Logical inference is often seen as an exclusively human and language-dependent ability, but several nonhuman animal species search in a manner that is consistent with a deductive inference, the disjunctive syllogism: when a reward is hidden in one of two cups, and one cup is shown to be empty, they will search for the reward in the other cup. In Experiment 1, we extended these results to toddlers, finding that 23-month-olds consistently approached the non-empty location. However, these results could reflect non-deductive approaches of simply avoiding the empty location, or of searching in any location that might contain the reward, rather than reasoning through the disjunctive syllogism to infer that the other location must contain the reward. Experiment 2 addressed these alternatives, finding evidence that 3- to 5-year-olds used the disjunctive syllogism, while 2.5-year-olds did not. This suggests that younger children may not easily deploy this logical inference, and that a non-deductive approach may be behind the successful performance of nonhuman animals and human infants.

Three experiments investigated the representations that underlie 14-month-old infants’ and adults’ success at match-to-sample (MTS) and non-match-to-sample (NMTS) tasks. In Experiment 1, 14-month-old infants were able to learn rules based on abstract representations of sameness and/or difference. When presented with one of eighteen sample stimuli (A) and a choice between a stimulus that was the same as the sample (A) and a different stimulus (B), infants learned to choose A in MTS and B in NMTS. In Experiments 2 and 3, we began to explore the nature of the representations at play in these paradigms. Experiment 2 confirmed that abstract representations are at play, as infants generalized the MTS and NMTS rules to stimuli unseen during familiarization. Experiment 2 also showed that infants tested in MTS learned to seek the stimulus that was the same as the sample, whereas infants tested in NMTS did not learn to seek the different stimulus, but instead learned to avoid the stimulus that was the same as the sample. Infants appeared to only use an abstract representation of the relation same in these experiments. Experiment 3 showed that adult participants, despite knowing the words “same” and “different”, also relied on representations of sameness in both MTS and NMTS in a paradigm modeled on that of Experiment 2. We conclude with a discussion of how young infants may possibly represent the abstract relation same.

No abstract

In Call’s (2004) 2-cups task, widely used to explore logical and causal reasoning across species and early human development, a reward is hidden in one of two cups, one is shown to be empty, and successful subjects search for the reward in the other cup. Infants as young as 17-months and some individuals of almost all species tested succeed. Success may reflect logical, propositional thought and working through a disjunctive syllogism (A or B; not A, therefore B). It may also reflect appreciation of the modal concepts “necessity” and “possibility”, and the epistemic concept “certainty”. Mody & Carey’s (2016) results on 2-year-old children with 3- and 4-cups versions of this task converge with studies on apes in undermining this rich interpretation of success. In the 3-cups version, one reward is hidden in a single cup, another in one of two other cups, and the participant is given one choice, thereby tracking the ability to distinguish a certain from an uncertain outcome. In the 4-cups procedure, a reward is hidden in one cup of each pair (e.g., A, C); one cup (e.g., B) is then shown to be empty. Successful subjects should conclude that the reward is 100% likely in A, only 50% likely in either C or D, and accordingly choose A, thereby demonstrating modal and logical concepts in addition to epistemic ones. Children 2 1/2 years of age fail the 4-cups task, and apes fail related tasks tapping the same constructs. Here we tested a Grey parrot (Psittacus erithacus), Griffin, on the 3- and 4-cups procedures. Griffin succeeded on both tasks, outperforming even 5-year-old children. Controls ruled out that his success on the 4-cups task was due to a learned associative strategy of choosing the cup next to the demonstrated empty one. These data show that both the 3- and 4-cups tasks do not require representational abilities unique to humans. We discuss the competences on which these tasks are likely to draw, and what it is about parrots, or Griffin in particular, that explains his better performance than either great apes or linguistically competent preschool children on these and conceptually related tasks.