PROBABILISTIC REASONING IN DELUDED, DEPRESSED AND NORMAL SUBJECTS - EFFECTS OF TASK-DIFFICULTY AND MEANINGFUL VERSUS NON-MEANINGFUL MATERIAL

Background. Research indicates that deluded patients 'jump to conclusi ons' on probabilistic reasoning tasks. Two experiments were carried ou t with patients suffering from persecutory delusions and depressed and normal controls in order to determine whether this response bias is a ffected by task difficulty and the meaningfulness of the materials. Me thods. Tasks were variants of those employed by Hug et al. (1988) and Garety et al. (1991). In Experiment 1, subjects judged which of two ba gs a sequence of coloured beads had been taken from. Difficulty was ma nipulated by varying the ratios of coloured beads in the bags. In Expe riment 2, a neutral condition required judgements about coloured beads drawn whereas, in meaningful conditions, subjects had to judge whethe r personality characteristics described one of two individuals. Result s. In Experiment 1, estimates of certainty varied with task difficulty , and there was no evidence of 'jumping to conclusions' in the deluded group. In Experiment 2, all groups reached an initial level of certai nty and reduced their estimates of certainty following disconfirmatory evidence more quickly in the meaningful conditions. Both clinical gro ups expressed higher certainty levels in early trials, and a greater m agnitude of reduction in certainty following disconfirmatory informati on. These group differences were more evident in the meaningful condit ions than in the neutral conditions. Conclusions. Probabilistic reason ing is affected by task difficulty and meaningfulness of materials in both deluded and depressed subjects. Observed reasoning abnormalities were not specific to the deluded group.