  Adopting a population-based approach to genetic causation, where differences in genes are understood to explain differences in traits and not traits themselves, replaces the need for ceteris paribus clauses because they rely on the actual distribution of the necessary genetic and nongenetic background factors in specific populations. The case can be made that the first approach is indebted to the second, and that one never explains a property of an object tout court but only in relation to a reference class of an object or objects that lack the property (but share the necessary background factors). Writes Germund Hesslow (1983), “all explanations of individual facts of the form Fa—that is, where an object a has a certain property F—involve a comparison with other objects which lack the property in question” (p. 91). No trait can be labeled “genetic” in any absolute sense, but only relative to a specific population. For example, lactose intolerance is considered to be a genetic condition in northern European populations where ingestion of milk products is common and lactase deficiency rare, whereas in African populations, where ingestion of milk products is rare and lactase deficiency common, it is considered to be an environmental condition (Hesslow 1984). This is the second way in which genetic explanations are  context-dependent.[32]