 Francis Galton promoted a version of Darwin’s account of inheritance that clearly identified an internal, material contribution to hereditary traits. He proposed the distinction between nature and nurture, arguing that traits due to nature were the products of inherited biological material. He also was the first to propose the study of human twins as a way of understanding the contribution of nature, as opposed to nurture, to human traits (Burbridge 2001; see also Kronfeldner 2018). Galton’s early work set the basis for a paradigm in behavior genetics: twin and family studies. In studies of plants and other organisms, genetic makeup and environment can be systematically varied to assess the relative impact of genes and environment on phenotypic traits. Twin studies are human behavioral geneticists’ analogue to such experiments (See, e.g., Shaffner 2016). By tracking similarities and differences between groups of genetically related individuals reared in similar environments, twin and family studies permit some scientific investigation into sources of variance attributable to genetics and environment. The predominant tool for practitioners in the field is the statistical ‘ACE model’ (figure X)