Non-verbal rationality? 2-year-old children, dogs and pigs show unselective responses to unreliability, but to different degrees

Some philosophers argue that reflection, the ability to assess one’s reasons for beliefs, is the defining feature of rational thinking. Yet, by tying reflective thinking to language, they struggle to account for apparently rational behaviour in minimally verbal human infants and altogether exclude non-human animals. To assess capacities for a basic form of reflective thinking that does not require language, we investigated whether 2-year-old children, dogs, or pigs could acquire and respond to undermining defeaters. In an object-search task, subjects observed different informants act on one of two screens: one informant’s actions reliably indicated the reward location, while the other’s actions were independent of the reward location. The informants switched to using new actions twice during a sequence of repeated trials, which put subjects in the position to make an inference about the reliability of the informants based on the informativeness of their actions. Neither 2-year-olds nor animals responded differently to the Reliable and Unreliable informants. There was a reduction in subjects’ willingness to follow the indications of the informants in later trials, which was stronger in 2-year-olds than animals. While these results provide no evidence that 2-year-olds, dogs, or pigs made an inference about the reliability of each informant across the actions, reduced willingness to follow the informants’ indication suggests that they may be responding to either uncertainty or an undermining defeater like .