Systematic manipulation of experimenter’s nonverbal behaviors for the investigation of placebo effects

Abstract Objective Nonverbal behaviors (NBs) and characteristics of healthcare providers affect pain reports and placebo effects in patients/participants. However, as it is difficult to systematically control NBs in interactions, little experimental research has examined the impact of NBs of experimenters on pain and placebo effects. This study protocol reports a systematic manipulation of experimenters’ NBs to investigate its effects on placebo effects. Methods We propose an experiment in which videotaped experimenters (VEs) guide participants through a pain stimulation and a placebo treatment. The VEs express one positively enhanced NB and keep their other NBs neutral. Participants will be randomized to four groups in which they will be exposed to either the positive facial expressions (+ FE), tone of voice (+ TV), body movement (+ BM), or neither positive nor negative behaviors (i.e., neutral control condition; NC) of the VEs. As a pilot investigation and manipulation check, twenty-one coders (15 Norwegian and six US psychology students) viewed the videos and rated the degree of NBs of smiling, eye contact, gestures, positivity in tone of voice, and impressions of dominance, positivity, and expressivity from each NB. The manipulated NBs were shown to have high construct validity and reliability. Moreover, both + BM and + FE were rated as more dominant than + TV and the NC. +FE and + BM were rated as the most positive and expressive, respectively. Expected results Transmitting the desired NBs to patients by VEs can be challenging. However, it’s expected that + FE will have the largest placebo effects. General Discussion NB training can help to further control the NBs of experimenters or healthcare providers to examine their impact on important patient and participant outcomes. Researchers who wish to experimentally manipulate medically important impressions, should consider varying degrees of facial expressions, tone of voice, and body movement.