The emergence of reasoning by the disjunctive syllogism in early childhood

Abstract

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.

Keywords: Cognitive development; Disjunctive syllogism; Inference; Logic; Reasoning; Reasoning by exclusion.

Figure 1

Structure of training trials (left) and test trials (right) in Experiment 2. Symbols above the cups indicate the information that is available to the participant: the cup with the cross is empty, the cup with the checkmark is certain to contain a sticker, and the cups with question marks may or may not contain stickers.

Figure 2

Proportion of training trials (top) and test trials (bottom) in which children in each age group selected the target cup in Experiment 2. Error bars represent 95% confidence intervals, and the dotted line indicates chance (0.33).