of the co-twins and therefore tends to decrease twins’ similarity. In addition, genetic factors can be subdivided into additive (reflecting additive effects of genes contributing to the trait) of and non-additive (reflecting non-additive allelic interaction including within-locus dominance and between-loci epistatic interaction). These components of variance can be estimated by fitting linear structural equation models (SEM) to the observed twin data (see Box 1). Importantly, MZ twins share 100% of their segregating genes, whereas DZ twins share only 50% on the average, the same as non-twin siblings. However, both MZ and DZ twins reared together share their environment to the same extent (one of the key assumptions of the twin method). The model fitting approach provides tests of different models that explain the variance in the trait by some combination of genetic, shared environmental, and individual environmental factors. Goodness of fit of these models can be compared and parameter estimates obtained for the best-fitting model. These parameter estimates yield the measure of heritability.
