Vertebrate intelligence: the null hypothesis

Human cognitive capacities have so evolved that man is able to solve an extensive range of problems having very different properties. Comparative psychologists have endeavoured to throw light on the evolution and nature of this general intellectual capacity by exploring performance of non-human vertebrates in a variety of learning tasks, in the expectation of demonstrating superior intelligence in species more closely related to man. It has, however, proved difficult to establish that any observed difference in performance is due to a difference in intellectual capacity rather than to a difference in such contextual variables as, for example, perception or motivation. Three hypotheses that might account for the lack of experimentally demonstrable differences in intelligence amongst non-humans are discussed. The first proposes that the data currently available may have been misinterpreted: that, for example, the potential role of contextual variables has been exaggerated. According to the second hypothesis, the questions posed by comparative psychologists have been inappropriate: learning mechanisms are adaptations evolved for life in a specific ecological niche, so that mechanisms available to species from different niches are not properly comparable. It is argued that neither of these two hypotheses receives convincing empirical support. A third hypothesis proposes that there are, in fact, neither quantitative nor qualitative differences among the intellects of non-human vertebrates. It is argued that this null hypothesis is currently to be preferred, and that man's intellectual superiority may be due solely to our possession of a species-specific language-acquisition device.