Figure 162

The percentage of subjects endorsing the various conditional inferences from Byrne (1989a) when they are given simple (standard conditional) arguments, alternative arguments, and additional arguments.

suppression of the inferences (Elio, 1997; Thompson, 1994, 1995). In a similar vein, Chan and Chua (1994) have shown that, based on background knowledge, additional requirements can have a differential impact on suppression depending on their relative saliency or strength. For example, given:

If Steven is invited then he will attend the dance party. (Standard Premise) If Steven knows the host well he will attend the dance party. (Weak Additional) If Steven is invited he will attend the dance party. (Strong Additional)

then it seems as if all additional premises are not equal; suppression of the valid inferences is less with weak additional relative to strong additional premises (see Byrne et al., 1999, for further work on this finding).

Stevenson and Over (1995) have also shown that introducing a degree of uncertainty regarding the additional premises can undo the suppression effect. So, when subjects were presented with:

If John goes fishing, he will have a fish supper (Standard Premise) If John catches a fish, he will have a fish supper (Additional Premise)

John is * lucky when he goes fishing (Qualification) John goes fishing Therefore, ?

* One of the following words was used: always, usually, rarely, never.

Stevenson and Over found that, when always was used in the qualifying sentence, the suppression of modus ponens disappeared, but that it gradually increased as it moved from always to never.

Evidence from Wason's selection task

Another strand of evidence on reasoning with conditionals comes from research on Wason's selection task, which is not purely a deductive reasoning task, but concerns hypothesis testing using a conditional rule (see Evans, 1982, 1989; Evans et al., 1993b; Wason, 1966 for reviews). The original findings on the task were taken as evidence of people's tendency to confirm hypotheses in reasoning situations (see Chapter 15 on hypothesis testing). Although the task has now assumed a special place in reasoning research, there is still some controversy about its utility as an instrument to examine human reasoning (Sperber, Cara & Girotto, 1995).

The selection task, first proposed by Wason (1966), looks like an innocuous puzzle but it hides a multitude of difficulties. In the original version, subjects are shown four cards face-down with letters or numbers on each of them (see Figure 16.3). They are told that each of these cards has a letter on one side and a number on the other side and they have to name the cards that need to be turned over to test the following rule:

If there is a vowel on one side of a card, then there is an even number on the other side of the card.

Because you can turn over any of the four cards, there are four possible choices. However, subjects are asked to turn over only those cards that need to be turned over. The correct answer is to turn over just two cards, the E-card and the 7-card; but few subjects spontaneously pick these cards on this abstract version of the task. To understand why this is the correct answer, first consider why the 4-card and K-card choices are wrong (and keep the truth table for if..then in Table 16.1 handy).

Examples of the abstract and concrete (postal) versions of Wason's selection task, with an indication of how the different cards/envelopes are labelled for the classification of subjects' choices in experiments.

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