A pup with only two siblings, for instance, will already weigh more on day sixty-six than a pup with four siblings. Thus the difference in birth weights has two causes, and we want to disentangle them. How much of the 5.66 grams is due to spending an additional day in utero and how much is due to having fewer siblings to compete with? Wright answered this question by setting up a path diagram (Figure 2.8).
