r 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. An oversampling method was used with the goal of having 3,000 pairs remain in the 2nd wave survey (11). Given the low follow-up rate of low-income families, all 620 pairs whose annual household incomes was lower than five million yen were asked to participate in the 2nd wave survey. Of the remaining 3,858 pairs, 2,551 were randomly asked to participate in the 2nd wave. A total of 3,172 pairs were asked to participate in the longitudinal cohort study, resulting in 3,007 pairs participating in the second 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?”. We dichotomized the answers to “prosocial praise group” and “other praise group.” We did not score the degree of prosociality but just divided the answers to the two groups. 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 (12), 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
There may be children who behave prosocially but not be praised for their prosocial behaviors. There may also be children who are praised for their prosocial behaviors but do not recognize they are praised for their prosocial behaviors. The children's recognition of being praised for prosocial behavior does not necessarily correspond to the children's objective prosocial behavior. Objective prosocial behavior of the 10 year-old children was assessed by 