Counterexample retrieval and inhibition during conditional reasoning: Direct evidence from memory probing

In one of Bill Waterson’s hilarious ‘Calvin and Hobbes’ cartoons, Calvin is standing in the garden with a big can of water. The plants in the garden are on the verge of wilting and with an evil grin on his face, Calvin proudly exclaims that it is up to him to decide if they get water or not. He keeps on shouting that their very lives are in his hands and that he controls their fate. Calvin’s aspirations of absolute power are short-lived, however. In the next frame, we can see a very disappointed Calvin standing in the middle of a sudden rain shower that is providing the victorious plants with all the water they could ever dream of. This cartoon is decorating my office door. It is a superb illustration of a classic reasoning fallacy. Calvin knows that if you water plants, they stay healthy. Based on this conditional (e.g., ‘if, then’) knowledge he infers that if he doesn’t water the plants, it follows that they will die. In logic, this inference is known as the Denial of the Antecedent fallacy. A conditional links a specific precondition or cause (e.g., you water the plants) with a specific consequence or effect (e.g., the plants stay healthy). Logically speaking, a conditional utterance implies that if the precondition is met, the effect should always follow. However, this does not mean that if the precondition is not met, the effect cannot occur. There might be other conditions that can result in the occurrence of the effect (e.g., ‘it might rain’). Most adults will spontaneously think of such alternative causes and this will help them to avoid Calvin’s fallacious reasoning (and utter disappointment). Over the last two decades it has been argued that the search for such alternatives (or ‘counterexamples’) lies at the core of the conditional reasoning process. It is assumed that when faced with a conditional, people will spontaneously search their semantic memory for stored alternative causes. Retrieval of such alternatives helps people to reject invalid inferences such as the above illustrated DA or related Affirmation of the Consequent (AC) fallacy. Numerous studies showed that the easier it is for people to think of possible alternatives, the less they accept the invalid AC and DA inferences. In one of the classic studies Cummins, Lubart, Alksnis, and Rist (1991) measured in a pretest how many alternative causes people could generate for a set of conditionals. When Cummins et al., used these conditionals in a reasoning task with different participants, she found that the invalid inferences were less frequently accepted when a conditional had many vs. few possible alternatives. Likewise, Markovits and Quinn (2002) measured individual differences in people’s memory retrieval capacity (i.e., how easily they could come up with alternatives for a set of conditionals) in a pretest. A subsequent reasoning test showed that participants with a better retrieval capacity were less likely to commit the AC or DA fallacy. Retrieving stored alternatives from memory makes people better logical reasoners: It helps us to reject invalid inferences. Unfortunately, people’s tendency to take stored background