Summary Forty-four adult Ss evaluated conclusions for two types of disjunctive arguments. The disjunctive premise of each argument was an inclusive (e.g., either P or Q or both) or exclusive (e.g., either P or Q but not both) proposition. The presence or absence of the negative “not” was varied systematically in the components of each of these disjunctive premises. Contrary to previous findings in the area of concept attainment, the results indicated that exclusive disjunction arguments were significantly (p < .05) easier than inclusive disjunction arguments. In addition, the location of the negative(s) in the major premise had a significant (p < .001) effect on the number of reasoning errors, and a significant (p < .001) differential effect according to the type of disjunction. In general, the results replicated, clarified, and extended the findings of previous research on concept attainment and deductive reasoning.
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