Introduction

Psychology is defined as the study of the mind (Psyche), and comparative psychology as the study of animal minds. Comparative psychology began by testing animals individually on tasks presumed to reflect general cognitive abilities, or a general intelligence factor, and taxa were compared by their achievements on these tasks. Ethologists were less interested in capacities and more interested in species-typical behavior.

Having been schooled in both traditions, I was quick to accept Mason's suggestions in 1960 that the study of primates living in groups would yield more insight into their cognitive capacities than would individual testing. Whereas Chance and Mead (1952) and Jolly (1966), theorized that primate intelligence evolved in part because of the selective pressures of group living, it was a quarter of a century later that Cheney and Seyfarth (1985a,b) suggested that primate intelligence is extraordinary only in the social domain. Today, investigators regularly examine social interactions and their functions in a search for evidence of extraordinary cognitive capacities in nonhuman primates.

As early as 1880, Lauder hypothesized the existence of infanticide, reciprocity, and teaching in animals. He argued that animals understand death, express anger, jealousy, rage and revenge, commit suicide, suffer from moral insanity, and recognize and respond to criminal behavior. We may describe such writings as quaint, but similar terms, arguments, and supporting evidence are used in recent accounts of the abilities of nonhuman primates. These are often presented against a background of sophisticated evolutionary theory, but the evidence is often distressingly sparse.

In 1986 Carolyn Ehardt and I published a paper, ‘The influence of kinship and socialization on aggressive behaviour in rhesus monkeys (Macaca mulatta)’ The title of this paper is not unusual; it asserts that kinship is an independent variable that influences aggressive behaviour, as a dependent variable. This suggests that manipulation of the first will alter the second. Naturally we would be quick to deny a causal relationship and would repeat that science only demonstrates correlations, not causation. But why then did we not title the paper, ‘The influence of aggressive behaviour on kinship’? If we had done so, many would have quickly pointed to this as an example of a backward causal argument. In fact, despite our denials, we do attempt to suggest causal relationships in our correlations and, in this case, as in many others, there is a clear correlation between two variables but there may be no direct causal relationship.

Perhaps we have acknowledged the contributions of Tinbergen (1951) only to fail to apply them to our own thinking. All too often we confuse function with proximal cause, evolutionary cause, and even structure (as in definitions like ‘the process leading to’). Functional outcomes are argued to be the motivational cause of behaviour (the animal was ‘trying to drive the predator away’) and if something functions in an adaptive fashion we often assume that this is because it was selected for this function during evolution birds developed wings in order to fly south in the winter).