Not all syllogisms are created equal: Varying premise believability reveals differences between conditional and categorical syllogisms

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

The Role of Working Memory in Deductive Reasoning: A Dual Task and Individual Differences Approach

The ‘belief bias’ effect is one of the most pervasive findings in the study of syllogistic reasoning. Here, participants respond “valid” to more believable than unbelievable conclusions, regardless of the actual validity of the conclusion. There is also an interaction characteristic of the belief bias effect, in that conclusion believability plays a greater role when conclusions are invalid than when they are valid. The experiments reported in this thesis had two goals: first, to determine how individual differences in working memory (WM) capacity influence belief bias in reasoning; and second, to indentify which WM systems are involved in syllogistic deductive reasoning. To this end, both experiments employed a dual task paradigm. In Experiment 1, participants remembered spatial arrays whilst reasoning through syllogisms in order to load the visuospatial sketchpad. Results demonstrated that performance on the secondary spatial memory task suffered when participants reasoned through syllogisms of which the validity and believability of conclusions were incongruent (i.e., “conflict” problems), indicating that reasoning through conflict problems utilized limited visuospatial WM resources. Also, only participants with high WM capacities showed the typical belief-bias effect, with greater effects of conclusion believability on invalid than on valid conclusions. This interaction was not present for low WM span participants, because they made greater errors on problems with invalid, unbelievable conclusions. In Experiment 2, participants remembered digit sequences whilst reasoning in order to load the phonological loop. Both of the major results from Experiment 1 were replicated. Accuracy on the secondary digit recall task was impaired when participants reasoned through conflict problems, demonstrating that limited verbal WM resources were directed toward reasoning. Again, only high WM span participant showed the interaction between conclusion validity and believability characteristic of the belief bias effect. Effects were additive for low WM span participants because they made more errors on invalid, unbelievable syllogisms. Results from both experiments demonstrate first, that both visuospatial and verbal WM resources are involved in syllogistic reasoning, and second, that individuals with different amounts of available WM resources demonstrate differential belief bias. These results are discussed in terms of the mental models and mental logic theories of reasoning and in terms of dual process accounts of reasoning

Dopaminergic medication impairs feedback-based stimulus-response learning but not response selection in Parkinson\u27s disease.

Cognitive dysfunction is a feature of Parkinson\u27s Disease (PD). Some cognitive functions are impaired by dopaminergic medications prescribed to address the movement symptoms that typify PD. Learning appears to be the cognitive function most frequently worsened by dopaminergic therapy. However, this result could reflect either impairments in learning (i.e., acquisition of associations among stimuli, responses, and outcomes) or deficits in performance based on learning (e.g., selecting responses). We sought to clarify the specific effects of dopaminergic medication on (a) stimulus-response association learning from outcome feedback and (b) response selection based on learning, in PD. We tested 28 PD patients on and/or off dopaminergic medication along with 32 healthy, age- and education-matched controls. In Session 1, participants learned to associate abstract images with specific key-press responses through trial and error via outcome feedback. In Session 2, participants provided specific responses to abstract images learned in Session 1, without feedback, precluding new feedback-based learning. By separating Sessions 1 and 2 by 24 h, we could distinguish the effect of dopaminergic medication on (a) feedback-based learning and response selection processes in Session 1 as well as on (b) response selection processes when feedback-based learning could not occur in Session 2. Accuracy achieved at the end of Session 1 were comparable across groups. PD patients on medication learned stimulus-response associations more poorly than PD patients off medication and controls. Medication did not influence decision performance in Session 2. We confirm that dopaminergic therapy impairs feedback-based learning in PD, discounting an alternative explanation that warranted consideration