In the early 1930s the distinction between learned and inherited behaviour seemed clearer than it does now. The view that any bit of behaviour either was learned or simply developed without learning seemed straightforward. Studies based on these expectations led investigators to conclude that rat-killing behaviour among cats is learned rather than instinctive, that human fears are all acquired, or that intelligence is completely the result of experience. Learning theorists were saying then that most behaviour is learned and that biological factors are of little or no importance.

Forty years later this position seemed grossly untenable. The once-implied sharp distinction between learned and inherited behaviour had become badly blurred. For example, it has been found that the young of many animal species automatically will learn to follow the first large, moving, noisy object presented (as if it were their mother). This special form of learning is called imprinting and seems to occur only during a critical early stage of life. Among mallard ducklings imprinting is most feasible about 15 hours after hatching. During this period a duckling will imprint as easily on an old man or on a rubber ball as it will on a mother duck. Is this instinctive or learned behaviour? Manifestly it is both. The instinctive tendency to be imprinted is part of the duckling’s biological heritage; while the object on which it is imprinted is a matter of experience. What is significant for learning theory is that the contribution of biology cannot be ignored.

Learning theorists once ruled a number of concepts out of court on the ground that they seemed objectively unclean. Image, cognition, awareness, and volition, all are concepts that were denied acceptance on this basis. They sounded mentalistic, subjective, introspective, and unverifiable. Yet, in the late 20th century these were being given more serious scientific consideration.

For example, the concept of image in learning has begun to show real viability. It has long been reported that the more meaningful a list of words is, the easier it will be to learn. Degree of meaningfulness for a word may be defined by the objectively observed probability that people quickly can give another word in response. Using such empirical scales of meaningfulness, a reliable and substantial relationship has been found between meaningfulness and ease of learning. However, meaningful words also may evoke vivid images that subjects can describe when asked. When they do evoke such imagery, they seem to be learned and remembered even more easily. Thus, learning theory seems to be enriched when introspective data are used.

A final fault in much learning theory stems from earlier tendencies to use the laws of physics as a model. Theorists once sought general laws of wide applicability that tended to obscure differences among individuals. For example, so complete was Hull’s faith in universal “laws” of animal behaviour, that he based his hypothesis about the optimal interval for classical conditioning in humans, other mammals, and birds on the pattern of nerve conduction in the optic nerve of the horseshoe crab. There was little concern even for species differences. Within the same species, individual differences were viewed as a mere nuisance; it was believed that, by studying many subjects and by computing averages, basic laws of learning could be found. However, so-called laws were developed in this way that failed to represent even one individual whose behaviour contributed to the average. More than any other consideration, this has led learning theorists to take a belated look at the importance of individual differences and species differences in learning.