This study examined the effects of a minor tranquilizer, diazepam, on a cognitive task that involved the updating of beliefs. On this task, subjects are first asked to express their strength of belief in a hypothesis and then to update this on the basis of new evidence. Past research has shown that revision of beliefs can be affected by many variables including the strength with which the initial belief is held, whether new information is perceived as positive or negative vis-à-vis the hypothesis, and the order in which evidence is processed. The purpose of this study was to assess whether a tranquilizer, diazepam, would affect the updating of beliefs, and specifically whether it would dampen the extent to which people revise their opinions. 12 healthy subjects participated in four experimental sessions, in which they received diazepam (0.14 mg/kg) or placebo and completed an updating task in which they received information in either of two orders, positive-negative or negative-positive. In each session, subjects saw a different experimental stimulus determined by a Latin-square design, and also completed pre- and postdrug mood questionnaires. Analysis showed order of presentation induced a recency effect similar to that obtained in previous studies, and the diazepam produced significant tranquilizer-like effects on self-report questionnaires. However, the diazepam had no effect on the updating task. These findings indicate that previously reported order effects in the belief updating task are robust and unaffected by a drug that has known sedative and memory-impairing properties.
