Contextual and socioeconomic variation in early motor and language development

To compare early motor and language development of children <3 years of age growing up in high-income and low-income contexts.; Cross-sectional study.; We analysed differences in motor and language skills across study sites in Cambodia, Chile, Ghana, Guatemala, Lebanon, Pakistan, the Philippines and the USA.; Cognitive and language development assessed with the Caregiver Reported Early Development Instruments (CREDI) tool.; 4649 children aged 0-35 months (mean age=18 months) were analysed. On average, children in sites with a low Human Development Index (HDI) had 0.54 SD (95% CI -0.63 to -0.44) lower CREDI motor scores and 0.73 SD (95% CI -0.82 to -0.64) lower language scores than children growing up in high HDI sites. On average, each unit increase in national log income per capita was associated with a 0.77-month (95% CI -0.93 to 0.60) reduction in the age of motor milestone attainment and a reduction in the age of language milestone attainment of 0.55 months (95% CI -0.79 to -0.30). These observed developmental differences were not universal: no developmental differences across sites with highly heterogeneous socioeconomic contexts were found among children growing up in households with highly educated caregivers providing stimulating early environments.; Developmental gaps in settings with low HDI are substantial on average, but appear to be largely attributable to differences in family-level socioeconomic status and caregiving practices. Programmes targeting the most vulnerable subpopulations will be essential to reduce early life disparities and improve long-run outcomes.