Perspectives in Biology and Medicine

THE PHYLOGENY AND ONTOGENY OF INTELLIGENCE ARTHUR R. JENSEN* Phylogeny of Adaptive Behavior Are there qualitative as well as quantitative differences in the behavior -adaptive capabilities of animals at different levels of the phyletic evolutionary sequence? That is to say, are there differences not only in the speed of learning but also in the complexity of what the organism can learn at all, given any amount of time and training? Are there discontinuities as well as continuities in capacities to perceive , to learn, and to manipulate the environment as we ascend the phyletic scale? The answer to these questions is now empirically quite clear. There are indeed discontinuities and qualitative differences in learning (i.e., behaviorally adaptive) capabilities as we go from one phyletic level to another. Behaviorally, the phylogenetic hierarchy is best characterized in terms of an increasing complexity of adaptive capabilities and an increasing breadth of transfer and generalization of learning, as we move from lower to higher phyla. It is a fact that every animal, at least above the level of worms, has the capacity to learn, that is, to form stimulus-response associations or conditioned responses. But the degree of complexity and abstractness of what can be learned shows distinct "quantum jumps" going from lower to higher phyla. Simpler capacities, and their neural substrate, persist as we move from lower to higher levels, but new adaptive capacities emerge in hierarchical layers as we ascend the phyletic scale. Each phyletic level possesses all the learning capacities (although not necessarily the same sensory and motor capacities) of the levels below itself in addition to new emergent abilities, which can be broadly conceived as an increase in the complexity of information processing. For example, studies by Bitterman [1] of animals at various levels of * School of Education, University of California, Berkeley, California 94720. Perspectives in Biology and Medicine · Autumn 1971 | 37 the phyletic scale (earthworms, crabs, fishes, turtles, pigeons, rats, and monkeys) have clearly demonstrated discontinuities in learning ability among different species and the emergence of more complex abilities corresponding to the phylogenetic hierarchy. In the experimental procedure known as habit reversal, a form of learning to learn in which the animal is trained to make a discriminative response to a pair of stimuli and then has to learn the reverse discrimination and the two are alternated repeatedly, a fish does not show any sign of learning to learn (i.e., each reversal is like a completely new problem and takes as long to learn as the previous problems), while a rat improves markedly in its speed of learning from one reversal to the next. When portions of the rat's cerebral cortex are removed, thereby reducing the most prominent evolutionary feature of the mammalian brain, the learning ability of the decorticate rat is exactly like that of the turtle, an animal with little cortex, and would probably be like that of the fish if all the rat's cortex could be removed. Harlow and Harlow [2] have noted similar discontinuities at high levels of learning among rhesus monkeys, chimpanzees, and humans. Again, situations that involve some form of learning to learn are most sensitive to differences in capacity. No animals below primates have ever learned the so-called oddity-non-oddity problem no matter how much training they are given, and more complex variations of this type of problem similarly differentiate between rhesus monkeys and chimpanzees . The species' differences are not just in speed of learning but in whether the problem can be learned at all, given any amount of training. This is essentially what is meant by a hierarchical conception of learning ability. There is much evidence for this conception, which Jensen [3] has summarized more extensively elsewhere. The evolution of humans from more primitive forms is now believed to be intimately related to the use of tools and weapons [4]. The mental capabilities involved in the use of implements for gaining ever greater control of the environment, in lieu of sheer physical strength, were just as subject to the evolutionary effects of natural selection as are any genetically mutated organs. More specifically, according to Haskell [5, p. 475], "What primarily evolves in man is the nerve...