Costs of thinking about verbal probabilities: is processing directionality easier than processing probabilistic meaning?

Uncertain outcomes can be described by raw probabilities (e.g., "There is 40% chance"), and also by verbal probabilities (e.g., "There is a chance", "It is not absolutely certain"). Beyond their probabilistic meaning verbal probabilities also have a directionality (Teigen & Brun, 1995), i.e. can be positive or negative. In this study we made the first investigation into the potential differences in processing directionality and probabilistic meaning. Twenty participants chose between two outcomes described by verbal probabilities. In one third of the trials the probabilistic meaning was controlled and the directionality was varied. In another third the directionality was controlled and the probabilistic meaning was varied. In the last third both dimensions were different, reinforcing each other (congruent trials; e.g., a positive one carrying a high probabilistic meaning) or contradicting each other (incongruent trials; e.g. a negative one carrying a high probabilistic meaning). Accuracy, directionality of the actual answer and response time were recorded. When both dimensions differed, accuracy and response time indicated that participants found it easier to choose between congruent than between incongruent verbal probabilities. When one dimension or the other was held constant and the other varied, we found evidence that participants were influenced by both probabilistic meaning and directionality. Participants found easier to make decisions between positive verbal probabilities and between ones of high probabilistic meaning than between negative verbal probabilities and between ones of low probabilistic meaning. Finally when the probabilistic meaning was held constant, participants tended to choose the outcome with the positive verbal probability more often than chance. We discuss the possibility of a positivity bias influencing the process of verbal probabilities