 Fred Gifford (1990) claims that although all phenotypic traits are the result of an interaction between genes and the environment within which they are expressed, nonetheless it makes sense to distinguish certain traits as “genetic”; he argues in terms of populations that if it is genetic differences that make the differences in that trait variable in a given population, and if genetic traits can be individuated in a way that matches what some genetic factors cause specifically, then a trait (including a disease trait) can be understood as genetic. Kelly Smith (1992) disputes this, noting that the second condition depends on an extremely problematic distinction between causes (in this case genes) and mere conditions (e.g., epigenetic factors). Lisa Gannett (1999) argues for a “pragmatic” account of genetic explanation, claiming that when a disease is classed as “genetic”, the reasons for singling out genes as causes over other conditions necessarily include pragmatic dimensions inasmuch as they are relative to a given causal background (which includes both genetic and nongenetic factors), relative to a population, and relative to our present state of knowledge. More recently it has been argued that although explanatory reduction cannot be defended on metaphysical grounds, reductive explanations might be indispensable ways to address certain questions in the most accurate, adequate, and efficient ways (van Bouwel et al. 2011).