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.

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Over the last two decades increasing evidence for an acute sensitivity to human gestures and attentional states in domestic dogs has led to a burgeoning of research into the social cognition of this highly familiar yet previously under-studied animal. Dogs (Canis lupus familiaris) have been shown to be more successful than their closest relative (and wild progenitor) the wolf, and than man's closest relative, the chimpanzee, on tests of sensitivity to human social cues, such as following points to a container holding hidden food. The "Domestication Hypothesis" asserts that during domestication dogs evolved an inherent sensitivity to human gestures that their non-domesticated counterparts do not share. According to this view, sensitivity to human cues is present in dogs at an early age and shows little evidence of acquisition during ontogeny. A closer look at the findings of research on canine domestication, socialization, and conditioning, brings the assumptions of this hypothesis into question. We propose the Two Stage Hypothesis, according to which the sensitivity of an individual animal to human actions depends on acceptance of humans as social companions, and conditioning to follow human limbs. This offers a more parsimonious explanation for the domestic dog's sensitivity to human gestures, without requiring the use of additional mechanisms. We outline how tests of this new hypothesis open directions for future study that offer promise of a deeper understanding of mankind's oldest companion.

We hypothesize that selection during dog domestication targeted CNVs associated with hypersociability.