Early Language Acquisition: From Birth Order Effect to Child-to-Caregiver Ratio

Growing up with multiple siblings might negatively affect a child’s language development. This study examined the associations between birth order, sibling characteristics and parent-reported vocabulary size in 6,163 Norwegian 8–36-month-old children (51.4% female). Results confirmed that birth order was negatively associated with vocabulary, yet exhibited a U-shaped pattern. A ‘child-to-caregiver ratio’ in the household was introduced, in which old-enough siblings are considered caregivers for their younger siblings. This index explained variance in vocabulary better than birth order, and suggested that female siblings may adopt a caregiving role 1–3 years earlier than males, indicating sex differences in the age at which siblings contribute, rather than deplete, available resources. A child-to-caregiver ratio might better capture the interplay between language learning resources and demands within households.