Probabilistic and deductive reasoning in the human brain

Reasoning is a process of inference from given premises to new conclusions. Deductive reasoning is truthpreserving and conclusions can only be either true or false. Probabilistic reasoning is based on degrees of belief and conclusions can be more or less likely. While deductive reasoning requires people to focus on the logical structure of the inference and ignore its content, probabilistic reasoning requires the retrieval of prior knowledge from memory. Recently, however, some researchers have denied that deductive reasoning is a faculty of the human mind. What looks like deductive inference might actually also be probabilistic inference, only with extreme probabilities. We tested this assumption in an fMRI experiment with two groups of participants: one group was instructed to reason deductively, the other received probabilistic instructions. They could freely choose between a binary and a graded response to each problem. The conditional probability and the logical validity of the inferences were systematically varied. Results show that prior knowledge was only used in the probabilistic reasoning group. These participants gave graded responses more often than those in the deductive reasoning group and their reasoning was accompanied by activations in the hippocampus. Participants in the deductive group mostly gave binary responses and their reasoning was accompanied by activations in the anterior cingulate cortex, inferior frontal cortex, and parietal regions. These findings show that (1) deductive and probabilistic reasoning rely on different neurocognitive processes, (2) people can suppress their prior knowledge to reason deductively, and (3) not all inferences can be reduced to probabilistic reasoning.

Automated summary

Reasoning is the process of inferring new conclusions from given premises. Deductive reasoning preserves truth and conclusions are either true or false. Probabilistic reasoning is based on degrees of belief and conclusions can be more or less likely. However, some researchers have argued that what appears to be deductive reasoning may actually be probabilistic inference with extreme probabilities. An fMRI experiment was conducted to test this assumption, and the results showed that deductive and probabilistic reasoning rely on different neurocognitive processes, people can suppress prior knowledge to reason deductively, and not all inferences can be reduced to probabilistic reasoning.