Background. Research indicates that deluded patients
‘jump to conclusions’ on probabilistic reasoning tasks.
Two experiments were carried out with patients suffering from
persecutory delusions and depressed and normal controls in order to
determine whether this response bias is affected by task difficulty
and the meaningfulness of the materials.
Methods. Tasks were variants of those employed by Huq
et al. (1988) and Garety et al. (1991). In
Experiment 1, subjects judged which of two bags a sequence of coloured
beads had been taken from. Difficulty was manipulated by varying the
ratios of coloured beads in the bags. In Experiment 2, a neutral
condition required judgements about coloured beads drawn whereas, in
meaningful conditions, subjects had to judge whether personality
characteristics described one of two individuals.
Results. 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 certainty and reduced their estimates of
certainty following disconfirmatory evidence more quickly in the
meaningful conditions. Both clinical groups expressed higher certainty
levels in early trials, and a greater magnitude of reduction in
certainty following disconfirmatory information. These group
differences were more evident in the meaningful conditions than in the
Conclusions. Probabilistic reasoning 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.