 The previous section touched on the fact that judgments about the relative contributions of genetic vs environmental factors to trait differences are context sensitive. They are sensitive to both the allelic combinations that a researcher studies and also to range of environments that a researcher includes in her analysis. Quantitative geneticists recognize that a given genetic variant (allele) will have variable phenotypic effects across different environments. These responses are often nonlinear, such that gradual increases in an environmental parameter do not have correspondingly gradual phenotypic effects. At the same time, different genetic variants (alleles) compared against a single environment will often produce a range of phenotypic effects. Quantitative geneticists typically represent these interactions with a graphical tool called the norm of reaction. These diagrams were introduced at around the same time as the idea of the gene and the distinction between genotype and phenotype (Sarkar 1999) and have long been advocated as the clearest way to think about the role of genes in development (Hogben 1933; Lewontin 1974; Gottlieb 1995; Kitcher 2001). Suppose, for example, that with respect to some environmental variable (E) an organism with a given genotype (G1) will develop the same phenotype (P) way no matter what value the environmental variable takes (Figure 1).