Box 1 Definitions (see [50, 51])

Heritability refers to the extent of phenotypic trait variation in a population that is due to genetic variation

The genetic component (A) refers to narrow sense heritability, which describes the proportion of the variance of a trait that is explained by additive genetic effects

The shared environment component (C) refers to influences related to the common environment of twins that increase the correlation of twins in a trait

The non-shared environment component (E) refers to influences related either to the twins’ shared environment that are experienced differently, or to specific and incidental experiences of the twins that reinforce the twins' dissimilarity in a trait