Later he realized an even more startling fact: in generational comparisons, the temporal order could be reversed. That is, the fathers of sons also revert to the mean. The father of a son who is taller than average is likely to be taller than average but shorter than his son (see Figure 2.2). Once Galton realized this, he had to give up any idea of a causal explanation for regression, because there is no way that the sons’ heights could cause the fathers’ heights.
