism, and Compliance in all three
samples of twins.

We estimated the association between the 5-HTTLPR polymorphisms and each of the Neuroticism and Agreeableness scores
using Pearson's r in the sample of nontwin siblings (Table 6).
Significant (p < .05) positive correlations were found between
5-HTTLPR and all the Neuroticism scores except SelfConsciousness. Significant negative correlations between
5-HTTLPR and the Agreeableness facets Trust, Straightforwardness, and Compliance and the total domain score were also found.
The corresponding values of hz due to the 5-HTTLPR ranged from
.5% to 2.8%.
Table 7 presents the values of h^A that ranged from .03 to .22.
Between the overall Neuroticism and Agreeableness domain
scores, /J^A was estimated at .10, indicating that the serotonin
transporter locus is responsible for approximately 10% of the
covariation between these personality domains as compared with
1.6% to 2.1% of the variation in either domain alone.
Regression analysis according to the equation given in the
Method section showed that the genotype-phenotype interaction
term, B 3 , was significant for the overall Neuroticism and Agreeableness scales and for many of the corresponding facet pairs
(Table 8). These findings reject the hypothesis that the association
of the 5-HTTLPR with both Neuroticism and Agreeableness is
attributable to population stratification.

Discussion
The present study estimated that up to 10% of the total covariance between scales from the Neuroticism and Agreeableness
domains is directly attributable to variations in the serotonin transporter gene. The results also show that two facets in particular,
Angry Hostility from Neuroticism and Trust from the Agreeableness domain, appear to account for most of the overlap between the
two domains. These results could not be explained as an artifact of
population stratification, but are supported by the genetic correlations that yielded the same basic patterns of interrelationship in
three independent twin samples.

Table 4
Monozygotic (MZ) and Dizygotic (DZ) Intrapair Pearson Correlations for the Revised NEO
Personality Inventory Neuroticism and Agreeableness Facet Scales for the Twin Samples
Canadian twins
Domain and scale
Neuroticism
Anxiety
Angry Hostility
Depression
Self-Consciousness
Impulsiveness
Vulnerability
Agreeableness
Trust
Straightforwardness
Altruism
Compliance
Modesty
Tender-Mindedness
**P