Research shows that placebo analgesia can be induced through social observational learning. Our aim was to replicate and extend this result by studying the effect of the sex of both the model and the subject on the magnitude of placebo analgesia induced by social observational learning. Four experimental (1 through 4) and 2 control (5 and 6) groups were observed: groups 1, 3, and 5 were female; groups 2, 4, and 6 were male. All subjects received pain stimuli of the same intensity preceded by green and red lights. Before receiving pain stimuli, groups 1 and 4 observed a female model and groups 2 and 3 a male model; both models simulated responses to pain stimuli preceded by green lights as less painful than those preceded by red lights. Groups 1 through 4 also rated pain stimuli preceded by green lights as less painful. Further investigation revealed that in fact subjects in experimental groups rated red-associated stimuli as more painful than subjects from control groups who did not observe a model before receiving the same pain stimuli, indicating that nocebo hyperalgesia rather than placebo analgesia was induced. Empathy traits predicted the magnitude of nocebo hyperalgesia. Regardless of the sex of the subject, nocebo hyperalgesia was greater after the male model was observed. The results show that social observational learning is a mechanism that produces placebo effects. They also indicate that the sex of the model plays an important role in this process.

Due to the frequency of surgeries, acute postsurgical pain (APSP) is a common problem. However, the role of psychological factors in the experience of this kind of pain has not been well established. In this review, we focused on presurgical psychological factors associated with the experience of APSP. A systematic search of articles was performed using PsycARTICLES, PsycINFO, PubMed, MEDLINE, Scopus, Cochrane and DARE. For each study, we assessed the risk of bias, the level of evidence, the corresponding score points and the degree of association with APSP. Separate meta-analyses were performed for the selected variables. Fifty-three relevant publications were selected. Pain catastrophizing, optimism, expectation of pain, neuroticism, anxiety (state and trait), negative affect and depression were classified as likely associated with APSP. Only one of the analysed psychological variables - locus of control - was recognized as shown unlikely association with APSP. Results of meta-analyses suggested that pain catastrophizing was most strongly linked with APSP. Results of the studies reviewed suggest that patients who do not exaggerate the negative aspects of the situation and who have positive expectation of the future before undergoing surgery report lower levels of APSP than patients who catastrophize pain and expect negative events in the future. An increasing interest in preoperative positive psychological variables has been observed over the last few years in studies of surgical patients. WHAT DOES THIS REVIEW ADD?: Pain catastrophizing, optimism, expectation of pain, neuroticism, anxiety (state and trait), negative affect and depression were classified as likely associated with acute postsurgical pain, and locus of control was classified as unlikely associated with acute postsurgical pain. Anxiety was the psychological variable most frequently measured before surgery. Pain catastrophizing was most strongly linked with acute postsurgical pain.

The aim of this study was to examine the relationships among classical conditioning, expectancy, and fear in placebo analgesia and nocebo hyperalgesia. A total of 42 healthy volunteers were randomly assigned to three groups: placebo, nocebo, and control. They received 96 electrical stimuli, preceded by either orange or blue lights. A hidden conditioning procedure, in which participants were not informed about the meaning of coloured lights, was performed in the placebo and nocebo groups. Light of one colour was paired with pain stimuli of moderate intensity (control stimuli), and light of the other colour was paired with either nonpainful stimuli (in the placebo group) or painful stimuli of high intensity (in the nocebo group). In the control group, both colour lights were followed by control stimuli of moderate intensity without any conditioning procedure. Participants rated pain intensity, expectancy of pain intensity, and fear. In the testing phase, when both of the coloured lights were followed by identical moderate pain stimuli, we found a significant analgesic effect in the placebo group, and a significant hyperalgesic effect in the nocebo group. Neither expectancy nor fear ratings predicted placebo analgesia or nocebo hyperalgesia. It appears that a hidden conditioning procedure, without any explicit verbal suggestions, elicits placebo and nocebo effects, however we found no evidence that these effects are predicted by either expectancy or fear. These results suggest that classical conditioning may be a distinct mechanism for placebo and nocebo effects.