Relatively few studies have concentrated on the qualities or implications of the relationships between twins and their co-twins. We examined dependence between twin siblings and its impact on their social contacts, leisure-time activities and educational attainment in adolescence in a population-based sample of 5268 Finnish adolescent twins (FinnTwin16 study). We found that co-twin dependence varies significantly by zygosity and gender. Mothers also reported zygosity and gender differences in dependence that were similar to the twins’ self-reports of dependence. Dependence was associated with social contacts, leisure-time activities and educational attainment, as well as with mothers’ educational level and fathers’ socio-economic status. These differences in twin relationships might be associated with different psychosocial measures and should be taken into account in studies of twins’ behavioral traits.
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