Participants and procedures
We used TTC data for three waves (10 years old: 2012–2015, n = 3, 171; 12 years old: 2014–2017, n = 2,982, n school district = 97; 14 years old: 2016–2019, n = 2,543, n school district = 91. Shown in Table 1 and Fig. 2). This study was approved by the Institutional Review Board of the Faculty of Medicine, University of Tokyo (approval numbers: 3150, 10057, and 10069), Graduate University for Advanced Studies (SOKENDAI; 2012002), and Tokyo Metropolitan Institute of Medical Science (12–35). Informed consent was obtained from the children’s primary caregivers (usually their mothers) before participation.
To obtain information about the school district, we used the websites of the three municipalities (Setagaya: https://www.city.setagaya.lg.jp/; Chofu: https://www.city.chofu.tokyo.jp/; and Mitaka: http://www.city.mitaka.tokyo.jp/). As the school district was determined by the year, we set the area definition based on information from 2014, when the survey in the first wave was conducted. In addition, we contacted government offices for further information. Finally, we used the school districts of 99 elementary schools: 64 schools in Setagaya Ward, 20 in Chofu City, and 15 in Mitaka City (Fig. 1). To define the cohort members’ school districts using their addresses, we used a coding program to obtain their school districts and ran the program on an offline PC, which stored their personal information in the Tokyo Metropolitan Institute of Medical Sciences. Data sorted in school districts (1–99) were extracted and used for further analysis to maintain anonymity.
Measures
For every group, we obtained self-reported anonymous questionnaires from the children, including the Japanese versions of the Short Mood and Feelings Questionnaire (SMFQ) and the World Health Organization Well-Being Index (WHO-5). In the interview phase, at age 10, we conducted a short version of the Wechsler Intelligence Scale for Children-Third Edition, the Japanese version (WISC-III). Using anonymous questionnaires for the main caregivers, we obtained the Japanese version of The Strengths and Difficulties Questionnaire (SDQ) for their children and confounding variables. For every subscale and scale of the questionnaire comprising five or more items, the average value of the remaining item scores was substituted for analysis if the missing value was within 20% of the items; otherwise, it was coded as missing.
Depressive symptoms
The SMFQ was used to assess children’s depressive symptoms and consisted of 13 self-reported questions (e.g., I felt so tired I just sat around and did nothing)25,35,36. Respondents chose their answers using a 3-point Likert scale (2 = true; 0 = not true). Higher scores indicate more severe depressive symptoms.
Psychological well-being
The WHO-5 assessed children’s psychological well-being37–39. It is a 5-item questionnaire (e.g., I have felt cheerful and in good spirits). Respondents were asked to choose answers using a 6-point Likert scale (5 = all of the time; 0 = at no time). Higher scores indicate better psychological well-being.
The Strength and Difficulties questionnaire
The SDQ has been used to assess children’s self-regulation40–43. The scale consists of 25 items in five subscales: emotional symptoms (e.g., often complains of headaches, stomach-aches, or sickness), conduct problems (e.g., often loses temper), hyperactivity/inattention (e.g., restless, overactive, cannot stay still for long), peer problems (e.g., rather solitary, prefers to play alone), and prosocial behavior (e.g., considerate of other people’s feelings). The scale was assessed using parental questionnaires, in which parents were asked to choose answers using a 3-point Likert scale (2 = certainly true; 0 = not true). The higher the prosocial scores, the lower the children’s strength. For the remaining four subscales, the higher the score, the greater the child’s difficulty.
Confounding variables
Confounding variables included children’s age, gender, intelligence quotient (IQ), parental educational attainment as socioeconomic status (SES), depressive symptoms, and the total population of elementary school students in 2014. For children’s IQ at age 10 years, the 2-battery short version of the WISC-III were used (Information and Picture Completion)44,45. SES was estimated using the higher academic attainment of their care givers (“1 = junior high school graduate (or under),” “2 = high school dropout,” “3 = high school graduate,” “4 = technical school or 2-year college graduate,” “5 = four-year college graduate,” and “6 = graduate university or six-year college graduate”) (Kanata et al., 2016) 46. For the caregivers’ main depressive symptoms, the Japanese version of the Kessler 6 was assessed through an anonymous questionnaire47,48. The average value was substituted for any missing child’s IQ (n = 3, 0.0009%) and parental SES (nfather = 154, 0.05%nmother = 25, 0.08%). The total population of elementary school students in 2014, as stated on Gaccom, a school education website in Japan (https://www.gaccom.jp/), was used to measure the school scale (Fig. 1).
Statistical analyses
To test the effects of school district health conditions on each individual variable, we applied an HLM that allowed for the assumption of random effects20–24. The model included individual and school district psychological conditions as independent variables, corresponding individual psychological conditions at Wave 2 or 3 as dependent variables, and confounding factors (child’s age at baseline survey, sex, child’s IQ, parental educational attainment, main caregiver’s depressive symptoms, and total number of children in the school) using a restricted maximum likelihood. To control for multicollinearity between group- and individual-level scores, both individual- and school-district-level scores were centered on the group mean49,50.
Previous studies have found that the community influences a person’s health factors5–8, and experience and years of residence within a community may influence an individual’s health conditions. In this study, the intercept baseline is the same across school districts. Still, the slope differs across school districts since this study aims to determine whether health status at the school district level can be predicted from health status at the individual level. Therefore, the slope of each health indicator may vary according to the school district. Since this study focuses on the slope of whether health conditions at the school district level can be predicted from health conditions at the individual level, it was tested using a random slope model in which the intercept baseline was equal across school districts and the slope differed across districts (see Supplementary materials and Fig. S1).
Statistical analysis packages included summarytools51, mice52, dplyr53, psych54, effsize55, corrplot56, ggplot257, leaflet58, and lmerTest (optimizer = "bobyqa")59 packages in R version 4.1.060. The significance level was corrected using the Bonferroni method, as the analysis was performed using seven predictive health measures (p < 0.0071 = 0.05/7).