THE ENDORSEMENT OF THE PREMISES - ASSUMPTION-BASED OR BELIEF-BASED REASONING

Three experiments examined the effect on reasoning of the degree of belief in the premises. In Expt 1, 84 adult participants had to evaluate their degree of confidence in the truth of conditional statements, and to evaluate the conclusion of modus ponens (MP) arguments using these statements as the major premise. The same seven-point response formal was used for both tasks. In the inference task, 48 per cent of the participants accepted all the MP arguments even when they disagreed with their major premise. The other participants considered that some or all of the conclusions were not certainly true. Moreover, they assigned a degree of belief to each conclusion which was highly correlated with the one they attributed to the major premise. Thus two modes of responding emerged, one which assumed the truth of the premises irrespective of actual beliefs about them, and one which integrated the truth status given to the premises. In Expt 2, one replication and three controls confirmed that non-endorsement of the MP arguments was due to the lack of believability of the premises. In Expt 3, on the usual three-response format, 80 adults were invited to adopt one of these two responding modes with four MP arguments, and then to shift to the other one with a new set of four MP arguments. With the appropriate instructions, assumption-based responding was adopted by only 43 per cent of the participants, and belief-based responding by 98 per cent. Three hypotheses are considered about the second mode of responding.