The effect of older siblings on language development in Japan

The effect of older siblings on language development is a phenomenon in which secondborn children with an older sibling have lower levels of language skills than firstborn children without older siblings. Because this effect has been confirmed in only a few cultures (e.g., France and Singapore), it remains unclear whether the effect is robust and universal. Using data for 755 Japanese 4- to 5-year-olds from the Hamamatsu Birth Cohort for Mothers and Children, we investigated the effect of older siblings on children's language skills, measured as verbal IQ by the Wechsler Preschool and Primary Scale of Intelligence (WPPSI). Consistent with previous findings, results showed that secondborn children with an older sibling had significantly lower verbal IQ than firstborn children. Further analysis also confirmed previous findings: the more closely spaced the siblings' age gap, the higher the verbal IQ of the secondborn child. Therefore, the present findings suggest that the older sibling effect is robust in Japan and may be universal across cultures.