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The Search for Counterexamples in Human Reasoning

How do logically-untrained individuals establish that an argument is invalid? Some theories postulate that they search for, but fail to find, a formal proof for it. They could, however, establish invalidity by constructing a counterexample, that is, a mental model that satisfies the premises but refutes the conclusion. We report an experiment designed to answer the question. The participants used paper and pencil and thought aloud as they tested the validity of putative conclusions from premises based on the non-standard quantifier, "more than half". The analysis of their videotaped protocols showed that they used a variety of strategies, but that every participant constructed explicit counterexamples.