Published papers

The purpose of this study was to explore the development of adolescent intelligence and cohort effects on their intelligence. Data for the study were drawn from the archival data of the secondary school attached to the University of Tokyo's Faculty of Education. Participants were 3,841 adolescents (1,920 females, M＝12.21 years, SD＝0.49; range 12–17), and the analyzed measure was the Tanaka B Intelligence Test - New Edition (Tanaka, 1953). Structural change, rank-order stability, and mean-level change of adolescents' intelligence were analyzed. The results showed that the structure of intelligence was highly consistent through adolescence and that rank-order stability was comparable to that found in previous research. As for mean-level change, adolescents' intelligence scores increased linearly and birth cohort had a statistically significant effect on the mean and slope of change. However, the directions of the effects were inconsistent. Later-born cohorts showed an increase of the mean and slope in the perception-related domain. On the other hand, these cohorts also showed a decline in their capacity to grasp the objects' relationships.

We examined developmental trends and sources of stability and change in adolescent personality by using twin data collected from 1981 to 2010 (273 monozygotic (MZ) and 48 dizygotic (DZ) twin pairs) from a secondary school affiliated with the University of Tokyo. Phenotypic analyses showed high rank-order stability and substantial mean-level increases in neuroticism and declines in extraversion over the adolescent years. Longitudinal bivariate genetic analyses revealed that the best-fitting model for adolescent personality includes additive genetic and non-shared environmental influences. Heritability estimates ranged approximately from 0.30 to 0.60. Additionally, three-year stability in adolescent personality was influenced mainly by genetic factors, and there were both genetic and environmental innovations in mid-adolescence. Our findings suggest that both genetic and environmental effects have significant roles in the etiology of personality development across adolescence.

The present study examines the association between the General Factor of Personality (GFP) and working memory and its etiology with a behavioral genetic approach. The GFP, which explains the common variance among lower-order personality traits, is considered to reflect social effectiveness. Meanwhile, working memory also plays a significant role in social competence. Hence, we expected a substantial association between the GFP and working memory. A total of 253 Japanese twin pairs (124 monozygotic female; 52 monozygotic male; 28 dizygotic female; 17 dizygotic male; and 32 opposite sex twins) were included in the analyses. Phenotypic analyses confirmed a significant positive correlation between the GFP and working memory. Biometric analysis with a bivariate Cholesky decomposition model showed that the phenotypic correlation derived from additive genetic and non-shared environmental correlations. The present findings are in line with social effectiveness account of the GFP.

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.

© 2019 Elsevier Ltd Research has indicated that human morality is associated with cognitive ability. However, morality is not a unified concept but rather is a multi-faceted concept. Moral Foundation Theory suggests that human beings have at least five innate moral foundations: Care/Injury, Fairness/Deception, Loyalty/Betrayal, Authority/Subversion, and Sacred/Fallen. The present study aimed to investigate the associations between these moral values and cognitive ability. A large-scale cross-sectional survey was conducted online, which was completed by a total of 4863 Japanese adults (2922 females, 1935 males, and 6 “other;” Mage = 48.78, SDage = 10.93, range 20–70). Correlation and multiple regression analyses revealed that cognitive ability was positively linked to Care/Injury, Fairness/Deception, and Sacred/Fallen. Notably, the positive associations of cognitive ability with Loyalty/Betrayal and Authority/Subversion were statistically significant only for people below the age of approximately 50. These findings indicate that although moral foundations are based on emotions and intuitions, they are substantially associated with cognitive ability. In addition, the positive direction of associations of cognitive ability with Loyalty/Betrayal and Authority/Subversion was inconsistent with previous findings in Western countries, which suggests that these associations are culture-dependent.

Using social media, which provides ways to socialize and present oneself, has become normal for young people. Although previous research shows that self-presentation via social media is associated with a sense of identity, little attention has been given to the association with identity-development processes. Therefore, the present study examined these associations while controlling for offline self-presentation and tested the potential moderating effect of neuroticism. A sample of 647 Japanese young adults (401 women; Mean age = 22.70; SD age = 3.25) were recruited to complete an online questionnaire, including measures of identity-development processes, online and offline self-presentation, and neuroticism. A correlation analysis revealed that identity-development processes were associated with online surface-level self-presentation (SSP) but not online inner-level self-presentation (ISP). Further, a moderated regression analysis indicated that the negative associations between online ISP and identity processes were only found among those high in neuroticism. The results of this study suggest that neuroticism may produce variations in young people's identity development.

This study focused on effects of life experiences on change in personality and moderation effects of attachment security on the life experience influences. With a sample of Japanese university students (N = 1,000; 679 female; M = 19.72; SD = 1.26; age range, 18-25 years), the Big Five personality traits were assessed twice. The assessment interval was 20 weeks. Hierarchical linear modeling (HLM) analyses revealed that there were large between-individual differences in change and stability of the Big Five personality traits, which were accounted for by the amount of daily positive and negative life experiences during the interval period. Moreover, multi-group HLM analyses indicated that the life experience effects were moderated by participants' attachment security at Time 1. These findings suggest that the accumulation of small daily experiences may work for the personality development of university students and that environmental influences may vary by individual susceptibility to experiences, like attachment security.

We examined developmental trends in personality traits during adolescence by using data from the secondary school affiliated with the University of Tokyo (N = 3656; range 12-18; 1832 female), collected from 1981 to 2010. Hierarchical linear modeling analyses revealed cumulative mean-level changes in personality averaging approximately 1 SD across adolescence. Scales related to Neuroticism showed linear increases, and those related to Extraversion showed distinct developmental patterns: General Activity and Rhathymia did not change, while Ascendance and Social Extroversion declined. Additionally, we found significant gender and birth cohort effects on personality. Although the intercepts differed according to both factors, the trajectories were different only for the birth cohort. These findings suggest that personality development trajectories differ according to the sociocultural context. (C) 2015 Elsevier Inc. All rights reserved.

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.

Research has repeatedly shown that early adversities are a risk factor for psychological distress in adulthood. However, limited research has examined the association between early adversities and intraindividual changes in psychological distress among middle-aged adults. This study developed a self-report scale to retrospectively measure the level of adversity experienced in childhood (Early Environment Scale: EES) and investigated the association between the EES score and the change in psychological distress in middle-aged adults. Study 1 examined the reliability and validity of the nine-item EES using a sample of 337 Japanese adults (162 women; Meanage = 42.96, SDage = 10.55). In Study 2, a three-wave six-month longitudinal survey was conducted with a sample of 2000 Japanese adults (1000 women; Meanage = 40.94; SDage = 5.35). Psychological distress at all three waves was measured with the Kessler Psychological Distress Scale (K-6). The results from a latent growth analysis showed significant positive associations between early adversities and the intercept and slope factors of psychological distress. Specifically, individuals who reported more early adversity showed higher initial levels of psychological distress and a stronger increase in psychological distress among Japanese middle-aged adults. Early adversities might influence the development of dispositional sensitivity for psychological distress in adulthood.

The present study examined whether the General Factor of Personality (GFP) is associated with moral foundations. The GFP represents the shared variance among lower-level personality traits and is assumed to reflect social effectiveness. We compare two contrasting hypotheses regarding the associations between the GFP and moral values. The social effectiveness view suggests relative independence of the two constructs, whereas, based on Life History theory, relevant associations would be expected. Two samples of Japanese adults participated in this study online. The first sample (Sample 1) consisted of 500 Japanese (M = 45.42 years; SD = 8.53; range 20-60; 250 women), and the second sample (Sample 2) of 487 Japanese (M = 41.13 years; SD = 10.08; range 20-60; 244 women). The GFP was found in the HEXACO-60 in Sample 1 and was extracted from the NEO-FFI in Sample 2. Correlational and multiple regression analyses revealed that the GFP was modestly, yet significantly associated with Ingroup/Loyalty which refers to individual differences in trueness, patriotism, and self-abnegations for ones group members. The GFP was not substantially associated with the other moral foundations. These findings indicate the relative independence of the GFP from moral values, which is more congruent to the social effectiveness view. (C) 2017 Elsevier Ltd. All rights reserved.