When we engage in reasoning, we keep track of and evaluate the reasons for our actions. Here we show that great apes demonstrate this capacity, by double-checking the evidence for a decision when new evidence calls it into questions. Remarkably, while apes demonstrate the ability young children at three years of age did not. In a second study we show that when young children and great apes are faced with a similar challenge – but now involving a social partner – even the youngest children double-checked their evidence, while apes did not. Together these studies reveal the social foundation of human reasoning.


Several species can detect when they are uncertain about what decision to make—revealed by opting out of the choice, or by seeking more information before deciding. However, we do not know whether any nonhuman animals recognize when they need more information to make a decision because new evidence contradicts an already-formed belief. Here, we explore this ability in great apes and human children. First, we show that after great apes saw new evidence contradicting their belief about which of two rewards was greater, they stopped to recheck the evidence for their belief before deciding. This indicates the ability to keep track of the reasons for their decisions, or ‘rational monitoring’ of the decision-making process. Children did the same at 5 years of age, but not at 3 years. In a second study, participants formed a belief about a reward’s location, but then a social partner contradicted them, by picking the opposite location. This time even 3-year-old children rechecked the evidence, while apes ignored the disagreement. While apes were sensitive only to the conflict in physical evidence, the youngest children were more sensitive to peer disagreement than conflicting physical evidence.

First published: 23 March 2022  Ι  SCI Author : Cathal O’Madagain