Mean percentage correct for baseline and transfer in the experimental and control groups of pigeons (left panel) and rhesus monkeys (right panel) at each set size or at equivalent points in training in the S/D procedure. Error bars represent SEMs. Note. From Figure 2, “Same/different abstract-concept learning by pigeons,” by J. S. Katz and A. A. Wright, 2006, Journal of Experimental Psychology: Animal Behavior Processes, 32, p. 84. From Figure 7, “Mechanisms of same/different abstract-concept learning by rhesus monkeys (Macaca mulatta),” by J. S. Katz, A. A. Wright, and J. Bachevalier, 2002, Journal of Experimental Psychology: Animal Behavior Processes, 28, p. 366. Copyrights 2006 and 2002 by the American Psychological Association. Adapted with permission.