Pigeons were trained with 4 pairs of visual stimuli in a 5-term series-A+ B-, B+ C-, C+ D-. and D+ E-(in which plus[+] denotes reward and minus(-] denotes nonreward)-before the unreinforced test pair B D was presented. All pigeons chose Item 8, demonstrating inferential choice. A novel theory (value transfer theory), based on reinforcement mechanisms, is proposed. In Experiment 2, the series was extended to 7 terms. Performance on test pairs was transitive, and performance on training pairs accorded with the theory. The 7-term series was closed in Experiment 3 by training the flrst and last items together. In accordance with the theory, the Ss could not solve the circular series. The authors suggest that primates, including humans, also solve these problems using the value transfer mechanism, without resorting to the symbolic processes usually assumed. 1. Edith is fairer than Suzanne. 2. Edith is darker than Lili. 3. Who is the darkest, Edith, Suzanne, or Lili? Here the competent subject concludes that Suzanne is the darkest, although no direct information about the relationship between Lili and Suzanne was given. In this purely linguistic Juan D. Delius and Lorenzo von Fe~n were supported by the Deutsche Forschungsgemeinschaft, Bonn; C. D. L. Wynne was SUJ>ported by the Science and Engineering Research Council, London, and the Alexander von Humboldt.Stiftung, Bonn; and J. E. R. Staddon was supported by the National Science Foundation and the Alexander von Humboldt-Stiftung. We thank W. Mathaus for initial motivation, F. von Mi.inchow-Pohl for assistance in programming, and A. Lohmann for help in running the experiments. We are grateful to A. Elepfandt, B. Mc-Gonigle, J. Pearce, H. Terrace, and T. Trabasso for helpful comments on drafts of this article.

Pigeons were trained to match-to-sample with several new methodologies: a large number of stimuli, computer-drawn color picture stimuli, responses monitored by a computer touch screen, stimuli presented horizontally from the floor, and grain reinforcement delivered onto the picture stimuli. Following acquisition, matching-to-sample concept learning was assessed by transfer to novel stimuli on the first exposure to pairs of novel stimuli. One group (trial-unique), trained with 152 different pictures presented once daily, showed excellent transfer (80% correct). Transfer and baseline performances were equivalent, indicating that the matching-to-sample concept had been learned. A second group (2-stimulus), trained with only two different pictures, showed no evidence of transfer. These results are discussed in terms of the effect of numbers of exemplars on previous failures to find concept learning in pigeons, and the implications of the positive finding from this experiment on abstract concept learning and evolutionary cognitive development.Human cognitive behavior is characterized, in part, by our ability to abstract rules and form abstract concepts (Medin & Schaffer, 1978). Indeed, even our ability to effectively communicate with others depends upon this ability (Premack, 1978). One question that logically follows from discussions of human cognitive capabilities is: To what degree can animals form and learn concepts? Whether they can or not, bears upon one measure of how unique humans really are in the evolutionary hierarchy of cognitive abilities.The concepts, which are the focus of this article, are relational ones; they depend upon relations between pairs of items, for example, in situations in which subjects judge whether or not two items are identical (same) or nonidentical (different)-a same/different task-or ones in which they choose a comparison item to match a previously presented sample item-matching-to-sample. It is possible to learn a rule-based concept in these situations, so that any pair of items can be correctly judged. These relational concepts are to be contrasted to

Pigeons and humans chose which one of two alternative visual forms was identical to, or a mirror image of, a previously presented sample form. The two comparison forms were presented in various orientations with respect to the sample. The two species yielded similar accuracies, but although human reaction times depended linearly on the angular disparities, those of the pigeon did not. Humans appeared to apply a well-known, thoughtlike, mental rotation procedure to the problem, whereas pigeons seemed to rely on a more efficient automatic process that humans can use only in simpler rotational invariance tasks. Mirror-image forms may be better discriminated by the pigeon's visual system than by the human one.