 Folk biology also departs from scientific classification in its essentialist commitments. Across a wide range of cultures, people tend to assume that members of each species possess a common essence or causal nature that determines their appearance, behavior, and ecological roles (Medin and Atran 2004). Psychologists have found that even when people have no specific ideas about the nature of a particular species, they nonetheless assume that it must have an essence (i.e. they operate with what Medin and Ortony (1989) call an “essence placeholder”). It is plausible that folk essentialism has served our species well for most of its history. Medin and Atran (2004) propose that the existence of an essence placeholder might have motivated people to explore their environments, leading them to discover and share information about edible or medicinal plants, about dangerous plants and animals, and other salient aspects of the environment. The idea of an essence might also have assisted humans in tasks associated with agriculture and animal husbandry. Despite its utility, however, essentialism is scientifically inaccurate. No internal essence explains why members of a species share certain traits. Although genetic similarities account for some of the traits that are typical of a species, it is misleading to think of a species’ genome as its essence. Elliott Sober (1980) provides a detailed account of what is wrong with this idea. Essentialism, he notes, employs a Natural State Model of explanation which views every member of a species as directed toward a single developmental outcome (natural state). Essentialists recognize that individuals vary in the characteristics they actually express. However, such variability is explained according to essentialism by positing the effects of intervening forces that interrupt development and prevent the full realization of its goal. The reason that individuals of the same sex differ in their sexual preference, on this view, is because of slightly different environmental factors impinging on the development of these individuals. By contrast, on a neo-Darwinian model there is no natural state. Individual variability is partly explained in terms of genetic variation passed on from one generation to the next. Moreover, even given a specific genetic starting point, development is not aiming at some specific, natural state, but simply goes wherever local causes push it. The interaction between genetic and environmental factors is most accurately represented as a norm of reaction (see Section 3.2 below), where a spectrum of developmental outcomes is associated with each unique genotype in a population of genetically variable individuals. This neo-Darwinian perspective requires a fairly dramatic shift in conventional thinking about species. As Atran et al. (2002, 32) put it, “to understand modern biological science, people must unlearn universal dispositions to view species essentialistically”.