Effects of open and hidden administration of treatment-related information; a multi-experiment study.

Background: Two experiments (E1 and E2; N = 44 and N = 52, respectively) investigated the effect of positive (PI) and neutral information (NI) about a dental procedure, and if the delivery of the information by the treatment team (open administration) or unbeknownst to the treatment team (hidden administration), affected pain.

Methods: Using a mixed design, patients undergoing drilling in a molar were randomized to the NI or PI groups. Before, during, and after treatment, patients reported their pain and stress levels. In E1 the treatment team delivered the information. In E2, an assistant not engaged in the treatment delivered the information.

Results: In the PI group in E1, pain was reduced by 50 % compared to the NI group, and the effects of stress on pain were mitigated. These effects were abolished in E2. The dentist reported having displayed positive nonverbal behaviours (e.g. smiling and longer eye contact) in the PI group in E1, but not in E2.

Discussion: Positive information reduced pain only when administrated openly. There was no effect of positive information administrated hidden from the treatment team. As information was similar in both experiments, factors other than the information most likely reduced pain in the PI group in E1.

Conclusion: Delivering positive information by the treatment team may generate behavioural cues which generate placebo effects.