4 Not Much Can Be Said About Means

It is vital to remember that almost every result in this book, and every conclusion that others obtain using these methods, relate to the causes of human differences, and may have almost nothing to do with the processes that account for the development of the mean expression of a trait in a particular population. We are necessarily concerned with what makes people vary around the mean of the population, race or species from which they are sampled. Suppose, for example, we were to find that differences in social attitudes had a very large genetic component of variation among U.S. citizens. What would that imply about the role of culture in the determination of social attitudes? It could imply several things. First, it might mean that culture is so uniform that only genetic effects are left to account for differences. Second, it might mean that cultural changes are adopted so rapidly that environmental effects are not apparent. A trivial example may make this clear. It is possible that understanding the genetic causes of variation in stature among humans may identify the genes responsible for the difference in stature between humans and chimpanzees, but it is by no means certain. Neither would a demonstration that all human variation in stature was due to the environment lead us to assume that the differences between humans and chimpanzees were not genetic. This point is stressed because, whatever subsequent genetic research on population and species differences might establish, there is no necessary connection between what is true of the typical human and what causes variation around the central tendency. For this reason, it is important to avoid such short-hand expressions as ``height is genetic" when really we mean ``individual differences in height are mainly genetic."