Monkeys are Rational!

Five squirrel monkeys showed a significant Symbolic Distance Effect (SDE) when tested on procedures designed to incorporate reaction time (RT) measures within the five-term series (“transitive inference”) task. Taken together with the high levels of transitive choice obtained by these subjects, this result shows that monkeys have met choice and reaction time criteria perceived by many as indexical of “linear representation”. Explicit evaluation of the representational implications of the SDE was subsequently undertaken in three-choice transfer tests, which initially revealed consistent deviations from the ranks predicted on a linear model. Extensive testing with the triadic method, however, elicited spontaneous improvement in overall choice ranking according to a linear principle. Further decomposition of the RT data showed that the irregularities in the initial transfer behaviour are by no means paradoxical. The ordinal separation data structure, from which the SDE is derived, masks two separate populations of scores–-one fast and relatively undifferentiated, the other slow and based on judgements with poorly encoded items. And it is these factors, not inter-item distance, which, we argue, are the main determiners of the SDE. Whilst this phenomenon is thus only weakly indexical of principled linear representation, we contend that new (triad-based) ranking evidence suggests that monkeys may be capable of seriation as well as transitivity.