Fig 1.

Top panel: The six stimulus classes hypothesized to develop from the reinforced sample-comparison sequences for the two symbolic and form-identity baseline matching tasks for Group Identity. Ellipses highlight common class members. Bottom panel: Two 4-member stimulus classes hypothesized to arise from the merger of the stimulus classes shown in the top panel via their common elements. Arrows indicate sample-comparison sequences to which the Group Identity pigeons should preferentially respond in a reflexivity test. R  =  red, G  =  green, T  =  triangle, H  =  horizontal, 1  =  first ordinal position within a matching trial, 2  =  second ordinal position within a matching trial.