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Differences in difficulty of learning conceptual rules have been explained within Bourne's truth-table strategy theory by the naturalness-unnaturalness of various assignments of instance classes to response categories. However, since this factor cannot explain why conditional rules are learned more easily than biconditional rules, two other factors were tested. Results showed that when 6"s were encouraged by neutral labeling of response categories (a) to learn the rule for both positive and negative categories and thereby (&) to assign to the negative category only one instance class for the conditional rule compared to two for the biconditional, they learned the conditional rule more proficiently. However, when 6"s were constrained by positivenegative labeling (a) to learn the rule for the positive category and thereby (b) to assign to the positive category only two instance classes for the biconditional rule compared to three for the conditional, they learned the biconditional more proficiently.In conceptual rule-learning tasks, biconditional rules have been more difficult to learn than conditional rules (Bourne, 1967;Haygood & Bourne, 1965). Bourne's (1967) theory that 6"s use a truth-table strategy to learn conceptual rules posits a two-stage process in which: (a) Ss learn to reduce the population of instances to four instance classes, i.e., the TT class, in which the relevant values of both attributes linked by the rule to be learned are present; the TF class, in which only the relevant value of the first attribute is present; the FT class, in which only the relevant value of the second attribute is present; and the FF class, in which the relevant values of neither attribute are present; (&) 5s learn the proper assignment of these instance classes to the positive-instance and negative-instance response categories. For the conditional rule, the TT, FT, and FF classes are positive instances, whereas for the biconditional rule the TT and FF classes are positive instances.