A trend toward greater body size in dizygotic (DZ) than in monozygotic (MZ) twins has been suggested by some but not all studies, and this difference may also vary by age. We analyzed zygosity differences in mean values and variances of height and body mass index (BMI) among male and female twins from infancy to old age. Data were derived from an international database of 54 twin cohorts participating in the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins), and included 842,951 height and BMI measurements from twins aged 1 to 102 years. The results showed that DZ twins were consistently taller than MZ twins, with differences of up to 2.0 cm in childhood and adolescence and up to 0.9 cm in adulthood. Similarly, a greater mean BMI of up to 0.3 kg/m2 in childhood and adolescence and up to 0.2 kg/m2 in adulthood was observed in DZ twins, although the pattern was less consistent. DZ twins presented up to 1.7% greater height and 1.9% greater BMI than MZ twins; these percentage differences were largest in middle and late childhood and decreased with age in both sexes. The variance of height was similar in MZ and DZ twins at most ages. In contrast, the variance of BMI was significantly higher in DZ than in MZ twins, particularly in childhood. In conclusion, DZ twins were generally taller and had greater BMI than MZ twins, but the differences decreased with age in both sexes.

For over 100 years, the genetics of human anthropometric traits has attracted scientific interest. In particular, height and body mass index (BMI, calculated as kg/m2) have been under intensive genetic research. However, it is still largely unknown whether and how heritability estimates vary between human populations. Opportunities to address this question have increased recently because of the establishment of many new twin cohorts and the increasing accumulation of data in established twin cohorts. We started a new research project to analyze systematically (1) the variation of heritability estimates of height, BMI and their trajectories over the life course between birth cohorts, ethnicities and countries, and (2) to study the effects of birth-related factors, education and smoking on these anthropometric traits and whether these effects vary between twin cohorts. We identified 67 twin projects, including both monozygotic (MZ) and dizygotic (DZ) twins, using various sources. We asked for individual level data on height and weight including repeated measurements, birth related traits, background variables, education and smoking. By the end of 2014, 48 projects participated. Together, we have 893,458 height and weight measures (52% females) from 434,723 twin individuals, including 201,192 complete twin pairs (40% monozygotic, 40% same-sex dizygotic and 20% opposite-sex dizygotic) representing 22 countries. This project demonstrates that large-scale international twin studies are feasible and can promote the use of existing data for novel research purposes.

The relationship between self-report abilities and measured intelligence was examined at both the phenotypic (zero-order) level as well as at the genetic and environmental levels. Twins and siblings (N = 516) completed a timed intelligence test and a self-report ability questionnaire, which has previously been found to produce 10 factors, including: politics, interpersonal relationships, practical tasks, intellectual pursuits, academic skills, entrepreneur/business, domestic skills, vocal abilities, and creativity. At the phenotypic level, the correlations between the ability factor scores and intelligence ranged from 0.01 to 0.42 (between self-report academic abilities and verbal intelligence). Further analyses found that some of the phenotypic relationships between self-report ability scores and measured intelligence also had significant correlations at the genetic and environmental levels, suggesting that some of the observed relationships may be due to common genetic and/or environmental factors.

The University of British Columbia (UBC) Twin Project is a registry of approximately 2,000 pairs of reared-together twins recruited across the major Canadian provinces. The focus of the project is an investigation of the behavioral genetics of personality and its disorders, and their relationship to other forms of mental illness. The goal is to find evidence for current diagnostic structures and classification systems, or certainly provide data for their reform. The primary measures employed are full-form self-report questionnaires of major psychological and psychiatric instruments covering personality, mood, general health, schizotypy, and more recently the anxiety disorders from symptoms resulting from exposure traumatic events, obsessive–compulsive behavior and beliefs, as well as coping strategies and health anxieties. No DNA has been collected. Data from the project have been used in several collaborative projects worldwide, and collaborative projects with other groups and interested researchers are welcomed.

The purpose of this study was to examine the phenotypic, genetic, and environmental relationships between multiple dimensions of Type A behavior and personality. Adult twins (N = 247 pairs) completed a self-report personality measure and a work style questionnaire that measured six facets of Type A behavior: impatience, anger, work involvement, time urgency, job dissatisfaction, and competitiveness. Univariate results suggest that only the job dissatisfaction facet of Type A behavior was explained by non-genetic (environmental) factors. Multivariate results suggest that all of the genetically determined facets of Type A behavior show some correspondence with at least one of the personality factors found in the study, suggesting that some of the phenotypic (observed) relationships could be due to common genetic factors.

The University of British Columbia (UBC) Twin Project is a registry of approximately 1500 pairs of reared-together twins recruited from Vancouver, British Columbia and surrounding municipalities. The focus of the project is to examine personality and its disorders from a behavioral genetic perspective. The primary measures include self-report measures of variables from the major models of personality and personality disorders. Subsamples of the study have also been surveyed on a wide range of psychiatric conditions and symptoms, including, for example, substance use, mood, anxiety, coping, posttraumatic stress disorder, schizotypy, and several measures of the environment and experience. Also surveyed are general health and basic psychological processes including cognitive ability. This broad assessment has enabled us to examine not only the structure of personality, but also its potential role in psychopathology and other psychological processes. A feature of the project is that the measures selected reflect current thinking in the field as opposed to traditional psychiatric diagnostic criteria. The UBC Twin Project has been used in a number of collaborative projects on personality and psychopathology with other worldwide twin registries. At the present time, no DNA has been collected; however the facility to collect these data is available. Collaborative projects on this and future questionnaire studies are welcome.

The UBC Twin Project is an on-going study of personality and personality disorder that has collected personality data on approximately 1000 adult volunteer twin pairs. The primary purpose of the study is to develop and validate measures of the major forms of personality disorder as a quantitative alternative to the classificatory system first proposed by the DSM-III using the standard methods of personality and behavioural genetics research. To date, the study has explored the univariate and multivariate relationships between the major personality and personality disorder inventories, and their relationships to other psychopathologies including the mood and anxiety disorders and key psychological variables, such as cognitive ability, attachment and attitudes. The results have several implications for personality and personality disorders, most notably a rationalized diagnostic/nosological system to enhanced measurement instruments that index and reflect the influence of specific genetic and environmental influences.

The present research examined the extent to which relations between social values and personality are due to shared genetic or environmental factors. Using the Rokeach (1973) Value Survey and a scoring key from Schwartz and Bilsky (1990), seven value scores (enjoyment, achievement, self-direction, maturity, prosocial, security, and restrictive conformity) were derived in a sample of twins. As expected, all of the value scales were found to have a significant genetic component, with values ranging from 36% for enjoyment to 63% for prosocial, and there were numerous significant phenotypic correlations found between the value scales and personality scores. Most important, bivariate genetic analyses revealed that some of these phenotypic correlations could be attributed to common genetic or environmental factors.

Phenotypic research on leadership style has long considered the importance of individual differences in personality when identifying the behaviors associated with good leaders. Although leadership and many personality traits have been separately shown to be heritable, these constructs have not been examined with genetically informative data to identify common sources of heritability in the two domains. A logical extension to current research, therefore, is to examine the extent to which factors of personality are predictive of leadership dimensions and the extent to which unique genetic contributions to the relationship between personality and leadership style may be identified. Adult twin pairs (183 MZ and 64 same-sex DZ) completed the Multifactor Leadership Questionnaire (MLQ) and the Personality Research Form (PRF). Univariate analyses indicated that both leadership factors (transformational and transactional leadership) and all five of the “Big Five” factors (openness, conscientiousness, extraversion, disagreeableness, and neuroticism) were best fit by genetic models. Multivariate genetic analyses suggest that transformational leadership shows a statistically significant positive genetic correlation with conscientiousness, extraversion, and openness to experience. Transactional leadership shows a significant negative genetic correlation with conscientiousness and extraversion, and a significant positive genetic correlation with disagreeableness. These results underscore the importance of conscientiousness and extraversion in predicting leadership style, and illustrate important differences between transformational and transactional leaders.

People differ markedly in their risk for developing posttraumatic stress symptoms (PTSS) after exposure to traumatic events. Twin studies suggest that the trauma-PTSS relationship is moderated by genetic and environmental influences. The present study tested for specific types of genetic and environmental interaction effects on PTSS. A sample of 222 monozygotic and 184 dizygotic twin pairs reported on lifetime frequency of assaultive and nonassaultive trauma and associated PTSS. Biometric analyses indicated that in the case of nonassaultive trauma, PTSS were directly affected by environmental factors that also influence exposure to nonassaultive trauma. For assaultive trauma both genetic and non-shared environmental influences jointly affected PTSS, and the number of traumatic events moderated the severity of PTSS. Genetic factors were found to become less important beyond some threshold (e.g., 3 or 4 types of serious trauma) suggesting that genetic factors — which may confer either risk or resilience to PTSS — modify these symptoms within a range of human experience, beyond which environmental effects supervene.

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.

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 anobliquely 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. Multi-variate analyses indicatedthat most ofthe variables studiedshared substantial genetic covariance, suggesting a large overlap in the underlying genes responsible for the leadership dimensions.