Hindsight bias means knowing the outcome may profoundly influence the perception of past events and prevent a realistic appraisal of what actually occurred.

To evaluate the role of hindsight bias on psychiatrists' diagnosis.

A psychiatric vignette was made, and validated by expert psychiatrists in national board of psychiatry. They defined the most probable (Bipolar Mood Disorder) and the least probable diagnosis (Schizoaffective disorder). We designed three types of questionnaire, Type A: the basic vignette with an additional explain that in pervious admission the diagnosis was Schizoaffective disorder, Type B: the basic vignette with a suggestion of Bipolar mood disorder, and Type C: the basic vignette without any suggestion. These three forms were sent out to psychiatrists by randomization and concealment allocation in three groups to evaluate their diagnosis.

Of the 240 psychiatrists included,173 persons responded (response rate 72%).There were 52 persons in group A with Schizoaffective disorder suggestion, 63 in group B with Bipolar mood disorder suggestion and 58 in the control group. There was a significant increase of schizoaffective disorder diagnosis (Pv <0.001) but there was not a significant difference for Bipolar Mood Disorder or any other diagnoses. There was no difference in mean age, sex and duration of clinical practice between groups.

According to this study decision-making in psychiatry may be vulnerable to biases like hindsight bias. Insight via education appears the major means to avoid distorting decision-making processes.