Combining verbal forecasts: The role of directionality and the reinforcement effect

Recent research has shown that when people combine verbal probabilistic forecasts from two sources, they are not simply averaged but can reinforce each other; so when two advisors both said an event was “rather likely,” some listeners concluded that the event was “quite likely”. Conversely, when both said the event was “rather unlikely,” people concluded that it was “quite unlikely.” The present studies demonstrate that the direction of this effect is not evoked by high versus low probabilities, but by the directionality of verbal probability expressions. Some phrases are affirmative, directed towards occurrences (“there is a chance”), whereas others are negations, pointing to the possibility that the event might not occur (“it is not certain”). Two positive phrases are perceived to reinforce each other, even when they convey low probabilities, resulting in a higher combined probability estimate, whereas two negative phrases do the opposite, regardless of the probabilities they convey. We show that this effect occurs both for equal and unequal verbal phrases, regardless of the probability equivalents of the expressions. We also found a positive, but weaker, reinforcement effect of numerical probabilities.