Deductive rationality in human reasoning: Speed, validity and the assumption of truth in conditional reasoning

We proffer the thesis that, in the process of defeating an inference on the basis of a factual truth that falsifies it, people move from a hypothetical truth-value to a factual truth-value of the conclusion. We will present evidence that shows (a) that some people spontaneously make a truth assumption and constrain their inferences to logically valid inferences, (b) that people tend to abandon the truth-assumption when they have factual evidence to the contrary, (c) that people, however, can and do in fact reason logically when they are informed about the rules of the language game (i.e., the truth-assumption) and (d) that adhering to the truth-assumption in the face of conflicting evidence to the contrary requires an investment of time and effort. The findings are discussed in relation to contemporary theories of human reasoning.