Fig. 2: Schematic diagrams demonstrating the principles underlying commonly used genetically informative designs which separate genetic and environmental mechanisms of transmission in parent–offspring associations.

A In adoption and related designs, knowledge of the type of relationship shared between parent and offspring is leveraged to gain insight into genetic and environmental factors underlying parent-to-offspring associations. Lived-with biological parents can influence offspring through both genetic and environmental transmission, as they provide both genes and the rearing environment. Not-lived-with biological parents who have no contact with the offspring provide only genes, indicating genetic transmission, whereas adoptive or step-parents provide only the rearing environment, indicating environmental transmission. In children-of-twins studies, children of identical (monozygotic) twins are as genetically similar to their aunt/uncle as they are to their parents (50% shared genes), whereas children of fraternal (dizygotic) twins share 25% of genes with their aunt/uncle. Higher monozygotic than dizygotic avuncular correlations (between uncle/aunt and niece/nephew, i.e. between Twin 1 and Child 2 or Twin 2 and Child 1) are likely due to a higher proportion of shared genes, suggesting genetic transmission, whereas higher parent–offspring than avuncular correlation suggests environmental transmission of a parental factor, above and beyond the effect of shared genetic or environmental effects. B In sibling comparison studies, the association between a specific parental factor and offspring outcome is studied in exposed versus unexposed offspring, as siblings are naturally matched for parentally provided genes and a rearing environment. Environmental transmission is indicated if the parent–offspring association is observed only in the exposed offspring. C In molecular genetics studies, the effect of shared parent–offspring (i.e. transmitted) genes on offspring outcome indicates the presence of genetic transmission. However, both transmitted genes and non-transmitted parental genes can also have an indirect (i.e. environmentally mediated) effect on offspring through parental traits that are genetically influenced; this is otherwise known as genetic nurture.