Two genetic analyses to elucidate causality between body mass index and personality.

BACKGROUND/OBJECTIVES: Many personality traits correlate with BMI, but the existence and direction of causal links between them are unclear. If personality influences BMI, knowing this causal direction could inform weight management strategies. Knowing that BMI instead influences personality would contribute to a better understanding of the mechanisms of personality development and the possible psychological effects of weight change. We tested the existence and direction of causal links between BMI and personality. SUBJECTS/METHODS: We employed two genetically informed methods. In Mendelian randomization, allele scores were calculated to summarize genetic propensity for the personality traits neuroticism, worry, and depressive affect and used to predict BMI in an independent sample (N = 3 541). Similarly, an allele score for BMI was used to predict eating-specific and domain-general phenotypic personality scores (PPSs; aggregate scores of personality traits weighted by BMI). In a direction of causation (DoC) analysis, twin data from five countries (N = 5424) were used to assess the fit of four alternative models: PPSs influencing BMI, BMI influencing PPSs, reciprocal causation, and no causation. RESULTS: In Mendelian randomization, the allele score for BMI predicted domain-general (ß = 0.05; 95% CI: 0.02, 0.08; P = 0.003) and eating-specific PPS (ß = 0.06; 95% CI: 0.03, 0.09; P < 0.001). The allele score for worry also predicted BMI (ß = -0.05; 95% CI: -0.08, -0.02; P < 0.001), while those for neuroticism and depressive affect did not (P ≥ 0.459). In DoC, BMI similarly predicted domain-general (ß = 0.21; 95% CI:, 0.18, 0.24; P < 0.001) and eating-specific personality traits (ß = 0.19; 95% CI:, 0.16, 0.22; P < 0.001), suggesting causality from BMI to personality traits. In exploratory analyses, links between BMI and domain-general personality traits appeared reciprocal for higher-weight individuals (BMI > ~25). CONCLUSIONS: Although both genetic analyses suggested an influence of BMI on personality traits, it is not yet known if weight management interventions could influence personality. Personality traits may influence BMI in turn, but effects in this direction appeared weaker.

Genetic and Environmental Structure of Altruism Characterized by Recipients in Relation to Personality.

Background and Objectives: Altruism is a form of prosocial behavior with the goal of increasing the fitness of another individual as a recipient while reducing the fitness of the actor. Although there are many studies on its heterogeneity, only a few behavioral genetic studies have been conducted to examine different recipient types: family members favored by kin selection, the dynamic network of friends and acquaintances as direct reciprocity, and strangers as indirect reciprocity. Materials and Methods: This study investigated the genetic and environmental structure of altruism with reference to recipient types measured by the self-report altruism scale distinguished by the recipient (the SRAS-DR) and examine the relationship to personality dimensions measured by the NEO-FFI with a sample of 461 adult Japanese twin pairs. Results: The present study shows that there is a single common factor of altruism: additive genetic effects explain 51% of altruism without a shared environmental contribution. The genetic contribution of this single common factor is explained by the genetic factors of neuroticism (N), extraversion (E), openness to experience (O), and conscientiousness (C), as well as a common genetic factor specific to altruism. Only altruism toward strangers is affected by shared environmental factors. Conclusions: Different types of altruistic personality are constructed by specific combinational profiles of general personality traits such as the Big Five as well as a genetic factor specific to altruism in each specific way.

Phenotypic and aetiological architecture of depressive symptoms in a Japanese twin sample.

BACKGROUND: The phenotypic and aetiological architecture of depression symptomatology has been mostly studied in Western samples. In this study, we conducted a genetically informed factor analysis to elucidate both the phenotypic and aetiological architectures of self-reported depression among a Japanese adult twin sample. METHODS: Depressive symptoms assessed by Zung's Self-rating Depression Scale were self-rated by 425 twin pairs (301 monozygotic and 124 dizygotic twin pairs) in a community sample in Japan. RESULTS: An exploratory factor analysis extracted three symptom domains representing cognitive, affective and somatic symptomatology. A confirmatory factor analysis demonstrated that a bi-factor solution fitted better than the alternative solutions, implying that depression may be defined as a combination of a single general construct and three factors specific to each of the three symptom domains. A multivariate genetic analysis with the bi-factor solution showed that the general factor was substantially heritable (47%), and that only the affective symptom domain was significantly heritable (29%) among the three specific factors, their remaining variance being explained by non-shared environmental influences. CONCLUSIONS: Depression symptomatology appears to be adequately captured by a substantially heritable general factor. The heritability of this factor (47%) in a Japanese adult sample is in line with commonly reported heritability estimates for depression. The three specific factors - cognitive, affective and somatic - are mostly explained by non-shared environmental factors, which include measurement error. The extent to which these specific factors are uniquely associated with correlates of depression when the general factor is accounted for should be investigated in future studies.

Psychosocial Twin Cohort Studies in Japan: The Keio Twin Research Center (KoTReC).

The Keio Twin Research Center (KoTReC) was established in 2009 at Keio University to combine two longitudinal cohort projects - the Keio Twin Study (KTS) for adolescence and adulthood and the Tokyo Twin Cohort Project (ToTCoP) for infancy and childhood. KoTReC also conducted a two-time panel study of self-control and psychopathology in twin adolescence in 2012 and 2013 and three independent anonymous cross-sectional twin surveys (ToTcross) before 2012 - the ToTCross, the Junior and Senior High School Survey and the High School Survey. This article introduces the recent research designs of KoTReC and its publications.

Genetic and Environmental Influences on Traits of Gender Identity Disorder: A Study of Japanese Twins Across Developmental Stages.

The present study examined: (1) gender and age differences of mean gender identity disorder (GID) trait scores in Japanese twins; (2) the validity of the prenatal hormone transfer theory, which predicts that, in dizygotic (DZ) twin pairs, twins with an opposite-gender co-twin more frequently exhibit GID traits than twins with a same-gender co-twin; and (3) the magnitude of genetic and environmental influences on GID traits as a function of age and gender. Data from 1450 male twin pairs, 1882 female twin pairs, and 1022 DZ male-female pairs ranging from 3 to 26 years of age were analyzed. To quantify individual variances in GID traits, each participant completed four questionnaire items based on criteria for GID from the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR). Our most important findings were: (1) Japanese females exhibited GID traits more frequently than males and Japanese children exhibited GID traits less frequently than adolescents and adults (among females, the prevalence was 1.6 % in children, 10 % in adolescents, and 12 % in adults; among males, the prevalence was 0.5, 2, and 3 %, respectively); (2) the data did not support the prenatal hormone transfer theory for GID traits; and (3) a large part of the variance for GID traits in children was accounted for by familial factors; however, the magnitude was found to be greater in children than in adolescents or adults, particularly among females. This study suggests that although the prevalence is likely to increase, familial effects are likely to decrease as individuals age.

Why children differ in motivation to learn: Insights from over 13,000 twins from 6 countries.

Little is known about why people differ in their levels of academic motivation. This study explored the etiology of individual differences in enjoyment and self-perceived ability for several school subjects in nearly 13,000 twins aged 9-16 from 6 countries. The results showed a striking consistency across ages, school subjects, and cultures. Contrary to common belief, enjoyment of learning and children's perceptions of their competence were no less heritable than cognitive ability. Genetic factors explained approximately 40% of the variance and all of the observed twins' similarity in academic motivation. Shared environmental factors, such as home or classroom, did not contribute to the twin's similarity in academic motivation. Environmental influences stemmed entirely from individual specific experiences.

Genetic and environmental structure of individual differences in hand, foot, and ear preferences: a twin study.

Recent studies with large sample size have reported moderate heritability of hand preference. However, little is known about genetic and environmental factors for lateral preference. We examined the genetic and environmental factors for hand, foot, and ear preferences using a twin design study. A lateral preference questionnaire was administered to twin participants (N=956). Phenotypic correlation matrices of lateral preferences were computed for monozygotic and dizygotic twin pairs, and were subjected to Cholesky decomposition to compute additive genetic and unique environmental correlation matrices. Promax rotation factor analysis of each genetic and environmental correlation matrix yielded six genetic and four environmental factors. Factor-loading patterns for these factors indicated that hand and foot lateral activities were affected by different genetic factors. By contrast, each of the four environmental factors was mainly associated with hand, foot, or ear preference. These results suggest that the genetic structure for lateral preference may be more complex than the environmental structure. In particular, hand preference may be multidimensional in terms of genetic factors.

Bidirectional influences between maternal parenting and children's peer problems: a longitudinal monozygotic twin difference study.

This twin study examined the bidirectional relationship between maternal parenting behaviors and children's peer problems that were not confounded by genetic and family environmental factors. Mothers of 259 monozygotic twin pairs reported parenting behaviors and peer problems when twins were 42 and 48 months. Path analyses on monozygotic twin difference scores revealed that authoritative parenting (the presence of consistent discipline and lack of harsh parenting) and peer problems simultaneously influenced each other. Authoritative parenting reduced peer problems, and peer problems increased authoritative parenting. Neither consistent discipline nor harsh parenting alone was associated with peer problems. These results suggest that maternal authoritative parenting works protectively in regard to children's peer problems, and peer problems can evoke such effective parenting.

Two cohort and three independent anonymous twin projects at the Keio Twin Research Center (KoTReC).

The Keio Twin Research Center has conducted two longitudinal twin cohort projects and has collected three independent and anonymous twin data sets for studies of phenotypes related to psychological, socio-economic, and mental health factors. The Keio Twin Study has examined adolescent and adult cohorts, with a total of over 2,400 pairs of twins and their parents. DNA samples are available for approximately 600 of these twin pairs. The Tokyo Twin Cohort Project has followed a total of 1,600 twin pairs from infancy to early childhood. The large-scale cross-sectional twin study (CROSS) has collected data from over 4,000 twin pairs, from 3 to 26 years of age, and from two high school twin cohorts containing a total of 1,000 pairs of twins. These data sets of anonymous twin studies have mainly targeted academic performance, attitude, and social environment. The present article introduces the research designs and major findings of our center, such as genetic structures of cognitive abilities, personality traits, and academic performances, developmental effects of genes and environment on attitude, socio-cognitive ability and parenting, genes x environment interaction on attitude and conduct problem, and statistical methodological challenges and so on. We discuss the challenges in conducting twin research in Japan.

A multivariate twin study of early literacy in Japanese Kana.

This first Japanese twin study of early literacy development investigated the extent to which genetic and environmental factors influence individual differences in prereading skills in 238 pairs of twins at 42 months of age. Twin pairs were individually tested on measures of phonological awareness, kana letter name/sound knowledge, receptive vocabulary, visual perception, nonword repetition, and digit span. Results obtained from univariate behavioral-genetic analyses yielded little evidence for genetic influences, but substantial shared-environmental influences, for all measures. Phenotypic confirmatory factor analysis suggested three correlated factors: phonological awareness, letter name/sound knowledge, and general prereading skills. Multivariate behavioral genetic analyses confirmed relatively small genetic and substantial shared environmental influences on the factors. The correlations among the three factors were mostly attributable to shared environment. Thus, shared environmental influences play an important role in the early reading development of Japanese children.

Genetic and environmental relationships between head circumference growth in the first year of life and sociocognitive development in the second year: a longitudinal twin study.

Although growth in head circumference (HC) during infancy is known to predict later childhood outcomes, the mechanisms underlying this association with later sociocognitive abilities remain undetermined. Thus, using a sample of 241 pairs of normally developing Japanese twins, this study investigated the underpinnings of the association between HC growth (difference between HC at birth and at 10 months) and sociocognitive abilities at 19 months as measured by 10 items from the M-CHAT. Phenotypic correlations between HC at birth and sociocognitive abilities and between HC growth and sociocognitive abilities were marginal and not significant. However, multivariate genetic analyses using Cholesky decomposition revealed that genetic influences on HC growth and those on sociocognitive abilities were negatively associated. On the other hand, shared and nonshared environmental influences on HC growth were positively associated with influences on sociocognitive abilities. Genetic and environmental influences on HC at birth were not significantly associated with influences on sociocognitive abilities. These results help to clarify the role of brain growth during infancy in the subsequent development of sociocognitive abilities and highlight the importance of examining the different roles of genetic and environmental influences in studies of these areas.

The second to fourth digit ratio (2D:4D) in a Japanese twin sample: heritability, prenatal hormone transfer, and association with sexual orientation.

The second to fourth digit ratio has been argued to reflect prenatal hormonal influences and is reportedly associated with various psychological and behavioral traits, such as sexual orientation, cognitive abilities, and personality. We examined genetic and environmental influences on the second to fourth digit ratio (2D:4D) using a Japanese twin sample (N=300). The genetic analysis showed substantial additive genetic influences for both right and left hand 2D:4D. The rest of the variance was explained mainly by environmental influences not shared within twin pairs. These findings were, in general, in accordance with preceding studies with primarily Caucasian twin samples. The bivariate genetic analysis revealed that the additive genetic influences were largely shared between the right and left hand, while the non-shared environmental influences were largely unique to each hand. Results from a comparison of opposite-sex and same-sex twins were not significant, although they were in the predicted direction according to the prenatal hormone transfer hypothesis. Female monozygotic twin pairs discordant in sexual orientation showed significant within-pair differences in left hand 2D:4D, where non-heterosexual twins had lower (more masculinized) 2D:4D. In addition, we found that non-heterosexual male MZ twins had larger (more feminized) 2D:4D than their heterosexual co-twins. These results suggest the existence of non-shared environmental influences that affect both 2D:4D and sexual orientation.

The effects of sibling relationships on social adjustment among Japanese twins compared with singletons.

This study examined the link between sibling relationships and children's social adjustment by comparing twin siblings and siblings with different ages (singleton siblings}, and clarified the role of reciprocity in sibling relationships on children's social development. Mothers of 58 monozygotic twin pairs, 48 dizygotic twin pairs, and 86 singleton sibling pairs reported their children's sibling relationships and social adjustment.This study showed that the effects of sibling relationships on the prosocial behaviors and conduct problems of each child are stronger for twins than for singleton siblings. Moreover, positivity toward one's sibling increased peer problems only among monozygotic twins. The opposite tendency was present among dizygotic twins and singleton siblings. This study suggests the importance for children's social development of having many interactions with siblings and establishing reciprocity in sibling relationships. Moreover, our results suggest that the quality of sibling relationships among monozygotic twins may be different from those among dizygotic twins and singleton siblings.

Using non-normal SEM to resolve the ACDE model in the classical twin design.

One of the biggest problems in classical twin studies is that it cannot estimate additive genetic (A), non-additive genetic (D), shared environmental (C), and non-shared environmental (E) effects, simultaneously, because the model, referred to as the ACDE model, has negative degrees of freedom when using Structural Equation Modeling (SEM). Therefore, instead of the ACDE model, the ACE model or the ADE model is actually used. However, using the ACE or ADE models almost always leads to biased estimates. In the present paper, the univariate ACDE model is developed using non-normal Structural Equation Modeling (nnSEM). In SEM, (1st- and) 2nd-order moments, namely, (means and) covariances are used as information. However, nnSEM uses higher-order moments as well as (1st- and) 2nd-order moments. nnSEM has a number of advantages over SEM. One of which is that nnSEM can specify models that cannot be specified using SEM because of the negative degrees of freedom. Simulation studies have shown that the proposed method can decrease the biases. There are other factors that have possible effects on phenotypes, such as higher-order epistasis. Since the proposed method cannot estimate these effects, further research on developing a more exhaustive model is needed.

Genetic and environmental sex differences in mental rotation ability: a Japanese twin study.

Sex differences in mental rotation ability have been observed in many countries. A previous study of Finnish participants reported that genetic and environmental influences on mental rotation ability differ between sexes. In this study, we assessed genetic and environmental influences on variance in mental rotation ability in 649 Japanese twins using a mental rotation test. To explain the influence of sex on variance in mental rotation ability, we applied genetic analysis using the sex limitation model. The following two factors explained variance in mental rotation ability: (1) the additive genetic factor, which reflects the accumulated small influence of many genes, and (2) the unique environmental factor, which is a type of environmental factor that differs between co-twins. The shared environmental factor, a type of environmental factor common for co-twins, could not explain the variance in mental rotation ability. Furthermore, the additive genetic factor was the same between sexes (i.e., not qualitative sex differences for the additive genetic factor), indicating that the same genes affect mental rotation ability in both sexes. Despite this observation, the additive genetic influence was greater in males than in females. In contrast, the unique environmental influence was not different between sexes. Considering the current results and those of a previous study, the quantitative sex difference for the additive genetic influences in mental rotation ability may be universal, while the unique environmental differences may depend on the characteristics of specific populations.

Genetic and environmental architecture of conscientiousness in adolescence.

Using a genetically informative design (about 2000 twin pairs), we investigated the phenotypic and genetic and environmental architecture of a broad construct of conscientiousness (including conscientiousness per se, effortful control, self-control, and grit). These four different measures were substantially correlated; the coefficients ranged from 0.74 (0.72-0.76) to 0.79 (0.76-0.80). Univariate genetic analyses revealed that individual differences in conscientiousness measures were moderately attributable to additive genetic factors, to an extent ranging from 62 (58-65) to 64% (61-67%); we obtained no evidence that shared environmental influences were observed. Multivariate genetic analyses showed that for the four measures used to assess conscientiousness, genetic correlations were stronger than the corresponding non-shared environmental correlations, and that a latent common factor accounted for over 84% of the genetic variance. Our findings suggest that individual differences in the four measures of conscientiousness are not distinguishable at both the phenotypic and behavioural genetic levels, and that the overlap was substantially attributable to genetic factors.

Etiological pathways of depressive and anxiety symptoms linked to personality traits: A genetically-informative longitudinal study.

BACKGROUND: The comorbidity of depression and anxiety is associated with an increased risk of prolonged adverse mental health status. However, little is currently known about their genetic and environmental influences that help to explain both the comorbidity and distinctiveness. Using longitudinal twin data, the present study investigated both the overlapping and distinct relationships between depression and anxiety viewed from the perspective of Gray's Reinforcement Sensitivity Theory (RST): two personality traits of the Behavioral Inhibition and Activation Systems (BIS and BAS). METHODS: A total of 422 twin pairs (298 monozygotic and 124 dizygotic pairs) participated by completing a personality questionnaire at wave 1, and mood symptoms questionnaires at wave 2. The waves were on average 2.23 years apart. RESULTS: Multivariate Cholesky decomposition indicated that the genetic variance of the personality traits (BIS and BAS) explained all of the genetic variance in depressive and anxiety symptoms. Additionally, genetic factors related to the BIS positively explained depressive and anxiety symptoms, whereas genetic factors related to the BAS negatively explained only depressive symptoms. LIMITATIONS: Limitations include shorter time interval and the reliance on self-reported data. CONCLUSIONS: The present study provided evidence explaining the overlap and differentiation of depressive and anxiety symptoms by using data on personality traits in a longitudinal, genetically-informative design. The findings suggested the personality traits from Gray's RST model played an important role in the prediction, and clarified the description, of both depressive and anxiety symptoms.

Direction of causation between shared and non-shared environmental factors.

Determining the direction of causation between two related variables is an interesting and challenging problem. A simple regression model is a frequently used statistical tool to find out whether a dependent variable is significantly predicted by an independent variable; however using a simple regression model cannot determine the direction of causation, because the model fit takes no account of this direction. As an approach to this problem, non-normal structural equation modeling (nnSEM; Shimizu and Kano, J Stat Plan Inference 138:3483-3491, 2008) using higher order moments (third, fourth,...) as well as first and second order moments, can be useful. This method enables us to determine the direction of causation using goodness of fit, even for a simple regression model. In this paper, nnSEM is applied to behavior genetics, in particular, to the genetic simplex model. In this context, nnSEM enables us to determine the direction of causation between C (shared environment) factors and between E (non-shared environment) factors. The efficiency of this method is illustrated by simulation studies and the analysis of real longitudinal twin data.

A general factor of personality from multitrait-multimethod data and cross-national twins.

In three studies, a General Factor of Personality (GFP) was found to occupy the apex of the hierarchical structure. In Study 1, a GFP emerged independent of method variance and accounted for 54% of the reliable variance in a multitrait-multimethod assessment of 391 Italian high school students that used self-, teacher-, and parent-ratings on the Big Five Questionnaire - Children. In Study 2, a GFP was found in the seven dimensions of Cloninger's Temperament and Character Inventory as well as the Big Five of the NEO PI-R, with the GFPtci correlating r = .72 with the GFPneo. These results indicate that the GFP is practically the same in both test batteries, and its existence does not depend on being extracted using the Big Five model. The GFP accounted for 22% of the total variance in these trait measures, which were assessed in 651 pairs of 14- to 30-year-old Japanese twins. In Study 3, a GFP accounted for 32% of the total variance in nine scales derived from the NEO PI-R, the Humor Styles Questionnaire, and the Trait Emotional Intelligence Questionnaire assessed in 386 pairs of 18- to 74-year-old Canadian and U.S. twins. The GFP was found to be 50% heritable with high scores indicating openness, conscientiousness, sociability, agreeableness, emotional stability, good humor and emotional intelligence. The possible evolutionary origins of the GFP are discussed.

Genetic effects on infant handedness under spatial constraint conditions.

Previous studies have reported a genetic influence on the individual differences in adult handedness; however, relatively little is known about genetic influences on the development of infant hand selection. In the current study, we examined whether genetic influences on handedness are expressed in various spatial locations in infants aged 18 months using the twin method. Infants were asked to respond to targets positioned in left, middle, and right locations using grasping movements. Results showed that similarities in hand selection within monozygotic twin pairs was more than two times higher than that of the dizygotic twin pairs in the middle location. In the left location, similarities in hand selection within monozygotic and dizygotic twin pairs were low. In addition, low individual differences in hand selection in the right location were also observed. These results suggest a non-additive genetic influence on handedness, and that spatial constraint is a crucial factor for the expression of genetic effects on handedness in infants.