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 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 (13).
The Short Mood and Feelings Questionnaire (SMFQ), a self-report questionnaire about depression (14, 15), 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 (16–18), and one study suggested socioeconomic status as a potential confounder in future studies (18). Therefore, we included children's sex, age, number of siblings, caregivers' age and education, and annual household income as potential candidates for 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 candidates for confounders. Children's IQ was estimated from two subsets (Information and Picture Completion) of the Wechsler Intelligence Scale for Children (WISC-III) (19). Psychological distress of primary caregivers was assessed by the Kessler Psychological Distress Scale (K6) (20). 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 f