Background: Little is known about the effects of paternal childcare on children's behavioral development. This study explored the association between paternal childcare during infancy in terms of childcare hours and type of caregiving behavior and subsequent behavioral problems in children aged 5.5 years stratified by sex.Methods: We analyzed data from the Longitudinal Survey of Newborns in the 21st Century (2001-2006), a population-based cohort survey in Japan (N = 27,870). Paternal childcare

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... as assessed at 18 months in terms of paternal childcare hours on weekdays or weekends and the frequency of each type of childcare (feeding, changing diapers, bathing, putting the child to sleep, playing with the child at home, and taking the child outside). Six categories of child behavioral problems were assessed when the children were 5.5 years old. Logistic regression analysis was applied to account for the known confounding variables.Results: Longer paternal childcare hours, on both weekdays and weekends in infancy, had a protective effect on behavioral problems at 5.5 years of age. The dose-effect relationships were found between the frequency of fathers taking their children outside and behavioral problems in boys, and the frequency of fathers playing with their children at home and behavioral problems in both boys and girls.Conclusions: Paternal childcare during infancy could prevent subsequent behavioral problems in children. Several specific paternal caregiving behaviors, such as taking their children outside and playing with them at home, may play an important role in preventing subsequent behavioral problems.