 Another norm of reaction is one in which genetic and environmental factors interact ‘additively’ (Figure 2). Genotype makes a constant difference across some range of environments. While the genetic variable does not determine the actual value of the trait in each individual, it does determine the differences between individuals. Moreover, when the norms of reaction have this form, heritability scores become relevant to the question of whether and how much a phenotype can be altered by environmental intervention, as discussed in the previous section. A famous diagram in the early days of behavior genetics depicted the relationship between IQ (P), genotype (G) and the ‘enrichment’ of the environment (E) as having roughly this form (Gottesman 1963a). If correct, this would mean that educational enrichment would cause every individual to get a higher test score, but would not change the ordering of their scores.