BEHAVIORAL GENETICS AND PERSONALITY: THE POSSIBLE
APPROACH TO UNDERSTANDING THE ETIOLOGY OF INDIVIDUAL
DIFFERENCES
Denis Bratko
Department of Psychology
Faculty of Social Sciences and Humanities
University of Zagreb, Croatia
Abstract
The paper presents the theoretical basis of behavioral genetics methods, as well as
the empirical evidence regarding individual differences in personality traits. The question
whether and how much genetic and environmental factors affect personality is one of the
possible approaches to our understanding of individual differences. Behavioral genetic
studies indicate that both genetics and environment contribute to the development of
individual differences in personality. The review of twin, family, adoption, and molecular
genetic studies of individual differences in personality traits is presented. Twin studies
indicate that genetic factors explain roughly 40-60% of the population variance in
personality traits. On the other hand, family and adoption studies consistently show lower
heritability of personality variables. Methods of molecular genetics which are used in
analysis of behavioral phenotypes are also presented. As an example of these methods,
studies of relation between novelty seeking and DRD4 gene are presented, as well as the
molecular genetic findings about the interaction of genes and environment.
Key words: personality, heritability, twin study, family study, genetics
Genetika ponašanja i ličnost: jedan od mogućih puteva za razumijevanje individualnih
razlika
Rad prikazuje teoretsku i metodološku podlogu genetike ponašanja, te empirijske nalaze
u vezi s individualnim razlikama u osobinama ličnosti. Traženje odgovora na pitanje jesu
li (i u kojoj mjeri) genetski i okolinski utjecaji povezani s ličnošću jedan je od mogućih
pristupa razumijevanju individualnih razlika. Istraživanja u području genetike ponašanja
upućuju na zaključak da individualnim razlikama u osobinama ličnosti doprinose i
genetski i okolinski faktori. Prikazan je pregled istraživanja blizanaca, obiteljskih
istraživanja, te istraživanja usvajanja u području ličnosti. Istraživanja blizanaca upućuju
na zaključak da genetski faktori objašnjavaju između 40 i 60% individualnih razlika u
osobinama ličnosti. S druge strane, obiteljske studije i studije usvajanja sustavno
pokazuju manje procjene heritabilnosti. Također, prikazane su metode molekularne
genetike koje se koriste u istraživanju bihevioralnih fenotipova. Kao primjer korištenja
tih metoda su prikazana istraživanja o odnosu između DRD4 gena i traženja novosti, te
nalazi o interakciji specifičnih gena i okolinskih utjecaja.
Ključne riječi: ličnost, heritabilnost, studija blizanaca, obiteljska studija, genetika
Behavioral genetics and personality: The possible approach to understanding the
etiology of individual differences
People differ. They differ in many different ways. Some of them are shy while
others are more assertive; some of them prefer high level of stimulation and take risks,
while others would rather choose to stay on the safe side, etc. The bottom-line question
that we can ask ourselves is: What is the origin of these differences? Why are humans so
different?
The issue of nature and nurture is one of the oldest questions in the behavioral
sciences. The modern form of this fundamental question is represented in the field of
behavioral genetics (see Plomin, De Fries & McClearn, 1990; Plomin, DeFries,
McClearn & McGuffin, 2000). Behavioral genetics includes both quantitative and
molecular genetic approaches to investigate genetic and environmental influences on
behavior. However, the questions whether and how much genetic factors affect particular
psychological dimensions represent only the first step in understanding the origin of
individual differences. Behavioral genetic research is also asking the questions about how
genetic mechanisms work (see Hamer, 2002), which part of the DNA is related to the
behavioral differences (see Plomin & Crabbe, 2000), what is the origin of behavioral
stability or change during development (see Plomin & Nesselroade, 1990; Bratko &
Butković, 2007), are genetic and environmental influences are mutually independent or
they are correlated or in interaction (see Caspi, 2003), etc.
Beside that, genetic research is changing the way we think about the environment
(for review, see Turkheimer & Waldron, 2000). Behavioral genetic research is not only
directed to genetic influences on behavior. It is equally relevant for the recognition of the
environmental contribution to the behavioral differences between individuals. Thus, the
name of the discipline, Behavioral genetics, does not exactly represent the contents of the
field. For a simple working definition of the behavioral genetic, we can state that this is a
field of study that analyzes genetic and environmental contribution to individual
differences in the behavior. In other words, behavioral genetic theory and methods
address the sources of individual differences among individuals.
Quantitative and molecular genetics: two worlds of behavioral genetics
Behavioral genetics as a discipline has two branches: quantitative and molecular
behavioral genetics. Historically, these two approaches were developed at different times,
and they use different methodology.
Theoretical foundation of quantitative genetics is Quantitative Genetic Theory
(see Falconer & MacKay, 1996). That theory represents the extension of the old Mendel's
laws of single gene inheritance to the complex multifactor behavioral traits. These
complex behavioral traits are most probably influenced by multiple genes as well as by
multiple environmental factors. For these reasons, behavioral traits are usually distributed
normally, following the well known bell-shaped curve. Even in case that particular trait is
influenced by three or four individual genes, its distribution in the population would be
close to the normal distribution (see Plomin, DeFries & Fulker, 1990).
Majority of behavioral genetic research was conducted using the quantitative
genetic approach. The logic of this research is straightforward: find genetically and
environmentally informative groups, i.e. groups in which the association between genes
and environments is distinctive, and compare their behavioral differences. The methods
of quantitative genetics, namely the twin, family and adoption study, were developed in
1920's (e.g. Merriman, 1924 in case of classical twin design), and many studies were
conducted since that time. Quantitative genetics does not study the genes directly. The
conclusions about genetic and environmental influences are drawn from the models (see
Neale & Cardon, 1992) which are based both on the quantitative genetic theory and on
the empirical evidence about the covariance between the members of the particular
groups, for example identical or fraternal twins, members of the biological or nonbiological (adoptive) families. Thus, the goal of quantitative genetics is to decompose the
observed individual variation into genetic and environmental sources of variation. It is
important to emphasize that genetic and environmental effect refers to the portion of the
population variance that may be explained by genetic or environmental differences in
population. Thus, genetic effect on the population variance does not imply genetic
determinism on individual level.
On the other hand, molecular behavioral genetics uses the recent advances in
biology, especially molecular genetic techniques which caused the revolution in the field.
Molecular genetic techniques study the DNA directly (see Watson, 2002, Craig &
McClay, 2002; Sham, 2002). The focus of molecular behavioral genetics is to find
particular genes which are related to individual differences, e.g. using the linkage or
allelic association methods. The goal of the molecular genetic approach is to find some of
many genes that contributing to the variance of the trait with varying effect sizes (Plomin
& Caspi, 1998; Plomin & Rutter, 1998). A huge research effort is currently focused on
finding genes associated with behavior, but still with limited success. Beside that, it is
equally important to understand how genes affect behavior. We don't know the fine
details of that process yet. However, it is certain that a model which would imply that one
gene causes one behavior is totally inadequate. Complex behaviors are most likely
influenced by multiple genetic and environmental influences, which are mediated in a
brain, and which change over time (Hamer, 2002).
Genetic differences, environmental differences, and behavioral differences
The Human Genome Project - probably the most influential scientific project in
genetics - came up with the several unexpected findings. The first surprise was related to
the estimate of the number of genes in the human genotype. It seems that there are fewer
genes in the human genotype than it was previously estimated. While previous estimates
were around 100 000 (McKusick, 1986), new data indicate that the number of genes in
the human genotype is roughly around 25-30 000 (Plomin & Crabbe, 2000). Beside that,
only 2% of the human DNA code involves gene in a traditional sense - that it is the gene
that codes for an amino acid sequence. The function of the vast majority of DNA code is
not clear yet. Genetic differences among people are extremely small. The DNA code is
the sequence of nucleotide bases (A, T, G, and C). However, the estimate is that 99, 9%
of these DNA sequences are the same for all people. Thus, genetic differences that are
interesting for behavioral genetics are extremely rare if we compare these differences
with the total number of DNA bases. However, the consequences of small genetic
differences can be extremely large at the phenotypic level. Human genotype is very
similar to the genotype of mice, or even genotype of the flies. However, humans can not
fly and they don't buzz around. The same reasoning applies for environmental differences
and their consequences. Small environmental differences among people can lead to large
behavioral differences.
Personality is a very complex phenotype
Behavioral genetics has studied personality almost exclusively in terms of
psychological trait theory. The reason for that is the nature of the concepts that are central
in these theories. Trait theory uses concepts of individual differences which are more
suitable for behavior genetic analysis than some other concepts, like process concepts
such as classical or instrumental conditioning. However, several competing trait models
and measures of personality do exist. Behavioral genetic studies usually use Eysenck's
PEN model (Eysenck & Eysenck, 1992), Costa's & McCrae's Five-factor model (Costa &
McCrae, 1992), or Tellegen Three-factor model (Tellegen, 1985; Tellegen, Lykken,
Bouchard, Wilcox, & Rich, 1988). On the other hand, Cloninger's psychobiological
model was particularly useful in the molecular genetic studies of personality (Cloninger,
Przybeck & Svrakic, 1991; Cloninger, Svrakic & Przybeck, 1993).
Genetic methodologies require a clear definition of the phenotype. For example,
medical genetics insists on the clear definition of the disorder when studying diseases.
Any misdiagnosis due to the overlapping diagnostic criteria alters the results of the
genetic study. However, personality as a phenotype is not very well defined. Sometimes
it looks more as “the elusive phenotype". Of course, the unclear definition of personality
does not help the behavioral genetic studies. How can we find the association between
DNA variation and basic personality dimensions when these basic personality dimensions
are different in different models? Beside that, same traits have sometimes different
position in various systems. For example, impulsivity is the facet of Neuroticism in
Costa's & McCrae (1992) Five-factor model, while Neuroticism measure from Eysenck
(1992) theory is lacking that behavior. Impulsivity moves from extraversion in the first
version of the theory to the psychoticism factor in the last version of Eysenck theory. In
MPQ three-factor model (Tellegen, 1985) impulsivity is part of Constraint factor, while
in Cloninger's psychobiological model (Cloninger, Svrakic & Przybeck, 1993) impulsive
behavior taps to the Novelty seeking scale. Thus, possible genetic effect on impulsive
behavior would obviously contribute to the same effect in conceptually very different
dimensions.
Heritability and environmentality
We are likely to observe a wide range of individual differences in personality, no
matter how personality is measured or which model is used. These differences may be
caused by genetic differences between individuals as well as by environmental
experiences. Components-of-variance approach that is used in quantitative genetics
examines the overall net effects of genetic and environmental influences. Heritability is a
statistic that describes the effect size of genetic influence. Heritability reflects the relative
contributions of genetic and environmental factors to differences in behavior between
individual members of a population (Erdle, 1990). The heritability concept does not
reflect the relative contribution of genetic and environmental factors on absolute levels of
behavior of one particular person. It is simply a statistic that describes the ratio of genetic
to phenotypic variance. Similarly, environmentality is a statistic that describes the effect
size of non-genetic influence or environmental influence (see also Wade & Tavris, 1987
p. 440). The simple formula expresses the idea of heritability concept:
Heritability = Vg/Vp,
where Vg is hypothetical population variance of genotypes, and Vp is observed
population variance of a particular measured behavior.
The concept of heritability is frequently misunderstood. Heritability is a
population parameter. It does not refer to one individual. For example, heritability of
weight is roughly 70 percent (Plomin & Colledge, 2001). That means that 70 percent of
variation observed in a particular population at this time is due to genetic differences. It
obviously does not mean that person who weighs 100 kilos has gained 70 kilos due to
genes and that other 30 were added by the environment. The problem arises from the fact
that almost every aspect of human behavior can be analyzed from the behavioral genetic
perspective, while, obviously, not every human behavior reflects some meaningful
concept. For example, Schönemann (1994) has shown that the conventional heritability
estimates can produce absurdly high values for variables that cannot possibly be genetic.
He used the results of Loehlin and Nichols (1974) study where question "Did you take a
bubble bath last year" has heritability of 90%. Thus, it is important to be aware of the fact
that behavioral phenotypes may be very complex and that behavioral genetic analysis
should be applied only to theoretically meaningful psychological concepts.
It is important to be aware of the limitations of the heritability concept. Since
heritability is a population parameter it can be different in different populations, and even
in the same population at different times. As an example of the concept, several
heritability estimates are given: fingerprint ridge count, which is a quantitative measure
of a fingerprint, 97%, systolic blood pressure 64%, heart rate 49% (Bouchard, 1990);
height 90%, weight 60 % (Plomin, DeFries, McClearn, & McGuffin, 2001), etc.
Heritability of personality traits is one of the goals of behavioral genetic studies.
However, behavioral genetic research moves far beyond that simple question and asks
questions about the nature-nurture interplay in the development of behavioral
characteristics, about the genetic mechanism involved in personality development, and
also, about the environmental contribution to individual differences. Although it may
look as a paradox, behavioral genetic methods provide the best insight into environmental
contribution to the development of individual differences. How do genetic designs
estimate the effects of the environment? Since genetic effects are controlled or measured,
environmental variance is simply defined as the variance not explained by genetics. Thus,
environmental effect can be estimated in a same way as a genetic effect, as a proportion
of the phenotypic variance:
Environmentality = Ve/Vp,
where Ve is hypothetical population variance of environmental influences, and Vp
is observed population variance of a particular measured behavior. The important
theoretical distinction is made between shared environmental influences which estimate
the net effect of influences which contributing to the similarity of family members, and
non-shared environmental influences which do not contribute to the similarities
(Turkheimer & Waldron, 2000; Stoolmiller, 1999; Hoffman, 1991; Baker & Daniels,
1990; Plomin, Chipuer & Neiderhisser, 1994).There is a lot of evidence indicating that
environmental influences explain a substantial portion of phenotypic variance in
personality traits, and that environment contributes more to differences of individuals
who live together than to their similarities (Plomin & Daniels, 1987; Plomin & Dunn,
1990; Rose, 1988; Loehlin, 1992).
The twin studies of personality
Twin study is the most popular behavioral genetic design. There are hundreds of
twin studies of personality. Most of them use the data from twins reared together.
However, the data from few studies of the twin reared apart are also available (Shields,
1962; Langenvainio et al., 1984; Tellegen et al., 1988; Pedersen et al, 1988; 1991;
Bouchard, Lykken, McGue, Segal, & Tellegen, 1990; Bergman et al. 1993).
Identical twins result from a single zygote that divides for unknown reasons. They
share, except in case of rare mutations, exactly the same genes. Expressed as a correlation
coefficient, their genetic similarity would be 1.00. Fraternal twins, as any other first
degree relatives, share half of their genes on average. Thus, their genetic similarity would
be 0.5. It should be noted that this is true only for genes which are located on
chromosomes. Mitochondria have their own DNA. For reasons no one has figured out
yet, all humans inherit only their mother's mitochondria genes. Thus, a fraternal twin has
identical mitochondria genes, and because of that twin studies investigate only the effect
of chromosomal genes.
The essence of the twin method is to investigate the evidence of genetic influence
by comparing identical and fraternal twins' similarities on a variable of interest. If
identical twins are more similar than fraternal twins, the particular trait is influenced by
genetic factors. Heritability estimate from twins reared together is obtained by simply
doubling the difference between identical and fraternal twins' correlations (see Plomin et
al. 1990). Beside that, path analytical procedures used to compare twin similarities are
also designed (see Neale & Cardon, 1992).
Although some peer-rating data recently became available (e.g. Riemann,
Angleitner and Strelau, 1997; Heath, Neale, Kessler, Eaves, & Kendler, 1992), the vast
majority of twin research on personality involves self-report questionnaires administered
to adolescents and adults. Results of these studies converge on two conclusions that are
now generally accepted in the field of personality. First, most of the personality traits are
moderately heritable. Identical twin correlations are greater than fraternal twin
correlations. This finding is consistent across different populations as well as across
different age of the participants or the instruments which are used for measurement of
personality. Second, there is no replicated evidence about differential heritability of
personality traits (Loehlin, 1982). Traits at different level of hierarchy are also heritable,
supporting the idea of hierarchical organization of human personality traits. At least two
studies have shown that heritability of traits of the Five-five factor model (FFM, Costa &
McCrae, 1992) is substantial, both at facets as well as at the factor level (Jang, McCrae,
Angleitner, Riemann, & Livesley, 1998; Loehlin, McCrae, Costa, & John, 1998). Even
more, there is empirical evidence that higher-order factors, known as “alpha” (aggregated
agreeableness, conscientiousness, and emotional stability) and “beta” (aggregated
extraversion and intellect) also show similar heritabilities (Jang et al., 2006). Beside that,
estimates of genetic and environmental effects are similar in different cultures, indicating
universal biological basis of FFM model in human species (Yamagata et al., 2006).
Although common sense would lead to a hypothesis that some traits are more
heritable than others, i.e. temperamental vs. character traits, it is simple not the case (see
also Onisczenko, Zawadzki, Strelau, Riemann, Angleitner, & Spinath, 2003; Rowe,
1997). No matter which traits are measured, we are likely to obtain the similar results.
For twins reared together, the typical pattern of correlations is from 0.40 to 0.60 for
identical and from 0.15 to .30 for fraternal twins (Plomin et al. 2001). Typical heritability
estimates from twin studies vary between 40 and 60 percent (Bouchard & Loehlin, 2001).
Several meta-analyses of the published twin correlations organized according to the
Eysenck's three-factor model (Eaves, Eysenck & Martin, 1989; Eysenck, 1990) as well as
according to the Big Five model are available (Loehlin, 1992; Bouchard, 1993; Johnson,
Vernon, & Feiler, in press).
Bouchard (1993) reports the mean intraclass correlations for MZ and DZ twins
according to the Eysenck PEN model. Intraclass correlation for MZ twins were .53, .44,
and .46 for extraversion, neuroticism, and psychoticism, respectively. . Intraclass
correlation for DZ twins were .24, .22, and .23 for extraversion, neuroticism, and
psychoticism, respectively. Doubling the difference between identical and fraternal twin
correlations gives estimates of heritability of 58, 44, and 46 percent for dimensions of
extraversion, neuroticism, and psychoticism, respectively. Organization of personality
according to the Big Five model yield similar results. . Intraclass correlation for MZ
twins were .52, .51, .44, .49 and .43 for extraversion, neuroticism, conscientiousness,
agreeableness and openness, respectively. Intraclass correlation for DZ twins were .25,
.22, .24, .23 and .19 for extraversion, neuroticism, conscientiousness, agreeableness and
openness, respectively. Doubling the difference between identical and fraternal twin
correlations gives estimates of heritability for the Big Five factors from 40 to 58 percent.
These older studies were carried out before the development of the NEO-PI-R and
NEO-FFI scales (Costa & McCrae, 1992), probably the most popular measures of the
Five-factor model. Two more recent twin studies (Jang et al., 1996; Riemann, Angleitner
and Strelau, 1997) using those scales yielded similar results (Table 1).
Table 1 about here
Family and adoption studies of personality
Surprisingly, family and adoption designs were used less often than the twin
design in behavioral genetic research on personality. Of course, phenotypic similarity of
the members of a nuclear family living together could be viewed only as compatible with
the genetic hypothesis, not as a proof of genetic influence. Since parents and offspring
share environment as well as heredity, genetic and shared environmental influences are
mixed in that design. However, family studies can be very useful because they provide an
upper-limit estimate of heritability obtained by doubling the correlations between parents
and offspring's. Similarly, heritability from adoption studies can be estimated by doubling
the correlations between biological parents and offspring.
The results of family and adoption studies are quite different from the results of
twin studies. Most family studies of personality have yielded relatively low parentoffspring correlations (Ahern, Johnson, Wilson, McClearn, & Vandeberg; 1982; Bratko
& Butković, 2003; Bratko & Marušić, 1997; Carmichael & McGue, 1994; Loehlin,
Willerman & Horn, 1985; Price, Vandeberg, Iyer, & Williams, 1982; Tambs, Sundet,
Eaves, Solaas, & Berg, 1991). For example, Ahern et al. (1982) in a widely cited Hawaii
Family Study reported parent-offspring correlations for 54 personality scales derived
from five questionnaires. The average parent-offspring correlation was .12. Carmichael &
McGue (1994) summarized the studies examining parent-offspring resemblance on
extraversion and neuroticism personality dimensions. Weighted average parent-offspring
correlation for 22 extraversion scales was .13, while for 19 neuroticism scales it was .12.
Loehlin (2005) compared upper-limit of heritability from family design with the
heritability estimates from twin studies for the Big five personality domains. The upperlimit of heritability estimates from family studies were .28, .22, .18, .26, and .34, for
Extraversion, Agreeableness, Conscientiousness, Neuroticism, and Openness,
respectively. The heritability estimates from the twin studies were .49, .35, .38, .41, and
.45, respectively. Thus, the average upper-limit of heritability from family design was
.26, whereas the average heritability estimate from twin studies was .42.
Correlations from family studies are very similar to correlations from the adoption
studies. Consider two broad personality dimensions, Extraversion and Neuroticism, as an
example. Similarly to above mentioned correlations between biological parents and their
living-together offspring, biological parents and their adopted-away offspring correlate
.16 for extraversion and .13 for neuroticism (Loehlin, 1992). On the other hand, adoptive
parents-adoptee correlation is not significantly different from zero, suggesting that
sharing the same environment does not contribute to the personality resemblance for
extraversion and neuroticism (Loehlin, Willerman and Horn, 1982; Loehlin, Willerman
and Horn, 1985; Plomin, DeFries and Fulker, 1988).
Results of family and adoption studies of personality traits suggest two
conclusions. First, heritability estimates are lower than those from twin studies. One of
the possible explanations of the low parent-offspring resemblance is the role of the
nonaditive genetic effect. Family and adoption studies capture only the genetic effects
which run in the family. However, it is possible that interaction of genes from different
loci also contributes to individual difference. These genetic effects would not contribute
to the family resemblance. Second, the correlations of first-degree relatives that share
family environment are almost identical to the correlations of those relatives that live
apart. Thus, the family environment does not contribute to the family resemblance.
Model-fitting of the world personality data
Twin, personality and adoption study converge on the conclusion that genetic as
well as nonshared environment substantially contribute to individual differences in
personality. On the other hand, the contribution of the shared environment is obviously
very weak, if any. However, the effect size of the genetic influence estimated from twin
studies differ from the estimates of family and adoption studies. One way to resolve this
puzzle is to analyze data from different behavioral genetic designs simultaneously using
the model-fitting approach. John Loehlin (1992) in a heroic enterprise analyzed all of the
published behavioral genetic studies organized according to the Big five model. He
estimated the heritability of .49, .41, .35, .38 and .45 for extraversion, neuroticism,
agreeableness, conscientiousness, and culture, respectively. The environmental
influences, including the measurement error, were also significant. However, the
important environmental influences were those of nonshared variety, which do not
contribute to the similarity of individuals who live together. Results of this meta-analysis
show that heritability of Big five personality traits is .49, .41, .35, .38, and .45 for
extraversion, agreeableness, conscientiousness, neuroticism and culture, respectively. The
effect size for shared environmental component of variance is .02, .07, .11, .07 and .06,
while for nonshared environmental component of variance is .49, .52, .54, .55 and .59 for
extraversion, agreeableness, conscientiousness, neuroticism and culture, respectively.
More recently Johnson, Vernon, & Feiler (in press) made a review of over 50
years of behavioral genetic research on the Big five and related personality traits. They
identified 145 studies which reported twin and other kinship correlations and estimated
the heritability and environmentality via model-fitting methods. In total, these studies
were based on data collected from 85,640 pairs of MZ twins, 106,644 pairs of DZ twins,
and 46, 215 pairs of other non-twin kinships, such as parents and their children and nontwin siblings. The results showed that individual differences in the Big five were
approximately equally attributable to genetic and nonshared environmental factors while
influence of the shared environment was essentially nonexistent. The mean heritability of
personality traits in this meta-analysis was .45.
Molecular genetics and personality
Development in quantitative genetics over the past decades demonstrated
moderate yet consistent genetic contribution to individual differences in personality.
However, the general statement that "genetic factors" contribute to individual differences
tells us little about the causal mechanisms which are responsible for genetic influence on
personality. Molecular genetic techniques are designed to answer the question which
genetic differences matter or how particular gene variants affect pathways that influence
phenotypes.
The complex phenotypes like personality are influenced by many genes
interacting with many environmental factors over time (see Plomin, Owen & McGuffin,
1994). Thus, it is very difficult to locate any particular DNA variation which will explain
substantial part of the personality variance. Genes for complex traits influenced by
multiple genes and multiple environmental influences are called QTL (quantitative trait
loci). The goal of the QTL approach is to find some of many genes which contribute to
the heritability of the trait (Plomin & Crabbe, 2000).
The molecular genetic studies usually use one of the two methods: linkage
method, which tracks the inheritance of DNA segments in families; and allelic
association method which refers to the correlation between alleles of a DNA marker, and
trait scores across unrelated individuals. In association study, researchers start with a
gene they have reason to believe may be important for particular trait and see if they can
establish the association between this "candidate gene" and trait of interest. As an
example of the molecular genetic approach few studies about the association between
trait novelty seeking and DRD4 marker will be presented.
Novelty seeking and DRD4
Novelty seeking is one of the four traits in a Cloninger (1986) psychobiological
model of temperament. Individuals high on that trait are impulsive, excitable, quicktempered, prone to explanatory behavior, etc. Although psychometrically not perfect, and
conceptually similar to the older Zuckerman sensation seeking trait (Zuckerman, 1979),
novelty seeking became very popular in the molecular genetic studies of personality. It
was predicted that metabolism of neurotransmitter dopamine is related to individual
differences in the novelty seeking (see also Plomin & Caspi, 1999). The DRD4 marker
exists in two forms. The first form is a short form which codes for a receptor that is more
efficient in binding dopamine. The second form is a long form which is less efficient. The
theory predicts that individuals with long repeat DRD4 alleles are dopamine deficient and
seek novelty to increase dopamine release. The first association between DRD4 and
novelty seeking was reported more than ten years ago (Ebstein, Novick & Umansky,
1996; Benjamin, Greenberg & Murphy, 1996). However, the replication studies
conducted since that time yielded mixed results. Recently, Kluger, Siegfried & Ebstein
(2002) conducted a meta-analysis of 20 studies of the association between DRD4
polymorphisms and novelty seeking (see also Schinka, Letsch & Crawford, 2002).
Thirteen reports suggested that a presence of a longer allele is associated with higher
novelty seeking scores while seven studies yielded different results. On average, there
was no conclusive evidence that association between DRD4 and novelty seeking exists.
However, molecular genetic approach will certainly be the focus of future studies.
Especially important line of research is identification of the gene-environment interaction
in the development of behavioral individual differences. Environment can have
differential influence on humans who are genetically different. For example, Avshalom
Caspi and his colleagues, in a fascinated series of studies, demonstrated that probability
of a trans-generational transmission of violence depended on MAOA gene variant that
was present in an individual (Caspi et al. 2002). Also, probability of depressive
symptoms as a reaction to the stress depended on s certain form of 5-HTT allele (Caspi et
al. 2003). Thus, it is obvious that genes do not determine our destiny; they rather interact
with our environment in very complex ways. We do not yet understand these interactions,
but behavioral genetic theory and methods certainly provide conceptual and
methodological ground for our understanding of the etiology of individual differences.
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Table 1. Mean intraclass correlations of identical and fraternal twins for Five Factor
model (Costa & McCrae, 1992)
Jang et al. (1996)
Riemman et al.(1997)
Dimension
rMZ
rDZ
rMZ
rDZ
Extraversion
.41
.18
.55
.28
Agreeableness
.55
.23
.53
.13
Conscientiousness
.58
.21
.42
.19
Neuroticism
.41
.26
.54
.18
Openness
.37
.27
.54
.35