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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 non-biological (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 parent-offspring 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 upper-limit 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 non-twin 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, quick-tempered, 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 |

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