 The scientific discipline that does meaningfully address questions of the relative contributions of genes and environment is quantitative genetics. It does so by shifting its focus from the individual to the population. Instead of investigating individuals, quantitative geneticists focus on populations comprised of genetically variable individuals. Instead of asking how genetic and environmental factors influence the development of an individual, this discipline focuses on how each factor influences phenotypic differences in a population. It is important to understand how this focus on population-level phenotypic differences impacts judgments about whether a given trait is “genetic.” An obvious limitation of this approach is that it can be applied only to traits that exhibit phenotypic variation. Take some trait that is invariant, such as all humans having one head. Without variation in this trait, it is not possible to conduct a population-level analysis to determine the extent to which this trait is genetically encoded. There are no phenotypic differences at the population level, and so no distinguishable genetic contribution to those differences (see  SEP entry  on heritability). This is, again, at odds with our vernacular notion of innateness, which would surely judge the human head to be an innate trait. Indeed, typicality (or invariance) is one of the driving factors in folk judgments of innateness (Griffiths, et al. 2009), so the conventional scientific method for evaluating genetic contributions to trait development is at odds with our folk intuitions for this reason alone.