 Research on heritability is currently undergoing a major transition. For nearly a century, heritability has been estimated in the ‘traditional’ sense, through twin and family studies, which are silent with respect to the underlying biological units of genetic transmission. Twin and family studies and the ACE model, that is, proceed from coarse-grained assumptions about genetic relatedness: MZ twin share roughly 100% of their genetic material, DZ just half. It wasn’t until Watson and Crick’s co-discovery of the double helical model of DNA that geneticists could refine heritability estimation with attention to biological details. Too much has happened since the discovery of DNA to detail here, such as linkage analysis and candidate gene studies, but a few key developments are integral to understanding the most recent approaches to estimating heritability.