Relative age effect in physical and psychological development in young Japanese children and associated problems for kindergarten teachers

In school classroom, a child born soon after the selection period’s cut-off date (early born children) may benefit by up to a full year in physical and psychological development compared with a child born just before the cut-off date (late born children). This phenomenon is called the “relative age effect” (RAE). RAE is defined as the consequence of age differences between individuals within the same cohort, either in school or on sports teams (Musch & Grondin, 2001). This study examines the effect size of RAE in physical and psychological development and discusses educational problems faced by kindergarten teachers due to this RAE. Data were collected from 100 five-year-old Japanese children (60 male, 40 female) and their kindergarten teachers. Significant main effects of birth month category on physical development and on psychological development were found (early born children were more developed than late born children). The effect size of RAE in physical development was greater than that in psychological development. We concluded that the RAE existed in physical and psychological development among young children. Further, RAE appeared greatly on physical than on psychological development. Thus, we propose that kindergarten teachers could benefit from an awareness of the RAE.