Figure 10.11: A geometric analogy to heritability It is senseless to say that a given rectangle’s area results more from its length than from its width, or vice versa. However, differences among rectangles in area can result primarily or entirely from differences in length (as shown in group A) or in width (as shown in group B). Similarly, one can imagine one group of people in which IQ variation results mostly from their differing genes and another group in which it results mostly from their differing environments.