There is a vast literature on the mysterious placebo effect. The latest paper to join this throng investigates how personality traits might play a role. According to the authors, optimism seems to enhance the effect.

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A recent study dives into the murky phenomenon of the placebo effect.

The placebo effect describes a beneficial or positive response to an inert intervention. Its counterpart — the nocebo effect — refers to a negative reaction to an inactive treatment.

Scientists often use placebo-controlled trials to evaluate medical interventions, so understanding how and why these effects arise is important. After all, if people experience relief from their symptoms or develop side effects when they have only taken a sugar pill, this makes interpreting the results of studies more difficult.

For instance, the authors of a meta-analysis of trials investigating antidepressant medications concluded that “the placebo effect accounted for 68% of the effect in the drug groups.”

The placebo response’s effect on the reliability of clinical trials is well-known. However, there is an even more important reason to study them: If an inert pill has the power to make someone feel better, we should try to harness it.

As the authors explain, scientists need to find ways of minimizing the placebo effect in clinical trials and maximizing it in the clinic.

Already, scientists have uncovered a range of factors that contribute to the placebo effect. So far, they have shown that genetics, learning and conditioning, and individual expectations of a treatment outcome play a part.

An addition to this list is personality, which was the focus of a recent review featuring in the Journal of Psychosomatic Research.

The authors of the review set out to understand whether certain broad personality traits might influence how likely someone is to experience the placebo or nocebo effect.

They focused on the so-called Big Five personality traits, which are neuroticism, extraversion, openness to experience, agreeableness, and conscientiousness. For this study, the authors also included optimism in their search.

To investigate, they searched for relevant papers that researchers had published between January 1997 and March 2018. In all, 24 studies met their criteria. The authors outline their key finding:

Optimism was relatively consistently associated with increased placebo responses, whereas pessimism was quite consistently associated with nocebo responses.”

They also found that fear and anxiety were associated with the nocebo response. Individuals experiencing these feelings were more likely to perceive any negative effects of the treatment.

Overall, the authors explain that they could not “identify a solitary personality trait that is exclusively related to placebo or nocebo responding.”

As the experiments that the authors included in the review were all very different, it proved difficult for them to pull out any reliable conclusions. They explain that “the diversity of study contexts and interventions may be responsible for [the] diverse results.”

In their paper, the authors discuss why a relationship exists between optimism and the placebo effect. They reference earlier research and theorize that it might be because optimists and pessimists differ in the way that they cope with stressful situations.

When discussing how anxiety might boost the nocebo effect, the authors write that it “might be explained by neuroendocrine and biochemical mechanisms, especially in nocebo hyperalgesia.” Hyperalgesia is an increased sensitivity to pain.

Alternatively or additionally, people who are prone to anxiety might misinterpret “symptoms of hypervigilance and anxiety as signs of illness,” explain the authors.

In general, the lack of agreement among studies demonstrates that a great deal more research is necessary before it is possible to make firm conclusions.

The authors note that while nearly every study assessed the Big Five in different ways, they almost always assessed optimism using the same questionnaire. The authors wonder whether this could be “an underlying reason for the unidirectional results.”

Although the review does not paint a clear picture of those who might be more susceptible to placebo or nocebo effects, it does demonstrate a sizable gap in our understanding of this complex, surprising, and meaningful set of phenomena.

The authors hope that they will inspire more similar work, writing, “In sum, further investigation and replication studies for optimism and pessimism seem warranted since these characteristics provided the most promising results.”