Family resemblances facilitate formation and expansion of functional equivalence classes in pigeons.

Four pigeons were given repeated reversal training and testing with photographs of human faces constituting two categories structured by family resemblances, each consisting of a prototype, good exemplars, and poor exemplars. Each of the good exemplars (AM, BM, and CM) was created by 50% morphing of the prototype (M) and one of the poor exemplars (A, B, and C, respectively) and thus was physically similar to the prototype and to the corresponding poor exemplar. The pigeons were first trained and tested for the formation of two (AM, BM, and CM) classes. Then, the stimulus sets were extended to include (1) M and the poor exemplars that were not physically similar to one another and (2) 50% morphs of the poor exemplars (AB, BC, and CA). In the sequentially introduced training and test phases, we successfully tracked expansion of the functional equivalence classes consisting of exemplars that had little similarity but could be linked together through other members of the class.