Genetic Studies Of Personality Research Paper

1. Behavioral Genetic Studies Of Personality

Twin, family, and adoption studies have been the primary source of information on the relative importance of genetic effects (‘heritability’) for personality. Behavioral geneticists study the similarity of individuals who share genes and environments to different degrees to quantify heritability. In the classical twin study, identical twins share all of their genes in common whereas fraternal twins share on average only half of their segregating genes in common. Thus, if genetic effects are important for a personality trait, identical twin pairs should be twice as similar as fraternal twin pairs. Similarity between parents and offspring or among siblings may also indicate the influence of genetic effects. The importance of the shared rearing environment can be evaluated by assessing the similarity of adoptees and their adoptive parents. Environmental influences can also be individual specific, or ‘non-shared,’ i.e., contributing to differences among family members. Behavioral geneticists typically apply structural equation modeling techniques to paper and pencil self-reports of personality when estimating heritability and the importance of shared and non-shared environmental influences.

1.1 Univariate Results

Most genetic studies of personality are based on very large twin and family samples. Personality has been assessed on over 50,000 pairs of twins in the national twin registries in Australia, Finland, Norway, Sweden, and The Netherlands, on volunteer twin registries in the USA, and on the 30,000 individuals in the Virginia study of twins and their families. Numerous smaller twin, family, and adoption studies have also been performed. Many of the studies (particularly those in the national twin registries) have assessed personality, using versions of the Eysenck Personality Inventory. However, during the 1980s and 1990s increasing amounts of data have become available concerning other personality inventories, such as the Five-Factor Model of Costa and McCrae (McCrae and Costa 1987) and the Seven-Factor Model of Cloninger (Cloninger 1998).

Remarkably, there is considerable consensus across studies and cultures. Genetic effects account for on average 35–40 percent of the variation in personality traits such as neuroticism and extraversion. (In other words, the heritability of these traits is 0.35–0.40.) Evidence from studies of twins reared apart and adoption studies indicate that shared environmental influences account for another 5–10 percent of the variation in most personality traits, and the remaining variation is individual specific or ‘non-shared’ environmental variation. In other words, people differ for personality. Most of the individual differences we observe are due to individual specific experiences not shared by other family members and about 40 percent of the differences are due to genetic differences among us. The extent to which family members are similar to one another reflects primarily their genetic similarity, and only in part being raised in the same home.

There is some heterogeneity in genetic effects on personality. Traits related to extraversion, such as impulsivity, openness to experience, and sensation seeking tend to have slightly higher heritability estimates than those related to neuroticism and instability. Furthermore, the genetic influences for the extraversion-related traits may in part be operating in a nonadditive manner. That is, there may be inter- actions within or between genetic loci, such that the effect of one gene may depend on another gene. The latter effect, known as epistasis, may complicate efforts to identify specific genes that are of importance for personality traits.

Heritability estimates may be on average 40 percent for most personality traits, but there are traits for which genetic effects are of less importance. Examples of these are ‘agreeableness,’ ‘guilt,’ feelings of control over ‘luck,’ and ‘cynic hostility.’ For these measures, familial similarity is primarily due to environmental influences shared by family members rather than genetic similarity.

1.2 Genetic Influences On Stability And Change

There has been a great deal of controversy as to whether personality is stable throughout the lifespan. Some developers of personality inventories argued that finding significant heritability for personality would support hypotheses of stability in personality. Their argument rested on the misconception that if genetic effects are important they are immutable and do not change. Despite the fact that humans are born with a full complement of genes, not all genes may be operating at all phases in life. Some genes turn on and others turn off at various transitional periods, while others may react dependent on external stressors. On the other hand, some genes may have a long-term effect that is stable across the lifespan. Thus, the issue of genetic contributions to stability for personality is empirically testable.

During infancy, there is less stability in temperament than is typically reported for adulthood. Nevertheless, results from longitudinal twin studies indicate that the moderate stability observed for infant temperament is influenced in part by genetic effects. During childhood, genetic effects are also important contributors to phenotypic (i.e., observable) stability. Longitudinal studies of adult twins confirm findings of substantial phenotypic stability in personality. Furthermore, the genetic effects themselves are very stable, i.e., the same genes appear to be influencing personality throughout adulthood. Nonetheless, genetic and environmental influences are equally important for phenotypic stability. Even though the genetic effects themselves are very stable, they are relatively less important than environmental effects, and thus the net influence on stability is reduced. Environmental effects are the most important influences on change. Thus, genetic influences on personality contribute to stability rather than to change. However not all personality stability can be explained by genetic influences alone.

1.3 Structure Of Personality

Developments in theories of personality have been paralleled with developments in statistical analyses of genetically informative data. During the 1980s and 1990s, the three-factor approach to personality was gradually extended to the five-factor theory and the seven-factor theory (McCrae and Costa 1987, Cloninger 1998). The factor analytic approach generally applied at the phenotypic level is also applicable at the level of genetic and environmental influences. The question to be addressed is whether there is a genetic factor structure or an environmental factor structure that is similar to the phenotypic factor structure. Do genetic or environmental effects mediate the associations among various aspects of personality? This line of analysis has only recently been applied and promises to be an important area for future research concerning the structure of personality.

1.4 Genetic Mediation Of Associations With Personality

Health psychologists have demonstrated that personality is important for health outcomes (such as cardiovascular disease, cancer, and osteoarthrosis) and response to treatment such as recovery from surgery. Current research efforts are focusing on the extent to which genetic and environmental effects mediate the associations between personality and health outcomes. By evaluating these associations, much can be learned about the mechanisms by which personality affects health. For example, neuroticism in young adulthood is predictive of joint pain and osteoarthritic symptoms some 25 years later in midlife. Genetic influences in common to neuroticism and joint pain and shared familial environmental influences mediate this association. There are no genetic influences unique to joint pain. Because neuroticism preceded the joint pain, it is likely that genetic effects are important for neuroticism which in turn influences joint pain and osteoarthritic symptoms through response to stress, immune, and inflammatory processes (Turk-Charles et al. 1999).

2. Molecular Genetic Associations With Personality Traits

The second major approach to the study of genetic influences on personality is to test whether specific genetic variants, or alleles, are associated with mean level differences in personality traits. Because personality traits are most often continuous measures and heritability analyses suggest that these traits are polygenic, the challenge is to find genes of medium effect size that influence personality levels. These genes are known as Quantitative Trait Loci or QTLs. Most efforts to identify which genes may be associated with personality traits are currently focusing on genes involved in neurotransmission, such as dopamine receptor genes and serotonin receptor and transporter genes. Many of the findings are only preliminary, and often with contrasting results or lack of replication in other studies or populations.

Finding genes that have relatively small effects will present great challenges to behavioral and genetic scientists, for several reasons. First, because many genes are likely to be involved, each with a small effect, very large sample sizes will be necessary to obtain sufficient statistical power. Second, many of the genes may be interacting with other genes, a phenomenon known as epistasis. For example, a particular allele for gene A may only influence personality negatively if a specific variant of gene B is present. Preliminary findings from large population-based studies suggest that some epistasis exists for personality and genes in the dopaminergic system. Third, there may well be substantial interactions between genes and environments that hamper the potential to find genotypic or even environmental associations. The impact of negative life events is greater in individuals with a genetic susceptibility to major depression, suggesting an interaction between genetic and environmental influences. Similarly, the effects of some allele may be enhanced or reduced depending on environmental circumstances. Again, large samples with genetic and detailed environmental characterization will be necessary to delineate the respective roles of genes and environments. In this fashion, scientists will come closer to understanding not only how much, but how genes influence personality.

3. Future Directions

Behavioral genetic analyses have provided a firm foundation for further studies on genetic influences on personality. Genetic influences are indeed important, and appear to be important for stability rather than change in personality. Further analyses of the genetic and environmental architecture of personality will expand our understanding of the associations within and among domains, such as the structure of personality and how personality influences health and other behavioral outcomes. Now that the Human Genome Project has provided a genetic map, the challenge to scientists will be to understand the function of the residents at the 30,000 addresses. Many genes, each of small effect, in interaction with each other and in interaction with the environment, are likely to influence variation in normal personality.

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