Not all syllogisms are created equal: Varying premise believability reveals differences between conditional and categorical syllogisms
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Deductive reasoning is a fundamental cognitive skill, and consequently has been the focus of much research over the past several decades. In the realm of syllogistic reasoning—judging the validity of a conclusion given two premises—a robust finding is the belief bias effect: broadly, the tendency for reasoners to judge as valid more believable than unbelievable conclusions. How the content believability of conclusions influences syllogistic reasoning has been the subject of hundreds of experiments and has informed several theories of deductive reasoning; however, how the content of premises influences the reasoning processes has been largely overlooked. In this thesis, I present 5 experiments that examine how premise content influences reasoning about categorical (i.e., statements with the words ‘some’ and ‘not’) and conditional (i.e., ‘if/then’ statements) syllogisms, which tend to be treated as interchangeable in deductive reasoning literature. It is demonstrated that premise content influences reasoning in these two types of syllogisms in fundamentally different ways. Specifically, Experiment 1 replicates and extends previous findings and demonstrates that for conditional syllogisms, belief bias results when premises are both believable and unbelievable; however, reasoners are more likely to judge that a conclusion is valid when it follows from believable than from unbelievable premises. Conversely, belief bias for categorical syllogisms results only when premises are believable; conclusion believability does not influence conclusion endorsement when premises are unbelievable. Based on these preliminary findings, I propose a theory that categorical and conditional syllogisms differ in the extent to which reasoners initially assume the premises to be true, and that this difference influences when in the reasoning process reasoners evaluate the believability of premises. Specifically, I propose that reasoners automatically assume that conditional, but not categorical, premises are true. It is proposed that, because the word “if” in conditional statements elicits hypothetical thinking, conditional premises are assumed to be true for the duration of the reasoning process. Subsequent to reasoning, premises can be “disbelieved” in a time-consuming process, and initial judgments about the conclusion may be altered, with a bias to respond that conclusions following from believable premises are valid. On the other hand, because categorical premises are phrased as factual propositions, reasoners initially judge the believability of categorical premises prior to reasoning about the conclusion. Unbelievable premises trigger the reasoner to disregard content from the rest of the syllogism, perhaps because the reasoner believes that the information in the problem will not be helpful in solving the problem. This theory is tested and supported by four additional experiments. Experiment 2 demonstrates that reasoners take longer to reason about conditional syllogisms with unbelievable than believable premises, consistent with the theory that unbelievable premises are “disbelieved” in a time-consuming process. Further, participants demonstrate belief bias for categorical syllogisms with unbelievable premises when they are instructed to assume that premises are true (Experiment 3) or when the word ‘if’ precedes the categorical premises (Experiment 4). Finally, Experiment 5 uses eye-tracking to demonstrate that premise believability influences post-conclusion premise looking durations for conditional syllogisms and pre-conclusion premise looking durations for categorical syllogisms. This finding supports the hypothesis that reasoners evaluate the believability of conditional premises after reasoning about the conclusion but that they evaluate the believability of categorical premises before reasoning about the conclusion. Further, Experiment 5 reveals that participants have poorer memory for the content of categorical syllogisms with unbelievable than believable premises, but memory did not differ for conditional syllogisms with believable and unbelievable premises. This suggests that unbelievable premise content in categorical syllogism is suppressed or ignored. These results and the theory of premise evaluation that I propose are discussed in the context of contemporary theories of deductive reasoning.