Same-different discrimination: the keel and backbone of thought and reasoning.

Discriminating same from different collections of items is central to human thought and reasoning. Recent comparative research suggests that same-different discrimination behavior is not uniquely human, does not require human language, is based on the variability of the collection of items, obeys fundamental psychophysical laws, and may be captured by quantitative models of the stimulus collection. The comparative study of same-different discrimination behavior sheds fresh light on the mechanisms and functions of abstract conceptualization. This study also has prompted the development of a theory-the Finding Differences Model-that successfully explains a wealth of findings in the comparative psychology of same-different discrimination behavior.