Study design and survey participants
The purpose of this study was to test the hypothesis that being praised for prosocial behavior would be longitudinally associated with decreased depressive symptoms in adolescents. This study used data from the Tokyo Teen Cohort study (TTC), a prospective population-based cohort study that is currently underway and aims to investigate the developmental trajectory of adolescents (7). Children born between September 2002 and August 2004 in three local governments in Tokyo (Setagaya, Mitaka and Chofu) were randomly extracted using the Basic Resident Register. Of the 10,234 pairs of children and their primary caregivers who were asked to participate, 4,478 pairs agreed to cooperate in the baseline survey (the 1st wave of data collection) at the age of 10 years. Of these, 3,171 pairs of children and primary caregivers were chosen and asked to participate in the longitudinal cohort study, and 3,007 pairs participated in the 2nd wave of data collection at the age of 12 years (follow-up rate was 94.8%).
In each of the 1st and 2nd waves of data collection, trained investigators visited the participants’ home twice and interviewed children and their primary caregivers. At the first visit, the investigators explained the research to both the child and the primary caregiver, obtained written consent, and asked them to complete the self-report questionnaire by the second visit. At the second visit, the child and the primary caregiver separately answered a self-report questionnaire containing sensitive content and sealed it immediately after completion. All questionnaires and data were collected anonymously. At this visit, the investigators also administered face-to-face psychological tests to the child.
TTC is a joint study of three institutions (Tokyo Metropolitan Institute of Medical Science, the University of Tokyo, and the Graduate University for Advanced Studies) and has been approved by the ethics committees of the three institutions.
The children answered self-report questionnaires including items on experiences of being praised, depressive symptoms and other variables, such as the number of siblings. Caregivers answered self-report questionnaires that included questions about the caregiver’s age, psychological distress, educational background and annual household income.
Experience of being praised for prosocial behavior
In the self-report questionnaire at the 1st wave, 10-year-old children were asked to freely describe answers to the question “What are you praised for?”. A qualitative classification was made by several researchers (DN, NT, HN, MT) as to whether prosocial behavior was included in the answers, double-checked by other researcher (DN, NT), and was finally confirmed by several experienced researchers (MM, TK). Based on a previous study (8), we defined prosocial behavior as voluntary, intentional behavior that results in benefits for another; the motive is unspecified and may be positive, negative, or both. Only children who clearly answered that they were praised for their prosocial behavior such as “helping with housework” were designated the “prosocial praise group”, and other children who did answer but did not include prosocial behavior in their responses such as “getting a good score on the exam” were designated the “other praise group”. Children who made multiple responses were also classified as the “prosocial praise group” if more than one of their answers included prosocial behaviors. Blank fields were treated as missing values.
Objective prosocial behavior
Objective prosocial behavior of the 10-year-old children was assessed by the Strength and Difficulty Questionnaire (SDQ), for which the primary caregivers answered in the self-report questionnaire in the 1st wave of data collection. The subscale score for prosocial behavior in the SDQ was calculated (9).
The Short Mood and Feelings Questionnaire (SMFQ), a self-report questionnaire about depression (10, 11), was used to investigate the degree of depression in children in the 1st (10 years old) and 2nd (12 years old) waves of data collection. Each of the 13 items had three response choices: “True” (2 points), “Sometimes true” (1 point), and “Not true” (0 points). The scores for each item were summed into the total score (0–26 points), and higher total scores meant stronger depression.
Previous studies on the relationship between praise for children and prosocial behavior adjusted for the children's sex, age, number of siblings, caregivers' age and educational history (12–14), and one study suggested socioeconomic status as a potential confounder in future studies (14). Therefore, we included children's sex, age, number of siblings, caregivers’ age and education, and annual household income as potential confounders. In addition, using external knowledge, we added children’s estimated intelligence quotient (IQ) and psychological distress of primary caregivers (mainly mothers) in the 1st wave as potential confounders. Children’s IQ was estimated from two subsets (Information and Picture Completion) of the Wechsler Intelligence Scale for Children (WISC-III) (15). Psychological distress of primary caregivers was assessed by the Kessler Psychological Distress Scale (K6) (16). Among these potential confounders, we regarded the variables that showed a significant association with the prosocial self-report as confounders.
For comparison of the demographic characteristics between the “prosocial praise group” and the “other praise group”, t tests, tests of differences in population ratios, or χ2 tests were used. To investigate the relationship between being praised for prosocial behavior and depressive symptoms, linear regression analysis was performed using the SMFQ at the 2nd wave (12 years old) as the objective variable and being praised for prosocial behavior as the main explanatory variable. In addition, the SMFQ score at the 1st wave (10 years old) and other confounders were treated as covariates, and multiple regression analysis was performed after supplementing missing values using the multiple substitution method (number of multiple imputations: m = 200). Furthermore, since there is a gender difference in the development of prosocial behaviors in adolescence (14), we examined the interaction effect of sex and prosocial self-reports on depressive symptoms. An interaction term of sex X prosocial self-report was added in the multiple regression analysis. For statistical analyses, open-source statistical software R (version 3.6.1) and the multiple substitution method calculation package mice (version 3.6.0) were used.