Academic journal article Memory & Cognition

Thinking about Conditionals: A Study of Individual Differences

Academic journal article Memory & Cognition

Thinking about Conditionals: A Study of Individual Differences

Article excerpt

Recent studies have shown the existence of two qualitatively distinct groups of people based on how they judge the probability of a conditional statement. The present study was designed to test whether these differences are rooted in distinctive means of processing conditional statements and whether they are linked to differences in general intelligence. In the study, each of 120 participants completed three separate cognitive tasks involving the processing of abstract conditional statements-the probability-of-conditionals task, the conditional truth table task, and the conditional inference task-in addition to completing a test of general intelligence (AH4). The results showed a number of predicted effects: People responding with conditional (rather than conjunctive) probabilities on the first task were higher in cognitive ability, showed reasoning patterns more consistent with a suppositional treatment of the conditional, and showed a strongly "defective" truth table pattern. The results include several novel findings and post challenges to contemporary psychological theories of conditionals.

Conditional sentences of the form if p then q are everywhere in ordinary language and communication and in technical and scientific discourse. In our view, such statements are of particular importance because if is used to initiate the imagination and simulation of possibilities, a process that we term hypothetical thinking (Evans, 2007; Evans & Over, 2004). Such imaginary thought is required whenever we entertain a hypothesis, attempt to forecast future events, or imagine the consequences of alternative actions to support our decision making. In view of this, it is not surprising that a huge amount of academic literature has been devoted to the study of if in both the philosophy and psychology (see Bennett, 2003; Evans & Over, 2004).

Most contemporary logicians have argued that the material conditional cannot be the ordinary conditional of everyday discourse, since this leads to unacceptable paradoxes. The material conditional renders if p then q equivalent to not p or q. In this case, the statement must be true whenever p is false or q is true. For example, we would have to believe that the statement "if Al Gore is president then George Bush is president" is true, when it appears quite clearly to be false (in this example, the antecedent is false and the consequent is true). Our own suppositional theory of conditionals builds on related psychological work (Braine & O'Brien, 1991; Marcus & Rips, 1979; Oaksford & Chater, 2001; Rips & Marcus, 1977), as well as on studies in philosophical logic (Edgington, 1995, 2003). In compliance with what philosophers call the Ramsey test (Ramsey, 1931), we propose that people think about a conditional statement in terms of p possibilities, rather than not-p or q possibilities. They do this by hypothetically supposing p and then running a mental simulation hi which they evaluate q.

Under the suppositional account, the paradoxical conclusion that the statement "if Al Gore is president then George Bush is president" is true is avoided. We can imagine a world in which Al Gore is president, but in that world George Bush could not also be president, so we would reject the conditional given above. Our work on this theory has led to a theoretical critique of the mental model theory of Johnson-Laird and Byrae (2002), together with the presentation of several empirical findings that we believe favor the suppositional account (Evans & Over, 2004; Evans, Handley, & Over, 2003; Evans, Over, & Handley, 2005; Handley, Evans, & Thompson, 2006; Over, Hadjichristidis, Evans, Handley, & Sloman, 2007). We believe that the model theory is at least partially committed to a material conditional, although this has been denied by some of its supporters (Schroyens & Schaeken, 2004). For the present purposes, however, we can focus on the noncontroversial claim that the model theory (1) does not support the suppositional view of conditionals and (2) differs from our account hi the prediction of certain key phenomena. …

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