characteristic or condition to additive genetic factors (the sum of individual genetic effects); non-additive genetic factors, such as gene×gene interactions; shared environmental experiences (events or experience that pertain to both twins), and non-shared environmental factors (events or experiences that are specific to an individual). These designs generate heritability estimates of the proportion of variation attributable to these different domains, such as the estimate of 50–60% heritability of addictive disorders (Goldman et al., 2005).
