The object of this paper is to show why recent research in the psychology of deductive and probabilistic reasoning does not have "bleak implications for human rationality," as has sometimes been supposed. The presence of fallacies in reasoning is evaluated by referring to normative criteria which ultimately derive their own credentials from a systematisation of the intuitions that agree with them. These normative criteria cannot be taken, as some have suggested, to constitute a part of natural science, nor can they be established by metamathematical proof. Since a theory of competence has to predict the very same intuitions, it must ascribe rationality to ordinary people.

Accordingly, psychological research on this topic falls into four categories. In the first, experimenters investigate conditions under which their subjects suffer from genuine cognitive illusions. The search for explanations of such performance errors may then generate hypotheses about the ways in which the relevant information-processing mechanisms operate. In the second category, experimenters investigate circumstances in which their subjects exhibit mathematical or scientific ignorance: these are tests of the subjects' intelligence or education. In the third and fourth categories, experimenters impute a fallacy where none exists, either because they are applying the relevant normative criteria in an inappropriate way or because the normative criteria being applied are not the appropriate ones.

A review of research with chicks, songbirds, rodents, and nonhuman primates indicates that the brain is lateralized for a number of behavioral functions. These findings can be understood in terms of three hypothetical brain processes derived from a brain model based on general systems theory: hemispheric activation, interhemispheric inhibition, and interhemispheric coupling.

Left-hemisphere activation occurs in songbirds and nonhuman primates in response to salient auditory or visual input, or when a communicative output is required. The right hemisphere is activated in rats when spatial performance is required, and in chicks when they are placed in an emotion-provoking situation. In rats and chicks interhemispheric activation and inhibition occur when there is an affective component in the environment (novelty, aversive conditioning) or when an emotional response is emitted (copulation, attack, killing). An interhemispheric coupling (correlation) found in rats and rabbits implies that the hemispheres are two major components in a control system with a negative feedback loop. Early-experience variables in rats can induce laterality in a symmetric brain or facilitate its development in an already biased brain.

It is predicted that functional lateralization, when present, will be similar across species: the left hemisphere will tend to be involved in communicative functions while the right hemisphere will respond to spatial and affective information; both hemispheres will often interact via activation-inhibition mechanisms when affective or emotional processes are involved. Homologous brain areas and their connecting callosal fibers must be intact at birth and must remain intact throughout development for lateralization to reach its maximum level. Injury to any portion of this unit will result in hemispheric redundancy rather than specialization. One major function of early experience is to provide stimulation during development, which acts to enhance the growth and development of the corpus callosum, thereby giving rise to a more specialized brain.

The measurement of sensory intensity has had a long history, attracting the attention of investigators from many disciplines including physiology, psychology, physics, mathematics, philosophy, and even chemistry. While there has been a continuing doubt by some that sensation has the properties necessary for measurement, experiments designed to obtain estimates of sensory intensity have found that a general rule applies: Equal stimulus ratios produce equal sensory ratios. Theories concerning the basis for this simple psychophysical rule are discussed, with emphasis given to the physical correlate theory, which considers judgments of subjective magnitudes to be based upon estimates of physical dimensions that vary regularly with changes in degree of stimulation. For the most thoroughly investigated sensory scales, brightness and loudness, the physical correlate is considered to be distance. Our “tacit knowledge” of the sensory effects of changing distance plays an essential role in matching motor activities to environmental conditions and in ensuring accurate perceptual evaluations through brightness and loudness constancies. In psychophysical experiments, subjects apparently use this same tacit knowledge when required to estimate relative subjective magnitudes. The evidence related to the physical correlate theory is summarized, and it is concluded that, while under appropriate conditions we demonstrate considerable skill in evaluating environmental relationships, we are quite unable to estimate the neurophysiological nature or quantity of sensory response. A psychophysics devoted to studying conditions required for accuracy and conditions producing errors in the perception of environmental relationships would seem to be more valuable than one devoted to subjective magnitudes.

Algorithmic model building in artificial intelligence provides a contemporary method for explaining certain kinds of human behavior. The paranoid mode of thought and action represents a kind of pathological behavior that has been well recognized for over twenty centuries. This article describes a computer simulation model embodying a theory that attempts to explain the paranoid mode of behavior in terms of strategies for minimizing and forestalling shame-induced distress. The model consists of two parts, a parsing module and an interpretation-action module. To bring the model into contact with the conditions of a psychiatric diagnostic interview, the parsing module attempts to understand the interview input of clinicians communicating in unrestricted natural language. The meaning of the input is passed to an interpretation-action module made up of data structures and production rules that size up the current state of the interview and decide which (linguistic) actions to perform in order to fulfill the model's intentions. This module consists of an object system which deals with interview situations and a metasystem which evaluates how well the object system is performing to attain its ends. The fidelity of the simulation has been tested using Turing-like indistinguishability tests in which clinical judges attempt to distinguish the paranoid model-patient from an actual paranoid patient. Since clinicians are unable to make the relevant distinctions, the simulation is considered successful at the input-output level of functional equivalence. Issues of underlying structural equivalence and the nature of generative pattern explanations are discussed in the light of the model's potential value in guiding clinicial decisions and intervention strategies in paranoid disorders.

Denenberg rightly stresses the importance of studying ethologically meaningful species-specific behavior in animals, and makes the interesting distinction between lateralization at an individual and at a population level. However, in the case of man, I believe Denenberg is wrong in arguing that lateralization in the individual increases with maturation. The overall evidence nowadays tends very much to the contrary. Moreover, with respect to a population, why should it become lateralized? If there is indeed an advantage for the individual in hemispheric specialization, why should the direction of such specialization be so consistent across a majority of individuals, whether human or, as Denenberg points out, other members of the phylum? Is there an evolutionary advantage in most animals' sharing the same direction, or is it a necessary consequence of some other preexisting, more fundamental anatomical, biochemical, or physical property of the organism and its constituents? If the former, why are not all members of the species, rather than just a majority, lateralized in the same direction? (Or, to put it another way, what is the evolutionary advantage to the species or individual of dimorphism, of retaining a minority who polarize in the opposite direction?) If the latter - i.e., if lateralization is a necessary consequence of some prior state - then there should not be any dimorphism, exceptions, or minority members, unless they are somehow disadvantaged in consequence. Indeed, there is some evidence of a cognitive deficit in sinistrals, though it is disputed (see Bradshaw 1980 for review), and others have even suggested that the species as a whole may benefit in some way from such an uneven dimorphism (Levy 1974), but what evidence is there for such propositions with respect to rats, apes, monkeys, or chicks? This is an issue that should be addressed in any general model that includes laterality in animals. [See Corhallis & Morgan: “On the Biological Basis of Human Laterality” BBS 1(2) 1978.]

The enigma of hemispheric specialization of the human brain continues to attract the attention of BBS readers. Although the lateralization of language is obviously specific to man, some scientists find the idea of human uniqueness unacceptable. Corballis and Morgan (1978) presented hemispheric dominance in man as a special case of a left-right maturational gradient, examples of which can be found throughout the animal kingdom. According to Denenberg, brain laterality can be induced in animals by nonlateralized environmental factors such as handling. Since nonlateralized influences can only unmask latent asymmetries, Denenberg's position is essentially similar to the views espoused by Corballis and Morgan (1978) and can, therefore, be criticized on the same grounds.

In his target article as well as in other writings, Denenberg presents a view of lateralization with which I fundamentally disagree: namely, that an affirmation of lateralization in a population is to be based primarily, if not exclusively, on observing a nonequiprobable distribution of asymmetric forms in that population.

This is a welcome attempt to seek common biological principles underlying laterality in animals and humans. I do not think it is wholly convincing, but this is partly because many of the results from nonhuman species are not yet firmly established or understood. I suspect that the author's characterization of lateralization, with the left hemisphere supposedly specialized for communicative functions and the right for spatial and affective functions, will require modification. For instance, it now seems fairly clear that the left hemisphere in humans plays a general role in the production and perception of sequences not restricted to communicative acts (Craig 1980; Kimura 1979), and indeed some of the examples of left-hemispheric specialization listed in Table 2 are not obviously communicative. Even so, there are some fairly striking parallels between humans and nonhumans with respect to the pattern of lateralization, and one suspects that common principles are operating. At the same time, in the enthusiastic search for functional asymmetries, we should not overlook the striking degree of bilateral symmetry that characterizes the brains of all animals, including humans.