Application of hierarchical genetic models to Raven and WAIS subtests: a Dutch twin study.

Hierarchical models of intelligence are highly informative and widely accepted. Application of these models to twin data, however, is sparse. This paper addresses the question of how a genetic hierarchical model fits the Wechsler Adult Intelligence Scale (WAIS) subtests and the Raven Standard Progressive test score, collected in 194 18-year-old Dutch twin pairs. We investigated whether first-order group factors possess genetic and environmental variance independent of the higher-order general factor and whether the hierarchical structure is significant for all sources of variance. A hierarchical model with the 3 Cohen group-factors (verbal comprehension, perceptual organisation and freedom-from-distractibility) and a higher-order g factor showed the best fit to the phenotypic data and to additive genetic influences (A), whereas the unique environmental source of variance (E) could be modeled by a single general factor and specifics. There was no evidence for common environmental influences. The covariation among the WAIS group factors and the covariation between the group factors and the Raven is predominantly influenced by a second-order genetic factor and strongly support the notion of a biological basis of g.

Intra- and extra-familial influences on alcohol and drug misuse: a twin study of gene-environment correlation.

AIMS: Genotype-environment correlation refers to the extent to which individuals are exposed to environments as a function of their genetic propensities. These correlations are important in the study of psychopathology because they identify environments that may maintain the expression of underlying genetic liabilities for a disorder. The present study examined the correlation between genetic liabilities for alcohol and drug misuse with perceptions of the social environments of the family of origin and the classroom. DESIGN: Postal survey data were collected from monozygotic and dizygotic twin pairs. SETTING: Twin pairs were recruited from Vancouver, British Columbia, Canada using newspaper advertisements and media stories. PARTICIPANTS: Eighty-five monozygotic and 77 dizygotic twin pairs were recruited from the general population. MEASUREMENTS: Twin pairs completed self-report measures of alcohol and drug misuse contained in the Dimensional Assessment of Personality Pathology, the Family Environment Scale, the Classroom Environment Scale, and the Traumatic Events Questionnaire. FINDINGS: Genetically indexed alcohol and drug misuse scores were regressed on the environmentally indexed FES and CES scales. Genetic liabilities for alcohol and drug misuse were associated with decreased perceived family moral-religious emphases, family cohesion and classroom task orientation and increased perceptions of classroom order and organization (strictness). CONCLUSIONS: Genotype-environment correlations, in particular, moral-religious emphases in the home, appear to be important in the development of substance misuse.

Covariance structure of neuroticism and agreeableness: a twin and molecular genetic analysis of the role of the serotonin transporter gene.

The Revised NEO Personality Inventory domains of Neuroticism and Agreeableness are considered factorially distinct despite several intercorrelations between these domains. The genetic correlation, an index of the degree to which these intercorrelations are caused by genetic influences, was estimated using data from 913 monozygotic and 562 dizygotic volunteer twin pairs from Canada, Germany, and Japan. The serotonin transporter gene, 5-HTTLPR, was assayed in a sample of 388 nontwin sibling pairs from the United States to determine the contribution of the serotonin transporter locus to the covariation between the Neuroticism and Agreeableness scales. In all four samples, genetic influences contributed to the covariance of Neuroticism and Agreeableness, with the serotonin transporter gene accounting for 10% of the relationship between these domains.

The heritability of attitudes: a study of twins.

The genetic basis of individual differences in attitudes was examined in a survey of 195 pairs of monozygotic twins and 141 pairs of same-sex dizygotic twins. A principal components analysis of the 30 attitude items in the survey identified 9 attitude factors, of which 6 yielded significant heritability coefficients. Nonshared environmental factors accounted for the most variance in the attitude factors. Possible mediators of attitude heritability were also assessed, including personality traits, physical characteristics, and academic achievement. Analyses showed that several of these possible mediators correlated at a genetic level with the heritable attitude factors, suggesting that the heritability of the mediator variables might account for part of the heritable components of some attitudes. There was also some evidence that highly heritable attitudes were psychologically "stronger" than less heritable attitudes.

Behavioural-genetic perspectives on personality function.

In the wake of the recent announcements that the human genome has been mapped, efforts to identify the genetic loci underlying personality function will grow and intensify. Much research has already been done in this area, but it has for the most part been limited to classical biometrical approaches designed to determine if personality has a heritable basis. These so-called "heritability" studies estimate how much of the individual differences in personality are attributable to genetic differences among people. Molecular-genetic approaches, on the other hand, are designed to identify specific putative loci, but have yielded mixed results. The inconsistency in research findings can be attributed in part to the lack of sufficient numbers of genetic markers in the chromosomal regions of interest--a problem that the creation of a map of the human genome will help to rectify. This map and its inevitable refinements, however, can only advance the search for the genes for personality to a limited degree. Serious unresolved problems in the conceptualization and definition of personality and its dysfunction remain, which will hamper the search for personality genes.

Personality disorder traits, family environment, and alcohol misuse: a multivariate behavioural genetic analysis.

AIMS: This study seeks to estimate the extent to which a common genetic and environmental basis is shared between (i) traits delineating specific aspects of antisocial personality and alcohol misuse, and (ii) childhood family environments, traits delineating broad domains of personality pathology and alcohol misuse. DESIGN: Postal survey data were collected from monozygotic and dizygotic twin pairs. SETTING: Twin pairs were recruited from Vancouver, British Columbia and London, Ontario, Canada using newspaper advertisements, media stories and twin clubs. PARTICIPANTS: Data obtained from 324 monozygotic and 335 dizygotic twin pairs were used to estimate the extent to which traits delineating specific antisocial personality traits and alcohol misuse shared a common genetic and environmental aetiology. Data from 81 monozygotic and 74 dizygotic twin pairs were used to estimate the degree to which traits delineating personality pathology, childhood family environment and alcohol misuse shared a common aetiology. MEASUREMENTS: Current alcohol misuse and personality pathology were measured using scales contained in the self-report Dimensional Assessment of Personality Pathology. Perceptions of childhood family environment were measured using the self-report Family Environment Scale. FINDINGS: Multivariate genetic analyses showed that a subset of traits delineating components of antisocial personality (i.e. grandiosity, attention-seeking, failure to adopt social norms, interpersonal violence and juvenile antisocial behaviours) are influenced by genetic factors in common to alcohol misuse. Genetically based perceptions of childhood family environment had little relationship with alcohol misuse. CONCLUSIONS: Heritable personality factors that influence the perception of childhood family environment play only a small role in the liability to alcohol misuse. Instead, liability to alcohol misuse is related to genetic factors common a specific subset of antisocial personality traits describing conduct problems, narcissistic and stimulus-seeking behaviour.

Individual differences in multiple dimensions of aggression: a univariate and multivariate genetic analysis.

Previous behaviour genetic studies of aggression have yielded inconsistent results: reported heritabilities for different types of aggressive behaviour ranging from 0 to 0.98. In the present study, 247 adult twin pairs (183 MZ pairs; 64 same-sex DZ pairs) were administered seven self-report questionnaires which yielded 18 measures of aggression. Univariate genetic analyses showed moderate to high heritabilities for 14 of these 18 measures and for a general aggression factor and three correlated aggression factors extracted from the measures. Multivariate genetic analyses showed sizeable genetic correlations between the different dimensions of aggression. Thus, individual differences in many types of aggressive behaviour are attributable to some extent to genetic factors and there is considerable overlap between the genes that operate on different types of aggressive behaviour.

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.

The genetic basis of the relation between speed-of-information-processing and IQ.

The relationship of speed-of-information-processing (SIP), as derived from reaction times (RTs) on experimental tasks, and intelligence has been extensively studied. SIP is suggested to measure the efficiency with which subjects can perform basic cognitive operations underlying a wide range of intellectual abilities. Observed phenotypic correlations between RT and IQ typically are in the -0.2 to -0.4 range, and the question is addressed to what extent this relationship is determined by genetic or environmental influences. In a group of Dutch twins the heritabilities for RT tasks at age 16 and 18 years were estimated longitudinally and the nature of the RT-IQ relationship was investigated. At age 16 years heritabilities for a simple reaction time (SRT) and choice reaction time (CRT) were 64 and 62% and the average phenotypic correlations between the RTs and IQ, assessed by the Raven standard progressive matrices, was -0.21. At the second test occasion lower heritabilities were observed for the RTs, probably due to modifications in administration procedures. The mean correlations between the RTs and WAIS verbal and per formal subtests were -0.18 and -0.16. Multivariate genetic analyses at both ages showed that the RT-IQ correlations were explained by genetic influences. These results are in agreement with earlier findings (Baker et al., Behav Genet 1991;21:351-67; Ho et al., Behav Genet 1988;18:247-61) and support the existence of a common, heritable biological basis underlying the SIP-IQ relationship.

The heritability of testosterone: a study of Dutch adolescent twins and their parents.

The heritability of total plasma testosterone levels, determined from blood samples, was examined in 160 adolescent twin pairs and their parents. Subjects were tested as part of a larger study of cardiovascular risk factors, conducted in Amsterdam. Each subject provided a sample of blood which was assayed to measure testosterone concentrations. Correlations of testosterone in monozygotic twins were higher than in dizygotic twins. No resemblance was found between testosterone values in fathers and those in their children and a moderate correlation was seen between mothers and their daughters. The lack of resemblance between family members of opposite sex suggests that different genetic factors influence plasma testosterone concentrations in men and women. In adolescent men, approximately 60% of the variance in testosterone levels is heritable. The lack of father-son resemblance suggests that different genetic factors may be expressed in adolescence and adulthood. In women, 40% of the variance in testosterone levels is heritable, both in adolescence and in adulthood.

Seasonal mood change and personality: an investigation of genetic co-morbidity.

Clinical observations and empirical studies suggest that Seasonal Affective Disorder (SAD) is related to personality. The present study estimates the genetic and environmental correlations between the Global Seasonality Score (GSS) from the Seasonal Pattern Assessment Questionnaire and personality measures, assessed using the NEO Five Factor Inventory (NEO-FFI) and the Dimensional Assessment of Personality Pathology (DAPP) in a volunteer sample of 163 monozygotic (MZ) pairs (102 female and 61 male pairs) and 134 dizygotic (DZ) pairs (70 female, 38 male and 26 opposite-sex pairs). Large genetic correlations were found between the GSS and NEO-FFI Neuroticism (0.52: 95% CI = 0.36-0.71) and DAPP-BQ Cognitive Dysregulation (0.50: 95% CI = 0.30-0.71), Affective Lability (0.49: 95% CI = 0.29-0.77), Anxiousness (0.37: 95% CI = 0.18-0.55) and Stimulus Seeking (0.45: 95% CI = 0.25-0.64) scales. The genetic correlations with the remaining scales, such as Extraversion (0.06: 95% CI = -0.16-0.26), Compulsivity (-0.09: 95% CI = -0.31-0.12) and Submissiveness (0.15: 95% CI = -0.05-0.34) were uniformly small. All environmental correlations between the GSS and personality scales were < or = 0.19. These results provide evidence that the observed correlations between these seasonality and personality dimensions are attributable to common genetic factors and that environmental influences are domain specific.

Nature vs nurture: are leaders born or made? A behavior genetic investigation of leadership style.

With the recent resurgence in popularity of trait theories of leadership, it is timely to consider the genetic determination of the multiple factors comprising the leadership construct. Individual differences in personality traits have been found to be moderately to highly heritable, and so it follows that if there are reliable personality trait differences between leaders and non-leaders, then there may be a heritable component to these individual differences. Despite this connection between leadership and personality traits, however, there are no studies of the genetic basis of leadership using modern behavior genetic methodology. The present study proposes to address the lack of research in this area by examining the heritability of leadership style, as measured by self-report psychometric inventories. The Multifactor Leadership Questionnaire (MLQ), the Leadership Ability Evaluation, and the Adjective Checklist were completed by 247 adult twin pairs (183 monozygotic and 64 same-sex dizygotic). Results indicated that most of the leadership dimensions examined in this study are heritable, as are two higher level factors (resembling transactional and transformational leadership) derived from an obliquely rotated principal components factors analysis of the MLQ. Univariate analyses suggested that 48% of the variance in transactional leadership may be explained by additive heritability, and 59% of the variance in transformational leadership may be explained by non-additive (dominance) heritability. Multivariate analyses indicated that most of the variables studied shared substantial genetic covariance, suggesting a large overlap in the underlying genes responsible for the leadership dimensions.

Testosterone is correlated with regional morphology of the human corpus callosum.

Theoretical speculation in humans (S.F. Witelson, Psychoneuroendocrinology 16 (1991) 131-153) and empirical findings in animals (R.H. Fitch, P.E. Cowell, L.M. Schrott, V.H. Denenberg, Int. J. Dev. Neurosci. 9 (1991) 35-38) suggest that testosterone (T) may play a significant role in the development of the corpus callosum (CC). However, there are currently no empirical studies directly relating T concentrations to callosal morphology in humans. The purpose of the present study was to investigate the relationship between free T concentrations as determined by radioimmunoassay, and the mid-sagittal area of the corpus callosum, as determined by magnetic resonance imaging (MRI). Subjects were 68 young adult (20-35 years), neurologically normal, right-handed males. All subjects underwent MRI and provided two samples of saliva for radioimmunoassay of T and cortisol. Anatomical regions of interest included total brain volume, left and right hemisphere volume and regional areas of the CC. CC regions were defined using two different measurement techniques, each dividing the CC into six sub-sections. Anatomical measurements were performed blind with respect to the hormone levels of subjects. A significant positive correlation between T concentration and cross-sectional area of the posterior body of the CC was found. This finding was consistent across the two measurement techniques and was not attributable to individual differences in total brain volume. All correlations between cortisol and CC sub-regions were non-significant. The results of this study are consistent with the notion that T, at an earlier stage in development, may play a significant role in modulating cortical/callosal architecture in humans.

The relationship between seasonal mood change and personality: more apparent than real?

A number of recent research reports have reported significant relationships between seasonal mood change (seasonality) and personality. However, some of the results are difficult to interpret because of inherent methodological problems, the most important of which is the use of samples drawn from the southern as opposed to the northern hemisphere, where the phenomenon of seasonality may be quite different. The present study examined the relationship between personality and seasonality in a sample from the northern hemisphere (minimum latitude = 49 degrees N). A total of 297 adults drawn from the general population (112 male and 185 female subjects) completed the Seasonal Pattern Assessment Questionnaire, and the results obtained confirmed most of the previously reported relationships and showed that these are reliable across (i) different hemispheres, (ii) different measures of personality and (iii) clinical and general population samples. However, the impact of the relationship seems to be more apparent than real, with personality accounting for just under 15% of the total variance.

Gender differences in the heritability of seasonal mood change.

The study estimated gender differences in the magnitude of genetic and environmental influence in seasonal mood change. The self-report Seasonal Pattern Assessment Questionnaire (SPAQ) was completed by 339 volunteer reared-together twinpairs (187 monozygotic pairs, 152 dizygotic pairs) and analysed using biometric genetic models. The SPAQ yields a global seasonality score (GSS) which is an index of change in sleep patterns, social activities, mood, weight, appetite, and energy level. The GSS was significantly heritable among males and females, estimated to account for 69% and 45% of the total variance, respectively. For the individual symptoms, changes in sleep patterns, social activities, mood, appetite, and energy levels were accounted for primarily by additive genetic effects in both males (median, 45.5%) and females (median, 30.5%). For both sexes, weight changes were not heritable. Sex-by-genotype analyses suggested that the genetic factors influencing female seasonality may not be the same as those influencing male seasonality.

Developmental changes in speed of information processing in young children.

This study investigated developmental increases in processing speed in young children, relative to adults, with only nonverbal stimuli. R. Kail's (1991) model of the rate of change in processing speed from childhood to adulthood was evaluated. Processing speed was measured in 34 children at 4 years, 37 at 5 years, and 38 at 6 years and in 43 adults, with a battery of 8 computer-administered tests. Results showed clear age-related increases in processing speed that cannot be attributed to increased accuracy and error rate monitoring. Kail's model adequately accounted for the observed rate of developmental change in processing speed; however, the parameter estimates of R. Kail and Y. Park (1992) provided more accurate predictions than did the meta-analytically derived estimates of Kail (1991). Findings support the global developmental trend hypothesis and suggest that this trend extends beyond the range of verbal skills evaluated in previous research.

Environmental predictors of personality differences: a twin and sibling study.

Nonshared environmental influences have consistently been shown to account for at least as much of the variance in personality as genetic factors, but the nature of these nonshared influences has largely remained unidentified. To identify environmental predictors of differential personality development, the Personality Research Form and 4 measures of people's perceptions of their background environments were administered to 143 adult twin pairs (93 monozygotic [MZ] and 50 dizygotic [DZ] and 66 pairs of same-sex nontwin (NT) siblings. Differences between MZ twins, DZ twins, and NT siblings in a number of dimensions of personality were significantly related to differences on the environmental measures, and phenotypic correlations between the personality and environment measures were themselves entirely attributable to correlated nonshared environmental effects.

Gender-specific etiological differences in alcohol and drug problems: a behavioural genetic analysis.

Although gender differences in the consumption and abuse of alcohol and drugs are frequently reported, the etiology of these differences has received limited attention. The present study applied biometric genetic analyses to determine whether self-report questionnaire items assessing problem alcohol and drug use are influenced by gender-specific genetic and environmental influences. The sample consisted of 693 volunteer general population twin pairs (209 monozygotic female, 138 monozygotic male, 170 dizygotic female, 82 dizygotic male, 94 dizygotic opposite-sex twin pairs). Heritability analyses showed that most aspects of drug and alcohol problems were differentially heritable by gender. Significant additive genetic effects were found only in males, whereas in females, most substance use problems were wholly determined by environmental factors. In both genders, non-shared environmental factors accounted for the largest proportion of the variance, and further gender-by-genotype analyses showed that these influences were not gender specific, but rather were common to males and females. Some preliminary evidence was also obtained that the use of illicit drugs may be etiologically distinct from the use of licit drugs in females and males.

Heritability of personality disorder traits: a twin study.

Genetic and non-genetic influences on the hierarchy of traits that delineate personality disorder as measured by the Dimensional Assessment of Personality Problems (DAPP-DQ) scale were examined using data from a sample of 483 volunteer twin pairs (236 monozygotic pairs and 247 dizygotic pairs). The DAPP-DQ assesses four higher-order factors, 18 basic dimensions and 69 facet traits of personality disorder. The correlation coefficients for monozygotic and dizygotic twin pairs ranged from 0.26 to 0.56 and from 0.03 to 0.41, respectively. Broad heritability estimates ranged from 0 to 58% (median value 45%). Additive genetic effects and unique environmental effects emerged as the primary influences on these scales, with unique environmental influences accounting for the largest proportion of the variance for most traits at all levels of the hierarchy.