Inferential reasoning by exclusion in pigeons, dogs, and humans.

The ability to reason by exclusion (which is defined as the selection of the correct alternative by logically excluding other potential alternatives; Call in Anim Cogn 9:393-403 2006) is well established in humans. Several studies have found it to be present in some nonhuman species as well, whereas it seems to be somewhat limited or even absent in others. As inconsistent methodology might have contributed to the revealed inter-species differences, we examined reasoning by exclusion in pigeons (n = 6), dogs (n = 6), students (n = 6), and children (n = 8) under almost equal experimental conditions. After being trained in a computer-controlled two-choice procedure to discriminate between four positive (S+) and four negative (S-) photographs, the subjects were tested with displays consisting of one S- and one of four novel stimuli (S'). One pigeon, half of the dogs and almost all humans preferred S' over S-, thereby choosing either by novelty, or by avoiding S- without acquiring any knowledge about S', or by inferring positive class membership of S' by excluding S-. To decide among these strategies the subjects that showed a preference for S' were then tested with displays consisting of one of the S' and one of four novel stimuli (S''). Although the pigeon preferentially chose the S'' and by novelty, dogs and humans maintained their preference for S', thereby showing evidence of reasoning by exclusion. Taken together, the results of the present study suggest that none of the pigeons, but half of the dogs and almost all humans inferred positive class membership of S' by logically excluding S-.