In this paper, we discuss what longitudinal research is, why it is necessary, and how to conduct it in real developmental-research context. Longitudinal research can be conducted using various research designs, but their common features are that data are collected from the same subject at multiple time points and analyses focus on change over time and relationships within individuals. Longitudinal approaches with these characteristics are beneficial to developmental research because they enable the researcher to (1) separate the effects of age, cohort, and period, (2) quantify trajectories of intraindividual change and how these differ between individuals, and (3) identify clues to causal relationships (especially temporal order). Taking a longitudinal approach increases the complexity of research design and data-analysis methodology. However, this cost is offset by the potential to broaden and deepen developmental research by generating creative research questions of a different kind from those generated by cross-sectional studies.

For human lifespan development research to be truly developmental, it is essential that the flow of time in which people live in a prospective manner is examined. Thus, longitudinal research should be enriched in developmental psychology. However, merely collecting longitudinal data does not reveal developmental continuity and changes from infancy to old age. Consequently, an in-depth examination of theories of developmental continuity and changes and use of appropriate methods based on these theories are imperative. In this paper, several long-term longitudinal studies on the lifespan development of human attachment are reviewed. Endeavors have been made to examine the continuity and changes of personal characteristics of attachment throughout life by employing the conceptual device of Internal Working Model. The difficulties of examining developmental continuity and changes, its aporia, and the possibilities and challenges of future longitudinal research are discussed.

A longitudinal cohort study design for health science research is most desirable for a life-course developmental approach. Longitudinal studies have revealed that early life stress may be associated with subsequent negative mental and physical health across individuals' lifespan. The author has provided additional evidence of long-term negative effects of adverse childhood experiences (both in domestic and out-of-home settings), including bullying, on adult health by employing United Kingdom longitudinal birth cohort data. Various studies have elucidated potential malleable protective/compensatory factors for mitigating these long-term negative effects. It is recommended that future research employ cross-cohort and cross-cultural comparisons to verify such findings. Advantages and disadvantages of conducting large-scale longitudinal studies in Japan are discussed.

Developmental research is a dynamic discipline aiming to clarify how and by what mechanisms various functions change over time. Longitudinal studies demonstrate their effectiveness when examining individual differences in interrelationships, including developmental changes. The idea that relationships between elements create developmental changes is also consistent with the recently developed dynamic systems theory. This paper first describes the relevant theoretical viewpoints and then discusses the challenges arising in actual longitudinal research and the potential for using longitudinal data in heuristic research. Finally, given that the current research environment makes longitudinal research unfeasible, we question whether open access could be provided to the data previously accumulated in longitudinal studies.

In this study, adversity experiences were examined from the perspective of longitudinal studies. Individuals' experiences of adversity, including early developmental abuse and family dysfunction, may cause a variety of physical and psychological problems in adulthood. Group adversity experiences may include traumatic experiences that result in the bereavement of family members and/or acquaintances, damage to buildings, and even social system crises. These traumatic experiences may have profound effects on children, including posttraumatic stress disorder, and may cause cascade effects in their interactions with those around them. However, resilience may preserve children's health and encourage posttraumatic growth through adversity. Although it is beneficial to examine the developmental mechanisms of adversity from a variable-focused approach employing multivariate analysis, it is essential to consider a new approach, which pays attention to the person-focused approach of development that takes into account the uniqueness of human development by combining prospective and retrospective perspectives freely and frames adversity in life constructively.

The author examines Endo (2021)'s proposal that a person-based bottom-up long-term longitudinal study (BULS) can take a balance of researchers' interests with the pursuit of a person's well-being in comparison with a top-down longitudinal study based on researchers' interests. He insists, (1) BULS provides analysis based solely on intra-individual variability, indicating that every person has a way to develop; (2) BULS can be analyzed according to the timing of individual development, so it is possible to understand the reality of research objects inherent in individual context-dependent processes; (3) in BULS, a researcher can grasp the “uniqueness” of the life for each person, because of their long-term experiences shared with research participants. As the limitation of BULS, the influence of the historical times is confounded due to the long-term duration of investigation, but analyzing cohort effects can lead to clarification of the plasticity of personal development embedded in society.

The twin method is an approach within behavioral genetics that reveals both genetic and environmental effects on traits by comparing the behavioral similarities of monozygotic twins, who share the heredity and family environment, with those of dizygotic twins, who share half their genes but have the same family environment similarity as monozygotic twins. In classical twin studies, genetic factors are treated as latent variables rather than at the molecular level, and the focus is on variance rather than mean values. There is a vast accumulation of twin studies in the various fields of psychology that show significant and substantial genetic effects on all behaviors, but no trait has been found to be 100% genetic. It has been universally shown that most environmental factors are not shared within families. From a developmental psychology viewpoint, evidence for dynamic changes in genetic effects, such as the expression of new genetic factors (genetic innovation) and the increase in the heritability of intelligence with age, is of particular interest. Stability between ages in many traits, moreover, has generally been found to be mainly attributable to heredity. The specific examples of these findings will be introduced through a meta-analysis of large-scale cross-sectional twin studies and the results of our longitudinal twin projects, using the Cholesky decomposition model, latent growth model, cross-lagged model, and the analysis of differences in monozygotic twins.

I Research on child development considers it important to understand each child's developmental process as well as that of the entire population. Therefore, child development scholars must be proficient in two methodologies—longitudinal research and longitudinal analysis—with the latter based on drawing developmental trajectories. This study focuses on describing the population's average developmental trajectory while capturing individual deviations from the average. To this end, this study introduces the growth curve model and latent class growth analysis, highlighting findings from the Hamamatsu Birth Cohort (HBC) Study. The growth curve model introduces the mixed-effects and latent class approaches using an example question of whether an individual's birth weight affects their expressive language development. Latent class growth analysis emphasizes the parallel-process approach, which processes multiple domains in parallel and the joint model, which can be used to examine links between the developmental trajectories of two outcomes.

Since Hamaker, Kuiper, and Grasman's (2015) critique, the random intercept cross-lagged panel model (RI-CLPM), which includes stable trait factors as unobserved heterogeneity to reveal within-person processes, has been widely applied in psychology. However, although various longitudinal models that examine reciprocal relations exist in different contexts and disciplines, their conceptual and mathematical relations have not been well recognized, and scholars continue to discuss the issues of model comparison and model choice. This study provides an overview of the RI-CLPM and then introduces other longitudinal models to explain their relations as well as potential difficulties to infer within-person processes. It describes that stable trait factors with time-invariant impacts on observations are modeled separately from regression models, making this factor conceptually and mathematically different from common factors in many other models. We also highlight that the presumed uncorrelatedness between stable trait factors and within-person processes is the key to understanding the RI-CLPM and how it is mathematically related to dynamic panel models, which could be a useful candidate.

Since the 1960s, researchers have widely used single-case experimental designs (SCEDs) as a standard method in applied behavior analysis, but this research method has rarely attracted the attention of scholars in other research fields. Recent years, however, have seen an increased interest in SCEDs in various research fields, including clinical psychology and developmental psychology, partly because of the demand for evidence-based practice. Therefore, this article provides a basic description of SCEDs and explains their fundamental properties as well as the characteristics and problems encountered in typical SCEDs often implemented in applied research. In addition, this study will explain the visual inspection and statistical methods for examining data obtained from SCEDs. It will also describe the standards for evaluating the quality of single-case experimental design studies, especially the criteria to satisfy “Meets WWC SCD Standards without Reservations” proposed by WWC standards version 4.1.

The importance of longitudinal studies in developmental psychology is widely recognized. Nevertheless, in Japan, they have not become the mainstream of developmental research due to the difficulty of conducting them. Therefore, we have been conducting a longitudinal study on the development of social cognition in early childhood by launching a project in collaboration with the local community. In this study, we discuss the typical developmental stages of joint attention and differences between typical development and autism spectrum disorder based on a longitudinal study of children from 8 to 18 months. Next, we explain the screening test for autism spectrum disorder, which was developed based on the characteristics of joint attention in children diagnosed with later autism spectrum disorders, and discuss its potential and shortcomings as a tool for early detection. Additionally, we introduce the project's efforts as a practical example of early support based on early detection. Finally, we discuss the significance and problems of longitudinal studies based on this project's findings.

Response to instruction (RTI) is the prevention and remediation model through research-based, effective instructions, and increasingly intense interventions. In the USA, the Individuals with Disabilities in Education Act（IDEA）in 2004 allows school districts to implement RTI models for LD identification. In this paper, we reviewed the studies based on longitudinal data to reveal recent issues in the RTI model. We also introduce school-based practices and studies conducted in Japan. In the USA, where RTI intervention is closely linked to LD identification, the determination of non-responders and prediction of long-term outcomes are important topics for studies, and a substantial number of longitudinal studies have been conducted. On the other hand, in Japan, RTI intervention is emphasized as an “intervention without identification,” and research mainly focused on the effect of interventions. Although only a few studies have targeted Tier 3 intervention, both in the US and Japan, our study revealed consistent results with those in the USA. Further studies will be required to reveal how the remediation of word-reading skills relates to future reading comprehension.

The COVID-19 pandemic has significantly changed social conditions, with concerns about its impact on children's mental health and socioemotional development . However, only a few Japanese studies have offered a comparison of children's behaviors before and during the pandemic or analyzed longitudinal changes as the pandemic was taking place. Therefore, we present cross-sectional and longitudinal studies, conducted by the authors, that examined socioemotional behavior (among children 4 to 9 years old) and psychological distance to caregivers and others (among children 0 to 9 years old) at four time points: prepandemic (T0), April 2020 (T1), October 2020 (T2), and February 2021 (T3). According to the results of these studies, children's socioemotional behavior hardly changed at these four time points and that their psychological distance from caregivers and others was influenced by the pandemic. These findings were discussed considering the issues surrounding cross-sectional and longitudinal research methods.

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 aimed to analyze long- and short-term longitudinal data sets (first wave N＝1,448 and 91, respectively) to identify profiles of change in depression during adolescence and adulthood using a group-based trajectory approach and investigate the extent to which two personality traits (Behavioral Activation and Inhibition Systems; BAS and BIS) measured at the first wave predicted these trajectories. Three distinct trajectories, low, moderate, and high depression, were identified via group-based trajectory modeling . Multinomial regression analysis revealed that the two personality traits contributed to the development of depression, with lower BAS and higher BIS sensitivity leading to greater levels of depression in both longitudinal data sets. These results indicate that there is heterogeneity in longitudinal profiles of change in depression and that personality traits are predictive of the patterns in the development of depression. These findings may facilitate the early identification of those at greater risk of following the higher-depression trajectory.

This study examined the relationship between scolding and negative emotions among mothers and how abuse anxiety, marital satisfaction, and children's birth order moderate it. Ninety-nine mothers (53 primiparas and 46 multiparas) caring for 2- and 3-year-old children were surveyed. Participants were given Android phones with an experience sampling method application installed. They were asked to answer questions when notified daily between 10 pm and 12 pm for four weeks. The results indicated that scolding typically leads to higher levels of negative emotion; however, none of the interactions were significant. While it has been suggested that feeling negative emotions during parenting may facilitate the development of mothers' parenting skills, it may also facilitate mental illness. Thus, relieving negative emotions after scolding their children may help promote mothers' mental health.

A challenge in dealing with gender dysphoria in children is the mixture of stable and temporary fluctuations in the feelings of gender dysphoria during development. This study examines the absolute and relative stability and the patterns of change of the feelings of gender dysphoria using data from a six-year longitudinal survey across three cohorts. The participants consisted of 2,031 people (999 boys and 1,032 girls) from fourth to ninth grades who completed the feelings of gender dysphoria scale. As a consequence of examining the changes in mean values by grade as absolute stability, the scores of boys slightly decreased from fourth to ninth grade; however, almost no significant change was found for girls. Latent profile analysis revealed 11 patterns including 22.6% of students who increased the score only within 1–2 years. The correlation between each grade as relative stability resulted in a positive correlation between each grade for boys and girls. In addition, the correlation coefficient increased as the grade increased, indicating that relative stability increased. The correlation coefficient was higher for girls than boys.

This study investigated the development trajectory of positive reappraisal and the relationship between positive reappraisal and emotional talk with parents and peers by 5-wave annual cohort-sequential design in adolescence. The study recruited 294 adolescents in their fourth year of elementary school, first year of junior high school, and first year of high school during the first wave. In each cohort, positive reappraisal at the four and five waves was higher than that at the first and second waves. Using an 11-wave random intercepts cross-lagged panel model, the between-person results indicated a strong trait effect for the positive association between positive reappraisal and emotional talk with parents and peers. Furthermore, the within-person results showed a longitudinal association between positive reappraisal and emotional talk with peers. Therefore, emotional talk with parents and peers positively affected this reappraisal.

Previous studies have not discussed the effect of class-size change that students experience accompanied by primary to secondary school transition. This study examined the impacts of the elementary school class size, the differences between class size and number of students in the same grade, and the transition to secondary school on norm-referenced standardized test score trends. We constructed the panel data of students' test scores in Japanese (n＝5,171), social studies (n＝4,109), and science (n＝4,994) from the fourth to eighth grades. Subsequently, we analyzed them using multilevel growth models. The results indicated that the small-class-size effect on fourth-grade scores in language, social studies, and science, and the interaction of enlarged class size and number of students in the same grade impacted decreasing trends of the scores in social studies and science. The underlying processes of these results remains unclear; however, without surveying teachers and students, this study analyzed five-year panel data covering the school transition period and introduced novel findings to the literature on class size.

In this study, the effects of promoting achievement goals on the processes and consequences of reciprocal teaching in the classroom (RTC) were investigated. The participants included 79 sixth graders who were engaged in mathematics group learning. Basic RTC (Machi & Nakaya, 2014) was adopted to enhance the participants' intellectual and social interaction. Their achievement goals were promoted through teachers' instructions in reduced/enlarged figure classrooms. While the teachers focused on the importance of understanding in the mastery goal-focused groups, they emphasized quick answers and correctness in the performance goal-focused groups. Quantitative and qualitative data analyses demonstrated that the mastery goal-focused groups demonstrated more understanding and engaged in process-oriented discourses in comparison to the performance-focused groups. Based on the results of the analyses, the important role of students' motivation in interactive learning based on RTC is discussed.

Parenting factors are theoretically considered important environmental precursors of narcissism. Although three theoretical hypotheses (Horton et al., 2006) lead many empirical studies, few attempts have been made to aggregate empirical findings. This meta-analysis integrated research on the relationship between childhood parenting and narcissism following adolescence. Forty-three articles (47 studies) reporting statistics that could be replaced by r or r were collected. As there are various indicators/scales assessing narcissism and parenting, the indicators were first organized. Scales of narcissism were broadly divided into grandiosity and hypersensitivity, and scales of grandiosity were further divided into the Narcissistic Personality Inventory and others. The indicators of parenting were classified into five categories: warmth, monitoring, psychological control, spoiling/inconsistent, and rejection/indifference. The results indicate that correlations cannot be interpreted as evidence of a direct relationship between narcissism and parenting. Therefore, it is necessary to examine the indirect relationships, such as the combined effects, multiple effects of parenting indicators, or the transactional effects, on the characteristics of self-evaluation that children already possess.