Neurobiology of the structure of personality: dopamine, facilitation of incentive motivation, and extraversion

Extraversion has two central characteristics: (1) interpersonal engagement, which consists of affiliation (enjoying and valuing close interpersonal bonds, being warm and affectionate) and agency (being socially dominant, enjoying leadership roles, being assertive, being exhibitionistic, and having a sense of potency in accomplishing goals) and (2) impulsivity, which emerges from the interaction of extraversion and a second, independent trait (constraint). Agency is a more general motivational disposition that includes dominance, ambition, mastery, efficacy, and achievement. Positive affect (a combination of positive feelings and motivation) is closely associated with extraversion. Extraversion is accordingly based on positive incentive motivation. Parallels between extraversion (particularly its agency component) and a mammalian behavioral approach system based on positive incentive motivation implicate a neuroanatomical network and modulatory neurotransmitters in the processing of incentive motivation. A corticolimbic-striatal-thalamic network (1) integrates the salient incentive context in the medial orbital cortex, amygdala, and hippocampus; (2) encodes the intensity of incentive stimuli in a motive circuit composed of the nucleus accumbens, ventral pallidum, and ventral tegmental area dopamine projection system; and (3) creates an incentive motivational state that can be transmitted to the motor system. Individual differences in the functioning of this network arise from functional variation in the ventral tegmental area dopamine projections, which are directly involved in coding the intensity of incentive motivation. The animal evidence suggests that there are three neurodevelopmental sources of individual differences in dopamine: genetic, "experience-expectant," and "experience-dependent." Individual differences in dopamine promote variation in the heterosynaptic plasticity that enhances the connection between incentive context and incentive motivation and behavior. Our psychobiological threshold model explains the effects of individual differences in dopamine transmission on behavior, and their relation to personality traits is discussed.

Genetic influences on impulsivity, risk taking, stress responsivity and vulnerability to drug abuse and addiction

Genetic variation may partially underlie complex personality and physiological traits--such as impulsivity, risk taking and stress responsivity--as well as a substantial proportion of vulnerability to addictive diseases. Furthermore, personality and physiological traits themselves may differentially affect the various stages of addiction, defined chronologically as initiation of drug use, regular drug use, addiction/dependence and potentially relapse. Here we focus on recent approaches to the study of genetic variation in these personality and physiological traits, and their influence on and interaction with addictive diseases.

Fitness consequences of avian personalities in a fluctuating environment

Individual animals differ in the way they cope with challenges in their environment, comparable with variation in human personalities. The proximate basis of variation in personality traits has received considerable attention, and one general finding is that personality traits have a substantial genetic basis. This poses the question of how variation in personality is maintained in natural populations. We show that selection on a personality trait with high heritability fluctuates across years within a natural bird population. Annual adult survival was related to this personality trait (behaviour in novel environments) but the effects were always opposite for males and females, and reversed between years. The number of offspring surviving to breeding was also related to their parents' personalities, and again selection changed between years. The observed annual changes in selection pressures coincided with changes in environmental conditions (masting of beeches) that affect the competitive regimes of the birds. We expect that the observed fluctuations in environmental factors lead to fluctuations in competition for space and food, and these, in association with variations in population density, lead to a variation in selection pressure, which maintains genetic variation in personalities.

Meta-analysis of genetic association studies

Meta-analysis, a statistical tool for combining results across studies, is becoming popular as a method for resolving discrepancies in genetic association studies. Persistent difficulties in obtaining robust, replicable results in genetic association studies are almost certainly because genetic effects are small, requiring studies with many thousands of subjects to be detected. In this article, we describe how meta-analysis works and consider whether it will solve the problem of underpowered studies or whether it is another affliction visited by statisticians on geneticists. We show that meta-analysis has been successful in revealing unexpected sources of heterogeneity, such as publication bias. If heterogeneity is adequately recognized and taken into account, meta-analysis can confirm the involvement of a genetic variant, but it is not a substitute for an adequately powered primary study.

Genes, evolution, and personality

There is abundant evidence, some of it reviewed in this paper, that personality traits are substantially influenced by the genes. Much remains to be understood about how and why this is the case. We argue that placing the behavior genetics of personality in the context of epidemiology, evolutionary psychology, and neighboring psychological domains such as interests and attitudes should help lead to new insights. We suggest that important methodological advances, such as measuring traits from multiple viewpoints, using large samples, and analyzing data by modern multivariate techniques, have already led to major changes in our view of such perennial puzzles as the role of "unshared environment" in personality. In the long run, but not yet, approaches via molecular genetics and brain physiology may also make decisive contributions to understanding the heritability of personality traits. We conclude that the behavior genetics of personality is alive and flourishing but that there remains ample scope for new growth and that much social science research is seriously compromised if it does not incorporate genetic variation in its explanatory models.

The genetic basis of complex human behaviors

Quantitative genetic research has built a strong case for the importance of genetic factors in many complex behavioral disorders and dimensions in the domains of psychopathology, personality, and cognitive abilities. Quantitative genetics can also provide an empirical guide and a conceptual framework for the application of molecular genetics. The success of molecular genetics in elucidating the genetic basis of behavioral disorders has largely relied on a reductionistic one gene, one disorder (OGOD) approach in which a single gene is necessary and sufficient to develop a disorder. In contrast, a quantitative trait loci (QTL) approach involves the search for multiple genes, each of which is neither necessary nor sufficient for the development of a trait. The OGOD and QTL approaches have both advantages and disadvantages for identifying genes that affect complex human behaviors.

Personality and Major Depression

CONTEXT

Prior studies suggest that the personality traits of neuroticism and extroversion may be related to the liability to major depression (MD).

OBJECTIVE

To clarify the magnitude and nature of the association between neuroticism and extroversion and the risk for MD.

DESIGN

Longitudinal population-based twin cohort.

SETTING

General community.

PARTICIPANTS

A total of 20 692 members of same-sex twin pairs from the population-based Swedish Twin Registry who completed a self-report questionnaire assessing neuroticism and extroversion in 1972 and 1973 and were personally interviewed for lifetime history of MD more than 25 years later.Main Outcome Measure Lifetime history of modified DSM-IV MD.

RESULTS

Levels of neuroticism strongly predicted the risks for both lifetime and new-onset MD. Twin modeling indicated that the association between neuroticism and MD resulted largely from shared genetic risk factors, with a genetic correlation of +0.46 to +0.47. Levels of extroversion were weakly and inversely related to the risks for lifetime and new-onset MD. This effect disappeared when we controlled for the level of neuroticism. Twin modeling produced similar results.

CONCLUSIONS

Results from both longitudinal and genetic analyses support the hypothesis that neuroticism strongly reflects the liability to MD. This association arises largely because neuroticism indexes the genetic risk for depressive illness. However, substantial proportions of the genetic vulnerability to MD are not reflected in neuroticism. By contrast, extroversion is only weakly related to risk for MD.

Personality-dependent dispersal: characterization, ontogeny and consequences for spatially structured populations

Dispersal is one of the most fundamental components of ecology, and affects processes as diverse as population growth, metapopulation dynamics, gene flow and adaptation. Although the act of moving from one habitat to another entails major costs to the disperser, empirical and theoretical studies suggest that these costs can be reduced by having morphological, physiological or behavioural specializations for dispersal. A few recent studies on different systems showed that individuals exhibit personality-dependent dispersal, meaning that dispersal tendency is associated with boldness, sociability or aggressiveness. Indeed, in several species, dispersers not only develop behavioural differences at the onset of dispersal, but display these behavioural characteristics through their life cycle. While personality-dependent dispersal has been demonstrated in only a few species, we believe that it is a widespread phenomenon with important ecological consequences. Here, we review the evidence for behavioural differences between dispersers and residents, to what extent they constitute personalities. We also examine how a link between personality traits and dispersal behaviours can be produced and how personality-dependent dispersal affects the dynamics of metapopulations and biological invasions. Finally, we suggest future research directions for population biologists, behavioural ecologists and conservation biologists such as how the direction and the strength of the relationship between personality traits and dispersal vary with ecological contexts.

The evolution of personality variation in humans and other animals

A comprehensive evolutionary framework for understanding the maintenance of heritable behavioral variation in humans is yet to be developed. Some evolutionary psychologists have argued that heritable variation will not be found in important, fitness-relevant characteristics because of the winnowing effect of natural selection. This article propounds the opposite view. Heritable variation is ubiquitous in all species, and there are a number of frameworks for understanding its persistence. The author argues that each of the Big Five dimensions of human personality can be seen as the result of a trade-off between different fitness costs and benefits. As there is no unconditionally optimal value of these trade-offs, it is to be expected that genetic diversity will be retained in the population.

Heritability of cardiovascular and personality traits in 6,148 Sardinians

In family studies, phenotypic similarities between relatives yield information on the overall contribution of genes to trait variation. Large samples are important for these family studies, especially when comparing heritability between subgroups such as young and old, or males and females. We recruited a cohort of 6,148 participants, aged 14-102 y, from four clustered towns in Sardinia. The cohort includes 34,469 relative pairs. To extract genetic information, we implemented software for variance components heritability analysis, designed to handle large pedigrees, analyze multiple traits simultaneously, and model heterogeneity. Here, we report heritability analyses for 98 quantitative traits, focusing on facets of personality and cardiovascular function. We also summarize results of bivariate analyses for all pairs of traits and of heterogeneity analyses for each trait. We found a significant genetic component for every trait. On average, genetic effects explained 40% of the variance for 38 blood tests, 51% for five anthropometric measures, 25% for 20 measures of cardiovascular function, and 19% for 35 personality traits. Four traits showed significant evidence for an X-linked component. Bivariate analyses suggested overlapping genetic determinants for many traits, including multiple personality facets and several traits related to the metabolic syndrome; but we found no evidence for shared genetic determinants that might underlie the reported association of some personality traits and cardiovascular risk factors. Models allowing for heterogeneity suggested that, in this cohort, the genetic variance was typically larger in females and in younger individuals, but interesting exceptions were observed. For example, narrow heritability of blood pressure was approximately 26% in individuals more than 42 y old, but only approximately 8% in younger individuals. Despite the heterogeneity in effect sizes, the same loci appear to contribute to variance in young and old, and in males and females. In summary, we find significant evidence for heritability of many medically important traits, including cardiovascular function and personality. Evidence for heterogeneity by age and sex suggests that models allowing for these differences will be important in mapping quantitative traits.

A longitudinal twin study of personality and major depression in women

OBJECTIVE

To elucidate the nature of the etiologic relationship between personality and major depression in women.

DESIGN

A longitudinal twin design in which twins completed a time 1 questionnaire and, 15 months later, were personally interviewed for the occurrence of major depression during the last year and completed a time 2-questionnaire. Both questionnaires contained short forms assessing neuroticism and extraversion.

PARTICIPANTS

1733 twins from female-female pairs ascertained from the population-based Virginia Twin Registry.

RESULTS

Extraversion was unrelated to lifetime or 1-year prevalence of major depression. Neuroticism was strongly related to lifetime prevalence of major depression and robustly predicted the prospective 1-year prevalence of major depression in those who, at time 1, denied previous depressive episodes. However, controlling for levels of neuroticism at time 1, levels of neuroticism at time 2 were moderately elevated in those who had had an episode of major depression between times 1 and 2 ("scar" effect) and substantially elevated in those experiencing an episode of major depression at time 2 ("state" effect). In those who developed major depression, levels of neuroticism did not predict time to onset. In the best-fit longitudinal twin model, the proportion of the observed correlation between neuroticism and the liability to major depression that is due to shared genetic risk factors was estimated at around 70%, that due to shared environmental risk factors at around 20%, and that due to a direct causal effect of major depression on neuroticism (via both "scar" and "state" effects) at around 10%. Approximately 55% of the genetic liability of major depression appeared to be shared with neuroticism, while 45% was unique to major depression.

CONCLUSION

In women, the relationship between neuroticism and the liability to major depression is substantial and largely the result of genetic factors that predispose to both neuroticism and major depression.

Heritability of the big five personality dimensions and their facets: a twin study

The genetic and environmental etiology of the five-factor model of personality as measured by the revised NEO Personality Inventory (NEO-PI-R) was assessed using 123 pairs of identical twins and 127 pairs of fraternal twins. Broad genetic influence on the five dimensions of Neuroticism, Extraversion, Openness, Agreeableness, and Conscientiousness was estimated at 41%, 53%, 61%, 41%, and 44%, respectively. The facet scales also showed substantial heritability, although for several facets the genetic influence was largely nonadditive. The influence of the environment was consistent across all dimensions and facets. Shared environmental influences accounted for a negligible proportion of the variance in most scales, whereas nonshared environmental influences accounted for the majority of the environmental variance in all scales.

Genetic and environmental influences on human psychological differences

Psychological researchers typically distinguish five major domains of individual differences in human behavior: cognitive abilities, personality, social attitudes, psychological interests, and psychopathology (Lubinski, 2000). In this article we: discuss a number of methodological errors commonly found in research on human individual differences; introduce a broad framework for interpreting findings from contemporary behavioral genetic studies; briefly outline the basic quantitative methods used in human behavioral genetic research; review the major criticisms of behavior genetic designs, with particular emphasis on the twin and adoption methods; describe the major or dominant theoretical scheme in each domain; and review behavioral genetic findings in all five domains. We conclude that there is now strong evidence that virtually all individual psychological differences, when reliably measured, are moderately to substantially heritable.

A population-based twin study of the relationship between neuroticism and internalizing disorders

OBJECTIVE

The anxiety and depressive disorders exhibit high levels of lifetime comorbidity with one another. The authors examined how genetic and environmental factors shared by the personality trait neuroticism and seven internalizing disorders may help explain this comorbidity.

METHOD

Lifetime major depression, generalized anxiety disorder, panic disorder, agoraphobia, social phobia, animal phobia, situational phobia, and neuroticism were assessed in over 9,000 twins from male-male, female-female, and opposite-sex pairs through structured diagnostic interviews. Multivariate structural equation models were used to decompose the correlations between these phenotypes into genetic and environmental components, allowing for sex-specific factors.

RESULTS

Genetic factors shared with neuroticism accounted for between one-third and one-half of the genetic risk across the internalizing disorders. When nonsignificant gender differences were removed from the models, the genetic correlations between neuroticism and each disorder were high, while individual-specific environmental correlations were substantially lower. In addition, the authors could identify a neuroticism-independent genetic factor that significantly increased risk for major depression, generalized anxiety disorder, and panic disorder.

CONCLUSIONS

There is substantial, but not complete, overlap between the genetic factors that influence individual variation in neuroticism and those that increase liability across the internalizing disorders, helping to explain the high rates of comorbidity among the latter. This may have important implications for identifying the susceptibility genes for these conditions.

Prevalence, heritability, and prospective risk factors for anorexia nervosa

CONTEXT

Anorexia nervosa (AN) is a serious mental illness with marked morbidity and mortality.

OBJECTIVE

To explore the prevalence, heritability, and prospectively assessed risk factors for AN in a large population-based cohort of Swedish twins.

DESIGN

During a 4-year period ending in 2002, all living, contactable, interviewable, and consenting twins in the Swedish Twin Registry (N = 31 406) born between January 1, 1935, and December 31, 1958, underwent screening for a range of disorders, including AN. Information collected systematically in 1972 to 1973, before the onset of AN, was used to examine prospective risk factors for AN.

SETTING

Population-based sample of twins in Sweden.

PARTICIPANTS

Cases of AN were identified as those individuals who met full DSM-IV criteria by means of clinical interview of the Swedish Twin Registry, who had a hospital discharge diagnosis of AN, or who had a cause-of-death certificate including an AN diagnosis.

RESULTS

The overall prevalence of AN was 1.20% and 0.29% for female and male participants, respectively. The prevalence of AN in both sexes was greater among those born after 1945. Individuals with lifetime AN reported lower body mass index, greater physical activity, and better health satisfaction than those without lifetime AN. Anorexia nervosa was inversely associated with the development of overweight (odds ratio, 0.29; 95% confidence interval [CI], 0.16-0.54 [P<.001]). The heritability of narrowly defined DSM-IV AN (additive genetic effects) was estimated to be a(2) = 0.56 (95% CI, 0.00-0.87), with the remaining variance attributable to shared environment (c(2) = 0.05; 95% CI, 0.00-0.64) and unique environment (e(2) = 0.38; 95% CI, 0.13-0.84). Neuroticism measured about 3 decades before the diagnostic assessment was significantly associated with the development of later AN (odds ratio, 1.62; 95% CI, 1.27-2.05 [P<.001]).

CONCLUSIONS

The prevalence of AN was higher in both male and female participants born after 1945. Individuals who survive AN and who no longer have body mass indexes in the AN range appear to be at lower risk for the development of overweight. Prospectively assessed neuroticism was associated with the subsequent development of AN, the liability to which is under considerable genetic influence.

A meta-analysis of the association between the serotonin transporter gene polymorphism (5-HTTLPR) and trait anxiety

Studies of the association between polymorphisms of the serotonin transporter gene (5-HTT) and trait anxiety have produced inconsistent results, raising questions about the strength of the relationship and the methodological conditions under which the relationship holds. We conducted a meta-analysis of existing studies to provide formal statistical measures of the strength of the linked polymorphic region of the serotonin transporter gene (5-HTTLPR)-anxiety relationship. For the entire collection of 26 studies, results provided no support for a relationship between anxiety and the presence of the short form of the 5-HTTLPR polymorphism. There was strong evidence of the presence of moderating variables, however, and subsequent analysis revealed that choice of the measure of trait anxiety was significant. Studies using the Neuroticism scale of Costa and McCrae were found to produce a small positive effect (d=0.23). Other potential moderators (country of study origin, type of subject) did not have a meaningful impact on d statistics. These findings indicate that 5-HTTLPR may in fact have a small but reliable influence on personality, particularly in the manifestation of trait anxiety when measured with a neuroticism scale based on the five-factor model of personality. Our results suggest that the success of future personality genetics research will be maximized by the use of personality measures from both the psychobiological and five-factor models.

Phenotypic and genetic structure of traits delineating personality disorder

BACKGROUND

The evidence suggests that personality traits are hierarchically organized with more specific or lower-order traits combining to form more generalized higher-order traits. Agreement exists across studies regarding the lower-order traits that delineate personality disorder but not the higher-order traits. This study seeks to identify the higher-order structure of personality disorder by examining the phenotypic and genetic structures underlying lower-order traits.

METHODS

Eighteen lower-order traits were assessed using the Dimensional Assessment of Personality Disorder-Basic Questionnaire in samples of 656 personality disordered patients, 939 general population subjects, and a volunteer sample of 686 twin pairs.

RESULTS

Principal components analysis yielded 4 components, labeled Emotional Dysregulation, Dissocial Behavior, Inhibitedness, and Compulsivity, that were similar across the 3 samples. Multivariate genetic analyses also yielded 4 genetic and environmental factors that were remarkably similar to the phenotypic factors. Analysis of the residual heritability of the lower-order traits when the effects of the higher-order factors were removed revealed a substantial residual heritable component for 12 of the 18 traits.

CONCLUSIONS

The results support the following conclusions. First, the stable structure of traits across clinical and nonclinical samples is consistent with dimensional representations of personality disorders. Second, the higher-order traits of personality disorder strongly resemble dimensions of normal personality. This implies that a dimensional classification should be compatible with normative personality. Third, the residual heritability of the lower-order traits suggests that the personality phenotypes are based on a large number of specific genetic components.

Heritability of facet-level traits in a cross-cultural twin sample: support for a hierarchical model of personality

The common variance among personality traits can be summarized in the factors of the five-factor model, which are known to be heritable. This study examined heritability of the residual specific variance in facet-level traits from the Revised NEO Personality Inventory. Analyses of raw and residual facet scales across Canadian (183 monozygotic [MZ] and 175 dizogotic [DZ] pairs) and German (435 MZ and 205 DZ pairs) twin samples showed genetic and environmental influences of the same type and magnitude across the 2 samples for most facets. Additive genetic effects accounted for 25% to 65% of the reliable specific variance. Results provide strong support for hierarchical models of personality that posit a large number of narrow traits in addition to a few broader trait factors or domains. Facet-level traits are not simply exemplars of the broad factors they define; they are discrete constructs with their own heritable and thus biological basis.

Convergent translational evidence of a role for anandamide in amygdala-mediated fear extinction, threat processing and stress-reactivity

Endocannabinoids are released 'on-demand' on the basis of physiological need, and can be pharmacologically augmented by inhibiting their catabolic degradation. The endocannabinoid anandamide is degraded by the catabolic enzyme fatty acid amide hydrolase (FAAH). Anandamide is implicated in the mediation of fear behaviors, including fear extinction, suggesting that selectively elevating brain anandamide could modulate plastic changes in fear. Here we first tested this hypothesis with preclinical experiments employing a novel, potent and selective FAAH inhibitor, AM3506 (5-(4-hydroxyphenyl)pentanesulfonyl fluoride). Systemic AM3506 administration before extinction decreased fear during a retrieval test in a mouse model of impaired extinction. AM3506 had no effects on fear in the absence of extinction training, or on various non-fear-related measures. Anandamide levels in the basolateral amygdala were increased by extinction training and augmented by systemic AM3506, whereas application of AM3506 to amygdala slices promoted long-term depression of inhibitory transmission, a form of synaptic plasticity linked to extinction. Further supporting the amygdala as effect-locus, the fear-reducing effects of systemic AM3506 were blocked by intra-amygdala infusion of a CB1 receptor antagonist and were fully recapitulated by intra-amygdala infusion of AM3506. On the basis of these preclinical findings, we hypothesized that variation in the human FAAH gene would predict individual differences in amygdala threat-processing and stress-coping traits. Consistent with this, carriers of a low-expressing FAAH variant (385A allele; rs324420) exhibited quicker habituation of amygdala reactivity to threat, and had lower scores on the personality trait of stress-reactivity. Our findings show that augmenting amygdala anandamide enables extinction-driven reductions in fear in mouse and may promote stress-coping in humans.

Genetically driven variation in serotonin uptake: is there a link to affective spectrum, neurodevelopmental, and neurodegenerative disorders?

Serotonin (5-HT) is an important regulator of morphogenetic activities during early central nervous system development, including cell proliferation, migration, and differentiation. The 5-HT transporter (5-HTT) plays a pivotal role in brain 5-HT homeostasis. It is also the initial target for both antidepressant drugs and drugs of abuse, some of which are potent neurotoxins. A polymorphism in the 5'-flanking regulatory region of the 5-HTT gene that results in allelic variation of 5-HTT expression is associated with anxiety-related personality traits and may influence the risk of developing affective disorders. Progress in 5-HTT gene inactivation studies are also changing views of the relevance of adaptive 5-HT uptake function in brain development and plasticity as well as processes underlying drug dependence and neurodegeneration. Despite evidence for a potential role of the 5-HTT in the integration of synaptic connections in the mammalian brain during development, adult life, and old age, detailed knowledge of the molecular mechanisms involved in these fine-tuning processes is just beginning to emerge. Integration of various strategies, including molecular genetic, transgenic, and gene transfer techniques, will allow elucidation of the 5-HTT's role in brain development, plasticity, and degeneration as well as in affective illness, drug abuse, and dementia.

Association of the dopamine D4 receptor (DRD4) gene and approach-related personality traits: meta-analysis and new data

BACKGROUND

Two variants in the dopamine D4 receptor (DRD4) gene have been reported to be associated with human approach-related traits such as novelty seeking and extraversion. However, the strength of evidence for this association remains uncertain.

METHODS

We conducted a meta-analysis of published studies of the association between the DRD4 gene variable number of tandem repeats (VNTR) and C-521T polymorphisms and human approach-related personality traits, including novelty seeking, extraversion, and impulsivity, restricted to adult samples recruited from nonpsychiatric populations, and extended on this literature by attempting to confirm any evidence of association in a replication sample (n = 309) selected for extreme scores on the extraversion subscale of the Eysenck Personality Questionnaire from a large (n = 40,090) population-based sample.

RESULTS

Our initial meta-analysis supported the association of the DRD4 C-521T polymorphism, but not the VNTR polymorphism, with approach-related traits. This conclusion was qualified by evidence of significant publication bias and the failure to detect association in a replication sample comprising individuals at the extremes of the trait distribution. The association of the C-521T polymorphism observed in our initial meta-analysis was robust to the inclusion of these new data, but our revised meta-analysis indicated that the association was present for measures of novelty seeking and impulsivity but not for measures of extraversion.

CONCLUSIONS

The DRD4 gene may be associated with measures of novelty seeking and impulsivity but not extraversion. The association of the C-521T variant with these measures, if genuine, may account for up to 3% of phenotypic variance.