Sameness May Be a Natural Concept That Does Not Require Learning.

It has been assumed that when pigeons learn how to match to sample, they learn simple stimulus-response chains but not the concept of sameness. However, transfer to novel stimuli has been influenced by pigeons' tendency to be neophobic. We trained pigeons on matching ( n = 7) and mismatching ( n = 8) with colors as samples and, with each sample, one color as the nonmatching comparison. We then replaced either the matching or the nonmatching stimulus with a familiar stimulus never presented with that sample. Results suggest that for both matching and mismatching, pigeons locate the stimulus that matches the sample: If the task involves matching, they chose it; if it involves mismatching, they avoid it. Thus, the concept of sameness is the basis for correct choice with both tasks. This finding suggests that sameness is a basic concept that does not have to be learned and may have evolved in many species, including humans.