 In perhaps the most famous paper on this topic the geneticist Richard Lewontin (1974) argued that actual norms of reaction are likely to be non-additive (Figure 3). In that case, it makes no sense to talk of a particular genotype ‘determining’ a phenotypic difference. Genotype and environment jointly determine the outcome in the straightforward sense that the effect of each factor on the outcome is a function of the particular value taken by the other factor. Whether norms of reaction are typically non-additive and exactly what this implies is the subject of an extensive scientific and philosophical literature on ‘gene-environment interaction’, as discussed in the previous section.