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This paper presents comparative information on the socioeconomic status (SES) gradients in literacy skills at age 6-8, drawing on harmonized national datasets from France, Germany, Japan, the Netherlands, the United Kingdom, and the United States. We investigate whether understanding of comparative SES gradients in early-to-mid childhood depends on the operationalization of SES (parental education, income, or both); and whether differences in inequalities at the end of lower secondary schooling documented in international large-scale assessments are already present when children have experienced at most two years of formal compulsory schooling. We find marked differences in the SES gradient in early achievement across countries that are largely insensitive to the way SES is measured, and that seem to mirror inequalities reported for older students. We conclude that country context shapes the link between parental SES and children’s educational achievement, with country differences rooted in the early childhood period.

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.

This study used a behavioral genetic approach to examine the genetic and environmental etiology of stability and changes in self-esteem in relation to personality. Multiple genetic analyses were conducted on a longitudinal dataset of self-esteem and Big Five personality scores among young adult Japanese twins over the course of a decade. There were 1221 individuals for whom data were available on both self-esteem and the Big Five personality test at Time 1 and 365 at Time 2. The mean interval between the two times was 9.95 years. Genetic effects on self-esteem were robust, and the same genes were responsible for the stability of self-esteem in individuals over time. Nearly half of the variance in self-esteem was explained by a new genetic factor arising during the decade, suggesting that genetic innovation of self-esteem occurred in early adult life. The genetic and environmental covariance structure between personality and self-esteem in individuals was constant over a decade, providing evidence that the stability of self-esteem was largely attributable to personality. However, genetics for self-esteem independent of personality still contributed to stability over time, differentiating the concept of self-esteem from personality as a trait in terms of its genetic and environmental etiological levels.

The Child Allowance Policy (CAP) in Japan, a nationwide cash transfer program for families with children, was designed to increase household expenditures toward children. Using unforeseen changes in the CAP that occurred due to the electoral results as a source of exogenous variation in income in the early 2010s, this paper examines the causal impact of family income on households’ private educational expenditures and child outcomes in the short-run, based on a longitudinal parent-child survey. The ordinary least squares (OLS) and first-differenced (FD) results show that family income is in most cases positively correlated with child's cognitive outcomes, and, to a lesser extent, with families’ educational expenditure on their children. Based on the FD instrumental variable (FD-IV) estimation, using unexpected changes in CAP payments as an instrument, we find positive income effects on educational expenditure in the short-run. However, we did not find statistically significant impacts on children's cognitive outcomes.

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 cultural behavior genetic approach was introduced as a prospective means to describe psychological differences between cultures. We compared genetic and environmental influences on remembered parenting for samples of twins from Japan and Sweden. Data were collected from 720 pairs of young adult Japanese twins and 824 pairs of adult Swedish twins using the Parental Bonding Instrument. In both samples, a very similar phenotypic factor structure was developed for maternal and paternal parenting. However, the genetic and environmental contributions were different. Parenting in Japan showed more genetic influences, whereas parenting in Sweden showed more shared environmental influences. Moreover, covariation among the six dimensions of parenting (i.e., maternal and paternal Warmth, Protectiveness, and Authoritarianism) was due to genetic correlations in Japan and to shared environmental correlations in Sweden. These results are consistent with the cultural psychology argument that parenting practices are child centered in Japan but parent centered in the West.

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.

It has been reported that the ability to solve syllogisms is highly g-loaded. In the present study, using a self-administered shortened version of a syllogism-solving test, the BAROCO Short, we examined whether robust findings generated by previous research regarding IQ scores were also applicable to BAROCO Short scores. Five syllogism-solving problems were included in a questionnaire as part of a postal survey conducted by the Keio Twin Research Center. Data were collected from 487 pairs of twins (1021 individuals) who were Japanese junior high or high school students (ages 13–18) and from 536 mothers and 431 fathers. Four findings related to IQ were replicated: 1) The mean level increased gradually during adolescence, stayed unchanged from the 30s to the early 50s, and subsequently declined after the late 50s. 2) The scores for both children and parents were predicted by the socioeconomic status of the family. 3) The genetic effect increased, although the shared environmental effect decreased during progression from adolescence to adulthood. 4) Children's scores were genetically correlated with school achievement. These findings further substantiate the close association between syllogistic reasoning ability and g.

Using a behavioral genetic approach, we examined the validity of the hypothesis concerning the singularity of human general intelligence, the g theory, by analyzing data from two tests: the first consisted of 100 syllogism problems and the second a full-scale intelligence test. The participants were 448 Japanese young adult twins (167 pairs of identical and 53 pairs of fraternal twins). Data were analyzed for their fit to two kinds of multivariate genetic models: a common pathway model, in which a higher-order latent variable, g, was postulated as an entity; and an independent pathway model, in which the higher-order latent variable was not posited. These analyses revealed that the common pathway model which included additive genetic and nonshared environmental factors best accounted for the three distinct mental abilities: syllogistic logical deductive reasoning, verbal, and spatial. Both the substantial g-loading for syllogism-solving, historically recognized as the symbol of human intelligence, and the emergence of g as an entity at an etiological level, that is, at the genetic and environmental factor level, provide further support for the g theory.

Since natural selection produces genetically homogeneous populations with regard to adaptively important traits, the relatively high genetic variance associated with human personality is an enigma. In this article, we propose that humans adaptively control the activation of domain-specific mental mechanisms in accordance with personality. This process functions to reduce fitness differences among individuals with different genetic backgrounds associated with personality. Such control would facilitate the evolution of heritable personality traits. We conducted a twin study, showing that the level of general trust (trust of strangers) is controlled not only by environmental factors but also by personality factors, thus producing reactive heritability of general trust. This result supports our hypothesis.

Why does decision making differ among individuals? People sometimes make seemingly inconsistent decisions with lower expected (monetary) utility even when objective information of probabilities and rewards are provided. It is noteworthy, however, that a certain proportion of people do not provide anomalous responses, choosing the alternatives with higher expected utility, thus appearing to be more “rational”. We investigated the genetic and environmental influences on these types of individual differences in decision making using a classical Allais problem task. Participants were 1,199 Japanese adult twins aged 20–47. Univariate genetic analysis revealed that approximately a third of the Allais problem response variance was explained by genetic factors and the rest by environmental factors unique to individuals and measurement error. The environmental factor shared between families did not contribute to the variance. Subsequent multivariate genetic analysis clarified that decision making using the expected utility theory was associated with general intelligence and that the association was largely mediated by the same genetic factor. We approach the mechanism underlying two types of “rational” decision making from the perspective of genetic correlations with cognitive abilities.

In past decades, income inequality has risen in most developed countries. There is growing interest among economists in international comparisons of economic and educational mobility. This is aided by the availability of internationally comparable, large-scale data. The present paper aims to make three contributions. First, we introduce the Japan Child Panel Survey (JCPS), the first longitudinal survey of school-age children that includes cognitive and non-cognitive measures, and rich household information. The JCPS was developed to measure dynamic inter-relationships between children's academic and social outcomes, their family background, and local policy and environment, in a way that allows comparison of the results with international data. Second, based on JCPS data, we present selected results of the dynamics of inequality in multiple indicators of children's educational and behavioral outcomes. We found that changes in cognitive achievement across parental income groups, the degree of mobility of cognitive test scores, and the correlation between the difficulty score and parental education in Japan are similar to other countries, such as the United States, United Kingdom, Australia, and Germany. Finally we discuss issues underlying the globalization of education research based on our experiences with the JCPS. We discuss reasons and strategies for further globalization of education research in Japan, and propose suggestions as to how Japanese education research can move toward better international collaboration, particularly in research on economic and educational mobility.

Highlights • Genes rather than environment contribute to family resemblance in academic motivation. • Environmental influences stemmed entirely from individual specific experiences. • Attending same vs. different classes did not affect twin similarity in motivation. • Results are similar across ages, countries and academic subjects., 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.

Prosociality is one of the most distinctive features of human beings but there are individual differences in cooperative behavior. Employing the twin method, we examined the heritability of cooperativeness and its outcomes on public goods games using a strategy method. In two experiments (Study 1 and Study 2), twin participants were asked to indicate (1) how much they would contribute to a group when they did not know how much the other group members were contributing, and (2) how much they would contribute if they knew the contributions of others. Overall, the heritability estimates were relatively small for each type of decision, but heritability was greater when participants knew that the others had made larger contributions. Using registered decisions in Study 2, we conducted seven Monte Carlo simulations to examine genetic and environmental influences on the expected game payoffs. For the simulated one-shot game, the heritability estimates were small, comparable to those of game decisions. For the simulated iterated games, we found that the genetic influences first decreased, then increased as the numbers of iterations grew. The implication for the evolution of individual differences in prosociality is discussed.

Since the pioneering work of Eaves and Eysenck (1974) appeared in Nature some 40 years ago, psychologists, sociologists, political scientists, and behavioral geneticists have investigated the effects of nature and nurture on the formation of social attitudes. It has consistently been found that manifestations of social attitudes (i.e., preferences, values, and beliefs pertaining to things such as politics, religion and the treatment of ingroups and outgroups) are genetically influenced. More recently, researchers have focused their efforts on the psychophysiological pathways between gene activity and attitudes. In particular, a broad body of research examines how personality traits may be a link between genetic factors and political orientations. The latter are typically treated as either a single left–right dimension or divided into two core aspects: resistance to change/authoritarian conservatism and acceptance of inequality/social dominance orientation. In this essay, we provide an overview of this research, present some findings from our recent international behavioral genetic study on the topic, and identify key issues for future research.We suggest that future studies treat attitude formation as a complex process in which genetic factors and the psychophysiological phenomena that stem from them are affected by the surrounding social environment and culture. Such research will require (i) international study designs capturing individual and cultural levels of variation and (ii) interdisciplinary collaboration among scientists and researchers in various fields of study such as genetics, psychology, sociology, political science, neuroscience, and human biology.

Since the pioneering work of Eaves and Eysenck (1974) appeared in Nature some 40 years ago,psychologists, sociologists, political scientists, and behavioral geneticists have investigated the effects of nature and nurture on the formation of social attitudes. It has consistently been found that manifestations of social attitudes (i.e., preferences, values, and beliefs pertaining to things like politics, religion, the treatment of ingroups and outgroups, etc.) are genetically influenced. More recently, researchers have focused their efforts on the psycho-physiological pathways between gene activity and attitudes. In particular, a broad body of research examines how personality traits may be a link between genetic factors and political orientations. The latter are typically treated as either a single left-right dimension or divided into two core aspects: resistance to change/authoritarian conservatism and acceptance of inequality/social dominance orientation. In this article, we provide an overview of this research, present some findings from our recent international behavioral genetic study on the topic, and identify key issues for future research. We suggest that future studies treat attitude formation as a complex process in which genetic factors and the psycho-physiological phenomena that stem from them are affected by the surrounding social environment and culture. Such research will require: (1) international study designs capturing individual and cultural levels of variation; and: (2) interdisciplinary collaboration among scientists and researchers in various fields of study such as genetics, psychology, sociology, political science, neuroscience, and human biology.

Social inequality is a social phenomenon which varies across cultures. However, it does not only depend on social and cultural influences (e.g., public ideological opinion), but also on basic individual attitudes and values that promote socio-political decisions and behaviour. Behaviour genetic studies across several nations have found that individual differences in core ideological attitudes toward social and economic equality versus hierarchy (e.g., rejecting versus accepting inequality or social dominance orientation) are to some degree genetically influenced and linked to specific personality traits (e.g., openness and agreeableness), albeit the effect size and correlations may vary among cultures. In addition, these studies have highlighted the significance of important social interaction partners (e.g., spouses) and social networks for opinion formation. Thus, attitudes toward inequality are shaped by both top-down (acquisition of opinions through exposure to social environments and culture) and bottom-up effects (underlying genetic basis that influence the individual receptiveness to specific opinions).

Human intelligence, as measured by intelligence quotient (IQ) tests, demonstrates one of the highest heritabilities among human quantitative traits. Nevertheless, studies to identify quantitative trait loci responsible for intelligence face challenges because of the small effect sizes of individual genes. Phenotypically discordant monozygotic (MZ) twins provide a feasible way to minimize the effects of irrelevant genetic and environmental factors, and should yield more interpretable results by finding epigenetic or gene expression differences between twins. Here we conducted array-based genome-wide DNA methylation and gene expression analyses using 17 pairs of healthy MZ twins discordant intelligently. ARHGAP18, related to Rho GTPase, was identified in pair-wise methylation status analysis and validated via direct bisulfite sequencing and quantitative RT-PCR. To perform expression profile analysis, gene set enrichment analysis (GSEA) between the groups of twins with higher IQ and their co-twins revealed up-regulated expression of several ribosome-related genes and DNA replication-related genes in the group with higher IQ. To focus more on individual pairs, we conducted pair-wise GSEA and leading edge analysis, which indicated up-regulated expression of several ion channel-related genes in twins with lower IQ. Our findings implied that these groups of genes may be related to IQ and should shed light on the mechanism underlying human intelligence.

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.

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.

This study investigated the possible effects of genetic and environmental gender differences in effect on individual differences by using the Bem Sex Role Inventory (BSRI) with twins. A sex/gender-limitation analysis, a behavior genetics methodology was used to the following: (a) effects of gender-specific genes, (b) gender differences in quantitative genetic effects, (c) effects of gender-specific shared environment, (d) gender differences of quantitative shared environment, and (e) gender differences of quantitative nonshared environment. Participants were adolescent and adult twins, including 111 identical male pairs, 241 identical female pairs, 36 fraternal male pairs, 65 fraternal female pairs, and 58 opposite-gender pairs. The results indicated that although masculinity and femininity were explained by genetic factors to some extent, there were no significant gender differences in the genetic factors. Moreover, because our data did not support a model which explained gender differences in the effects of specific common environment factors, no evidence was found to support the prenatal hormonal hypothesis or the existence of parenting which encouraged children's gender role personality.

Since established in 1998, the Keio Twin Project (KTP) has been dedicated to investigating genetic and environmental sources contributing to human psychological traits in adolescence and young adulthood. A population-based twin registry was constructed by the KTP through the use of official residential records in the Tokyo area, and to date requests to participate in our research have generated 1040 pairs of twins and triplets of age 14 to 30, forming one of the largest twin registries in Asia. Our comprehensive datasets, obtained through questionnaires, performance tests, and physical measurements, cover a wide range of human traits: personality, psychiatry, mental health, sociality, cognition, and physical index. Demographic variables and environment of upbringing are also sought by twins and by some parents. This extensive information allows us to clarify the genetic and environmental overlap across multiple traits as well as specificities unique to single traits. Adding an evolutionary psychology perspective to the behavior genetics framework is currently being attempted in order to develop a grand theory of human genetics.