Expectation and the Placebo Effect in Inflammatory Skin Reactions

Abstract

reaction (weal size and symptoms). During the experiment a number of questionnaires measuring stress, mood, perceptions, and expectation were administered. Self-reported symptoms were assessed along with the physiological measures of weal size and heart rate. The two main outcome variables were weal size and symptom reporting as generated by the histamine-induced skin reaction. Results showed that the manipulation of expectation was successful with a lower expected weal size in the expectancy group as compared to the control group, but there was no difference in expected symptoms. There was no difference between groups in weal size, or symptoms reported, but there was a significant decrease in heart rate in the expectancy group after manipulation of expectation. Given the contextual cues and the nature of the information provided to participants regarding heart rate, this finding is thought to be attributable to an expectation-induced placebo effect. Relationships were also found between symptom reporting and negative arousal (current mood state), perceived stress, and heart rate. The findings from this study indicate that while an expectation-induced placebo effect on skin reactivity was not demonstrated, it does not necessarily indicate that this is not possible. The methodological limitations associated with administration of the skin reactivity test, as well as those associated with the manipulation of expectation, may have impeded demonstration of this effect. The modulation of skin reactivity by expectation may be possible and future research should be carried out to investigate this with the noted limitations addressed. The modulation of heart rate demonstrated in this study should be replicated and extended with the aim of investigating mechanisms that might be involved.