Outcome Knowledge, Regret, and Omission Bias

Subjects made or evaluated decisions in hypothetical scenarios. We manipulated knowledge about the outcome and act vs omission in four cases. In case 1 (production processes), acts (changing the process) were considered better than omissions when the decision maker did not know the outcome or knew that it was better than the status quo. Acts were considered worse than omissions once the decision maker learned that the foregone option would have led to an even better outcome. In case 2 (medical treatment), act vs omission again interacted with gain vs loss (relative to the status quo) unless the outcome of the foregone option was known, in which case act vs omission interacted with better vs worse (of the two options). In case 3 (fetal testing), subjects tolerated less risk of miscarriage when a potential for regret was present (because the test with the risk of miscarriage, although better, might miss disorders that another test would detect). This effect was greater for actions than omissions. In case 4 (vaccination), subjects showed less tolerance of vaccine risk when the decision maker would know about the outcome of vaccination or nonvaccination. Thus, the bias toward omission (not vaccinating) is greater when potential regret is present, and potential regret is greater when knowledge of outcomes is expected.