Reduced data gathering in individuals with delusions has
been repeatedly demonstrated using probabilistic reasoning
tasks based on a Bayesian model of probabilistic inference.
22–28 On a typical probabilistic reasoning task
(the beads task), participants are asked to request as
many pieces of evidence (colored beads) as they would
like before making a decision (from which of 2 hidden
jars the beads are drawn). The participants are shown
that the jars have beads of 2 different colors and are informed
of the proportions of each colored bead in the
jars. In the original version of the task, one jar has 85
black beads and 15 yellow beads and the other jar has
the opposite ratio of black and yellow beads. In a
more difficult version, the beads are in the ratio 60:40.
The key variable employed is the number of items requested
before making a decision. Individuals with delusions
request fewer beads before making their decision than psychiatric or nonclinical controls. The extreme
form of the bias—‘‘jumping to conclusions’’ (JTC)—has
been operationalized as when a decision is made after 2
or fewer beads. Approximately 40% of people with delusions
jump to conclusions even when the beads are in the
difficult ratio of 60:40.