Published papers

This study presents the first quantitative evaluation of the quality of early childhood education and care (ECEC) in Japan to make a significant contribution to the body of knowledge accumulated on ECEC in countries where research has been limited. We observed 30 classes comprising 3-year-olds, 28 classes comprising 5-year-olds, and 30 classes comprising mixed-ages from publicly provided nursery centers under the jurisdiction of the Kanto metropolitan area, Japan. An internationally-recognized quality rating scale for ECEC called the Early Childhood Environment Rating Scale, 3rd edition, which consists of six subscales, was used for this study. In contrast to previous studies conducted in the US, the results of this study showed that the Japanese ECEC is characterized as showing higher scores in the two subscales, “Personal Care Routines” and “Interaction,” and showing lower score in the subscale, “Learning Activities.” In addition, this study showed that the quality of ECEC varied across nursery centers. Furthermore, with regard to the two subscales, “Interaction” and “Language and Literacy,” the degree of variation within centers differed across nursery centers. This study analyzed how these characteristics of Japanese ECEC can be partly produced by the existence of national guideline for nursery centers authorized by the Japanese government. In addition, mechanisms producing differences in the quality of ECEC among and within centers were also discussed.

Using a large scale of administrative records provided by a local government, this study showed that experiencing family financial difficulties before school entry, signaled by receiving public assistance, and a child's own risk on health/development evaluated at the time of the Health Checkup for 3-year-old children lowered academic performance. It was deduced that the negative association between past financial difficulties of families and academic performance was greater than that between children's health and developmental risks and academic performance. Furthermore, the longer the period of exposure to family financial difficulties is, the greater the impact is. Additionally, the absence of health checkups for 3-year-old children was negatively related to academic performance. It was discussed that information on administrative records related to the welfare, health, and education of all children aggregated across jurisdictions and clarifying the relationship between developmental risk factors and outcomes would be extremely important for the effective implementation of scientifically based outreach and push-type support.

This study investigated the role that daily communication between parents and teachers in childcare centers plays in enhancing parental self-efficacy, which in turn decreases the child's problem behaviors and improves the parent–child relationship. The data were drawn from 239 children (boys: N = 141; girls: N = 98; mean (SD) age = 4.95 (0.99) years) in formal childcare settings from the Japan Child Panel Survey-Preschool Survey. The results showed that daily communication between parents and teachers was positively related to parental self-efficacy, which was negatively related to child's problem behaviors and the conflictive parent–child relationship. Mediation analysis showed that parental self-efficacy mediated the association between parent–teacher communication and child's outcome and parent–child relationship. Furthermore, the association between parent–teacher communication and parental self-efficacy was moderated by socioeconomic status (SES), suggesting that higher-SES parents benefited more from daily parent–teacher communication compared with lower-SES parents. Child's daily experiences at childcare centers were not related to parental self-efficacy or any other outcomes. These findings suggest that daily communication between parents and teachers in childcare centers plays an indispensable role in supporting parents, which ultimately leads to positive child development and family well-being.

[Importance] Although a growing number of studies have reported negative associations of the COVID-19 pandemic with academic performance among school-aged children, less is known about the pandemic’s association with early childhood development. [Objective] To examine the association between the COVID-19 pandemic and early childhood development. [Design, Setting, and Participants] In this cohort study conducted in all accredited nursery centers in a Japanese municipality, baseline surveys of children aged 1 and 3 years (1000 and 922, respectively) were conducted between 2017 and 2019, and participants were followed up for 2 years. [Exposure] Children’s development was compared at age 3 or 5 years between cohorts that were exposed to the pandemic during the follow-up and a cohort that was not. [Main Outcome and Measure] Children’s developmental age was measured by nursery teachers using the Kinder Infant Development Scale (KIDS). Data were analyzed between December 8, 2022, and May 6, 2023. [Results] A total of 447 children (201 girls [45.0%] and 246 boys [55.0%]) aged 1 year at baseline were followed up to age 3 years, and 440 children (200 girls [45.5%] and 240 boys [54.5%]) aged 3 years at baseline were followed up to age 5 years. During the follow-up, the cohorts that were exposed to the pandemic were 4.39 months behind in development at age 5 compared with the cohort that was not (coefficient, −4.39; 95% credible interval, −7.66 to −1.27). Such a negative association was not observed in development at age 3 years (coefficient, 1.32; 95% credible interval, −0.44 to 3.01). Variations in development were greater during the pandemic than before the pandemic regardless of age. Additionally, the quality of care at nursery centers was positively associated with development at age 3 years during the pandemic (coefficient, 2.01; 95% credible interval, 0.58-3.44), while parental depression appeared to amplify the association between the pandemic and delayed development at age 5 (coefficient of interaction, −2.62; 95% credible interval, −4.80 to −0.49; P = .009). [Conclusions and Relevance] The findings of this study showed an association between exposure to the pandemic and delayed childhood development at age 5 years. Variations in development widened during the pandemic regardless of age. It is important to identify children with developmental delays associated with the pandemic and provide them with support for learning, socialization, physical and mental health, and family support.

  This paper provides an overview of research presented from July 2022 to June 2023 that targeted infancy and childhood and consists of two parts. The first part summarizes the research themes presented in the "Development" section at the 65th Annual Meeting of the Japanese Association of Educational Psychology and compares them with recent trends. As reflected in the symposium "The Present and Future of Research Using Longitudinal Data" at the 65th Annual Meeting of the Japanese Association of Educational Psychology and in the special issue of The Japanese Journal of Developmental Psychology entitled "How Do Longitudinal Studies Contribute to Our Understanding of Development" (Vol. 33, No. 4, 2022), interest in longitudinal research has entered a new phase. In the second part, an overview of longitudinal research in Japan is provided. Specifically, it focuses on longitudinal studies among articles published in The Japanese Journal of Educational Psychology, The Japanese Journal of Developmental Psychology, The Japanese Journal of Psychology, Japanese Psychological Research, and international journals covering the period from infancy to childhood. Finally, research trends and prospects are discussed.

© 2018 American Psychological Association. Identifying the cognitive capacities associated with the development of school readiness is indispensable to support children's successful school transition. It has been shown that executive function (EF) in preschoolers is associated with both concurrent preacademic and subsequent academic skills. However, most research has controlled for the effect of general cognitive ability (GCA), and little research has examined non-English speaking children, which hinders the understanding of the development of school readiness in culturally diverse contexts. In addition, behavioral genetic research on the genetic and environmental influences on this association has been conducted with school-age children. Thus, the genetic and environmental influences on associations between GCA, EF, and preacademic skills during preschool age are unknown. We conducted multivariate behavioral genetic analyses on two waves of longitudinal data at 42 and 60 months of age from 171 and 135 pairs of monozygotic and dizygotic Japanese twins. The findings suggested that a genetic factor influenced GCA at 42 months and mediated preacademic ability at 60 months. In addition, another genetic factor emerged by 60 months that genetically mediated EF and math ability, independent of GCA. Preacademic ability at 60 months was affected by shared environments that influenced EF, rather than GCA, at 42 months. Moreover, shared environments that influenced preacademic ability at 42 months affected later GCA at 60 months. Finally, nonshared environments that had emerged by 60 months mediated these relationships, though the magnitude of this effect was modest.

Using an ethological observation method (the PC-MC comparison method), this study investigated patterns of reconciliation after aggression among Japanese preschool children, focusing on factors that influenced its occurrence or acceptance, as well as on strategies and function. There were several factors among four-year-olds that facilitated reconciliatory attempts and acceptance, such as proximity after the aggression; among three-year-olds there were no such factors. Depending on the situation during and after aggression, children used 'explicit' (e.g., apologizing, compromising, and offering objects) and 'implicit' (e.g., being friendly, talking without apologizing, and touching the opponent gently) reconciliatory strategies. Reconciliation occurred more often among friends than among non-friends, particularly among four-year-olds. However, after considering the baseline affiliation level, the conciliatory tendency among non-friends was higher than that among friends in four-year-olds. Victims' self-directed behavior (SDB) - a behavioral index of stress - was elevated following aggression, but decreased following reconciliation. This suggests that reconciliation functions to reduce the post-conflict stress suffered by the targets of aggression. © 2005 Wiley-Liss, Inc.

Sibling relationships appear important in fostering young children's growing theory-of-mind skills, but the quality of sibling interactions has rarely been investigated directly in relation to children's mental-state awareness (e.g. as indexed by talk about perceptions, desires, feelings, cognitions, i.e. inner state talk; IST). This study explored individual differences in young siblings' IST in relation to (i) child characteristics (verbal ability and theory-of-mind task performance); (ii) family characteristics (sib-ship size; social disadvantage, maternal well-being); and (iii) the quality of the children's play with their siblings (frequencies of pretence and reciprocal play). A socially diverse sample of 111 2-year-olds was filmed at home playing with a sib (mean age = 4.92 years; SD = 1.67 years); these sessions were coded from video for reciprocal play and then transcribed and coded for IST and pretence. Verbal ability was assessed for both sibs; in addition, 2-year-olds completed theory-of-mind tasks. Age and verbal ability were correlated with frequencies of overall talk, IST, pretence and reciprocal play. (Theory-of-mind scores were also correlated with 2-year-olds' IST, but this relation fell below significance once verbal ability was controlled). Significant independent relations were found between IST and children's quality of play, even when age, verbal ability and overall rates of talk were controlled. Although there were no gender contrasts in mean rates or in the variety of IST, gender differences were found in their correlates: in particular, context effects were significantly more important for girls than for boys. Individual differences in children's IST reflect multiple factors, including the dynamic features of children's reciprocal play. © 2006 The British Psychological Society.

This article reports developmental changes relating to reconciliation and bystanders' affiliation with victims of aggression (i.e., consolation) among 3- to 5-year-old Japanese preschool children. Use of the postconflict-matched control (PC-MC) method revealed that the frequency with which reconciliation and consolation were offered to a victim increased steeply in 5-year-olds, compared with 3- and 4-year-olds. The complexity of contextual factors affecting the occurrence of reconciliation and the form of consolation increased with age. Consolation occurred more often before reconciliation than after among all but the 3-year-olds and occurred more often when no reconciliation occurred than when it did occur among all classes. These findings support the view that consolation functions as a substitute for reconciliation, lessening the tension experienced by the victim of aggression. Copyright 2006 by the American Psychological Association.

The Tokyo Twin Cohort Project (ToTCoP) is a large-scale longitudinal study of 5 years based on 1619 pairs of infant twins reared together. The purpose of the study is to construct a population-based twin registry in Japan and to investigate human growth and development and twin themselves. It covers behavioral, neurological, physical and environmental variables measured by questionnaire, home visiting and brain imaging technology. The full registry contains over 47,000 multiple births collected from the Basic Resident Register, and the targeted population is 3070 probable twins of 0 to 2 years old. Preliminary analysis of the entry questionnaire data showed no serious sampling biases. Descriptive statistics of parental characteristics (parental age, gestation age, parity and placentation, maternal weight, parenting stress) and children's characteristics (body size at birth, 4 and 10 months of age, milk consumption, and sleeping and social behavior) and their correlations, genetic and environmental contributions and correlations are reported.

In general, aggression is believed to destabilize social integration. The question of who plays a central role in stabilizing and facilitating social integration is important in studies of conflict management and resolution. We investigated that question by applying social network analysis to behavioral data of preschool children (3- and 4-year-olds) in four classes. We identified two kinds of social network structures in each class: 'affiliative network structure' (ANS) consisting of socially positive behavior (affiliation and prosocial behavior) and 'disruptive network structure' (DNS) consisting of socially negative behavior (aggressive or disruptive behavior). Next, we tested how the exclusion of each child, having different degrees of centrality within the ANS or DNS, influenced the 'density', or cohesiveness, of the DNS or ANS among the same children. We found that excluding the children with higher centrality in the ANS did not affect the density of the DNS. In contrast, excluding the children with higher centrality in the DNS reduced the density of the ANS to a greater extent. This suggests that aggressive children play not only a dispersive role but also a cohesive role in their networks. This counterintuitive result highlights the adaptive and dual roles of aggressive children in their social groups. © 2008 Koninklijke Brill NV, Leiden.

This study investigated the association between negative parenting (NP) and conduct problems (CP) in 6-year-old twins, taking into account the severity of hyperactivity/inattention problems (HIAP). Analyses of the data from 1,677 pairs of twins and their parents revealed that the shared environmental covariance between NP and CP was moderated by the level of HIAP but not by CP or NP, where the shared environmental covariance was larger in children with higher levels of HIAP than in children with lower levels of HIAP. The genetic covariance between NP and CP was not moderated by the level of HIAP, whereas it was larger in the group with lower levels of CP and NP than in the group with higher levels. These results suggest that severe HIAP strengthens shared environmental associations between NP and CP and that interventions focusing on the shared environmental component of NP would be effective for parents and children with severe HIAP. © Springer Science+Business Media, LLC 2011.

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. © 2012 Blackwell Publishing Ltd.

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. © 2012 Elsevier Inc.

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. Copyright © The Authors 2013.

Copyright © 2016 John Wiley & Sons, Ltd. Executive functions (EF) are an important predictor of later adaptive development. A number of environmental influences, such as parenting, have been suggested as important promoters of EF development. However, behavioural genetic research has demonstrated that many environmental influences could be affected by genetic influences. Therefore, it is important to consider genetic variations when investigating environmental influences on EF development in children. To date, few studies have used genetically informative designs to assess the etiology of EF development during the preschool years, a period of rapid development. As a result, it remains unclear how and to what extent the environmental influences that are not confounded by genetic influences affect EF development during this developmental period. The present study explored EF development during the preschool years using a longitudinal and genetically informative design and a non-Western population. Japanese twins were visited at their homes and individually tested on EF measures at 24, 36, and 48 months of age. Phenotypic correlations and confirmatory factor analyses suggested that EF are less cohesive at 24 months of age and emerge as a common single factor at 36 and 48 months. Additionally, longitudinal and multivariate behavioural genetic analyses indicated that the EF developmental change during this period is promoted by both shared and nonshared environmental influences as well as genetic influences, while EF stability is brought about by shared environments. The present findings elucidated the etiology of EF development during the preschool years and confirmed that this period is an important transitional stage for EF development. Copyright © 2016 John Wiley & Sons, Ltd.

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 10months) and sociocognitive abilities at 19months 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. © 2011 Blackwell Publishing Ltd.

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.