The possession-based placebo effect: how implicit association between the self and a placebo analgesic affects pain outcomes

Past research on the placebo effect has mainly focused on how the use of a placebo induces positive consequences. For example, in the field of placebo analgesia, previous research has shown that using a placebo analgesic can reduce pain. Recently, a new line of research has emerged documenting that merely possessing (without use) a placebo analgesic is enough to reduce pain (Yeung, Geers, & Kim, 2019; Yeung, Geers, & Colloca, 2020; Yeung & Geers, 2021; Yeung, 2022). In a series of studies that used the cold pressor task (CPT), a method that induces acute pain in the non-dominant hand of participants, Yeung and colleagues (Yeung et al., 2019, 2020, 2021, 2022) demonstrated that participants who merely possessed (but did not use) a placebo analgesic reported greater pain resilience. They had a higher pain threshold, tolerated pain longer, and reported less pain intensity, compared to participants who did not possess a placebo analgesic. In one of their studies (Yeung et al., 2020), they even showed that merely possessing a placebo analgesic boosted placebo analgesia to the same extent as using a placebo analgesic.

Yeung et al. (2019, 2020, 2021, 2022) speculated that the observed possession-based placebo analgesic effect was a result of participants' object-to-self expectancy. As ownership denotes a relationship between the self and the owned object, the association between the self and the object induces an expectation that the owned object can benefit the self, leading to placebo analgesia. Although Yeung et al. (2020) have shown that object-to-self expectancy is an underlying mechanism of the possession-based placebo effect, it is essential to provide more direct empirical evidence to demonstrate that people indeed form a cognitive association between the self and the owned analgesic object via ownership and how such an association affects placebo analgesia.

In this proposed study, a reaction time task will be used to tap into the implicit association between the self and the owned analgesic object and observe its effect on placebo analgesia. We will examine whether participants have an implicit association with the analgesic cream in different conditions (possession vs. no possession conditions) and investigate their effect on placebo analgesia. We hypothesize that, compared to participants who do not experience ownership of a placebo analgesic cream, those who merely possess a placebo analgesic cream will perceive a stronger implicit association between the self and the analgesic cream.