The role of negation in conditional inference

This paper addresses the issue of how negative components affect people's a bility to draw conditional inferences. The study was motivated by an attemp t to resolve a difficulty for the mental models theory of Johnson-Laird and Byrne, whose account of matching bias in the selection task is apparently inconsistent with Johnson-Laird's explanation of the double negation effect s in conditional inference reported by Evans, Clibbens, and Rood (1995). Tw o experiments are reported, which investigate frequencies of conditional in ferences with task presentation similar to that of the selection task in tw o respects: the presence of a picture of four cards and the use of implicit negations in the premises. The latter variable was shown to be critical an d demonstrated a new phenomenon: Conditional inferences of all kinds are su bstantially suppressed when based on implicitly negative premises. This phe nomenon was shown to operate independently of and in addition to the double negation effect. A third experiment showed that the implicit negation effe ct could be extended to the paradigm in which people are asked to produce t heir own conclusions. It is argued that these two effects can be explained within either the mental models theory or the inference rule theory, of pro positional reasoning, but that each will require some revision in order to offer a convincing account.