Rumbaugh, Savage-Rumbaugh, and Hegel (1987) reported that two chimpanzees (Pan troglodytes) could select, with better than 90% accuracy, the greater of two paired quantities of chocolate chips. In that study, no one quantity of chocolates (from 0 through 5) was used in both pairs on a given trial. We investigated the effect of having one quantity in common (CQ) in both pairs. Whether the other quantities (OQs) of chocolates were both less than or greater than the CQ, summation still occurred. Accuracy was primarily a function of the ratios of sums to be differentiated. This finding substantiated the earlier conclusion that summation was based on both quantities of each pair and not on some simpler process such as the avoidance of the tray with the smallest single amount or selection of the tray with the single largest amount.
This essay contributes to the celebration of the 50th anniversary of the founding of the Society for the History of Psychology (SHP) in its earlier form (Division 26 of the American Psychological Association). Ronald Mayer's history of the division is updated by providing a description of some of the noteworthy events and changes in the organization since his publication. (PsycINFO Database Record
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