Two great religious traditions, Christian and Hindu, provide sharply dichotomous views of humanity. For the Christian, unbridgeable spiritual gaps separate humans from animals. Humans have souls; animals do not. For the Hindu, animal and human minds represent different points on a continuous progression toward Nirvana. These two philosophical perspectives permeate scientific as well as religious concepts of the human mind.

Twenty-five years ago, the dominant anthropological and psychological paradigms followed the Christian tradition in considering humans and animals to be separated by qualitative, seemingly unbridgeable, behavioral gaps: Humans were the only animals who could symbolize, make a tool, or recognize their own self-images. Within that theoretical climate, Ralph Holloway proposed his now classic theory of the evolution of the human brain. Brain size alone cannot account for the behavioral distinctions of the human species. Rather, the human brain has been reorganized, thereby permitting language and other mental skills (Holloway, 1966, 1968). Holloway's theory met with wide acclaim. It seemed intuitively correct.

Nearly simultaneously, Harry Jerison set forth a quite different theory, more in keeping with the Eastern tradition of a continuum between animal and human minds: Human intelligence reflects changes in brain size and in total information-processing capacity (Jerison, 1973). Unlike Holloway, Jerison received a cool reception. Those who considered human behavior qualitatively distinct from that of the apes also expected qualitative, rather than quantitative, differences in brain structure.

In the intervening years, our knowledge of primate behavior has expanded.

The editors conceived this book with the double purpose of introducing to a larger public a new approach to the study of animal abilities and of stimulating additional studies in the new research program described in this volume. This research program, which we call “comparative developmental evolutionary psychology,” is based on the application of frameworks from human developmental psychology and evolutionary biology to comparative studies of primate abilities. All of the studies reported in this volume are either directly or indirectly developmental; that is, they either trace the ontogenetic development of specific abilities within developmental frameworks or use such frameworks to compare the nature and level of abilities of their subjects to those of human infants and children at various stages of development. None of the studies in this volume have been published elsewhere. The majority of the chapters grew out of presentations made at a 3-day workshop held as a presession of the Ninth International Congress of Primatology at Göttingen, West Germany, in July 1986. The additional chapters are by colleagues of the original contributors or by researchers whose work we discovered subsequent to the workshop.

This book is the second volume devoted specifically and exclusively to studies of animal intelligence and cognition based on comparative developmental (Piagetian, neo-Piagetian, and developmental psycholinguistic) perspectives. The first volume, which was published last year, reported on long-term studies of intellectual development in cebus, gorillas, and macaques at the Comparative Psychology Laboratory at the Consiglio Nazionale delle Ricerche (CNR) in Rome (Antinucci, 1989). Several members of that laboratory are contributors to this book.

A recent article in Behavioral and Brain Sciences by the British psychologist Euan Macphail (1987) argues that all nonhuman vertebrates are equally intelligent. All animal intelligence derives from one and only one intellectual mechanism: the ability to form associations. No qualitative or quantitative differences can exist among nonhuman vertebrates because none possesses any additional intellectual mechanism.

The view that a uniform level of intelligence characterizes all nonhuman vertebrates is difficult to reconcile with the anatomy of the structure that mediates intelligence: the brain (Gibson, P&G3). Vertebrate brain sizes range from less than a gram to several pounds (Jerison, 1973; Sacher & Staffeldt, 1974). Structurally, the brains of some species are dominated by the brain stem, others by enlarged basal ganglia, and still others by enlarged neocortices (Sarnat & Netsky, 1981). Similarly, species differ in their performances on standard Piagetian tests of human intelligence (Chevalier-Skolnikoff, 1977, 1983, 1989; Dore & Dumas, 1987; Parker, 1977; Parker & Gibson, 1977, 1979; Redshaw, 1978; Wise, Wise, & Zimmerman, 1974; Miles, P&G19; Russon, P&G14).

These considerations suggest caution in interpreting the results of laboratory tests that imply uniformity of intelligence across the vertebrate order. Even if Macphail's assessment that all animal learning and problem solving reflect the ability to form associations should prove correct, his conclusion that all animals are equally intelligent does not follow.