Footnotes

• ↵* “Heritability” is the ratio between the “genetic variability” and the total variability that is observed between individuals for a certain trait, or the proportion of a trait’s variation within a specified population that can be attributed to genetic differences. Heritability ranges between 0 and 1, with 1 indicating that all of the interindividual variation in a certain characteristic can be attributed to genetic variation. Heritability is estimated in studies in which systematic comparisons are made between individuals in similar family environments that are known to share their genetic make up to various degrees (such as monozygotic and dizygotic twins, or non-adopted and adopted children), and between genetically similar individuals that are known to share their family environment to various degrees (such as monozygotic twins reared together or apart). The assumptions made in estimating heritability from such studies may not always be correct. In research on twins, for example, it is assumed that twins who are reared together have identical environments, so that greater similarity between monozygotic and dizygotic twins can be interpreted as reflecting differences in genetic make up. This is not necessarily true, however, because monozygotic twins look more alike than dizygotic twins, and may therefore be treated more alike. (Adapted from Atkinson et al48).

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• Conflicts of interest: none declared.