Mapuche represents the largest indigenous group in Chile amounting to nearly 10% of the total population. In a longitudinal cohort of 12,398 children, we analyzed the role of ethnicity in physical and psychosocial development of Mapuche and nonindigenous Chilean toddlers (age 2.5 years), taking into account sociodemographic and caregiver characteristics. As indicated by our univariate analysis, the Mapuche developmental niche was characterized by lower income, lower maternal education, poorer quality of the home environment, longer breastfeeding, and higher parental stress. Physical development showed higher body mass index. Mapuche children showed less externalizing problems. We then analyzed the incremental contribution of ethnicity in a series of hierarchical regressions with the second wave of developmental measurements (age 4.5 years) as outcome variables, showing a significant but modest incremental contribution of ethnicity to the prediction of children's development between 2.5 and 4.5 years of age. Controlling for environmental variables, Mapuche showed less externalizing and internalizing, behavior problems. Socioeconomic status, quality of the home environment, and parenting stress were stronger predictors of socioemotional development than ethnicity per se.

This study is based on 145 pairs of twins included in a longitudinal study of a cohort of singletons born in 1953 and living in the Stockholm metropolitan area in 1963. Of these 145 pairs, 28 were classified as MZ, 103 as DZ and 14 pairs could not be classified. School achievement and ability test results have been collected for this group as well as data for the boys from physical and mental examinations on enrollment to military service. The purpose of this study was to compare within-pair similarity for MZ and DZ twins from birth onwards. There seems to be a divergent trend over time for MZ twin pairs to become more similar and DZ twin pairs to become less similar in height growth, school achievement and verbal ability. This is in accordance with previous research in this field. In relating similarity within pairs to birthweight, we found a drop in within-pair similarity for low birthweight MZ and DZ twin pairs (both twins ≤2.500 g), but not for normal weight twin pairs. A discussion is initiated concerning methodological implications of these results.