THE EVOLUTION OF INTELLIGENCE
Definition: Intelligence is the behavioral consequence of the total neural-information processing capacity in representative adults of a species, adjusted for the capacity to control routine bodily functions.
In the first edition of this chapter I suggested the broad definition of intelligence above, which remains my working definition. The adjustment is estimated from the regression of brain size on body size in appropriate samples of species. Intelligence is thus defined as equivalent to encephalization. If one can accept it, this definition enables us to present an evolutionary history of intelligence that can be tied directly to the fossil record of the brain. There is a fine fossil record, which can be analyzed as the evolution of the brain's information processing capacity. Relating information processing to brain size involves simple, straightforward, and unusually reliable bivariate regressions. (For many critical relationships one finds correlation coefficients r > .99.) That, in a nutshell, is the way I have always approached the problem (Jerison, 1973, 1982, 1985, 1991). Although inspired by the great psychologist Karl Lashley (1949), my definition is obviously broader than those of most psychologists, but I believe that the definition is fundamentally correct and that it provides useful and sometimes surprising insights for an intuitive understanding of the nature of human intelligence. At present, the contribution of an evolutionary analysis is primarily on the nature of the fundamental dimensions of human intelligence. Although variation is the stuff of evolution, the clearest insights are with respect to mean values of traits rather than their variability.