Expectancy is the belief that a treatment will help, and the real changes that belief can cause. It is one reason placebo groups often improve. In a self-blinding microdosing study that 191 people completed, both the microdose and placebo groups improved, with no significant differences between them (Szigeti et al., 2021).
Psychedelic trials face a special problem. Many participants arrive hopeful, and the drug’s effects are hard to hide, so people can often guess their group. A 2021 review argued that this mix of expectancy and broken blinding probably makes effect sizes look larger than they are (Muthukumaraswamy et al., 2021). A 2022 lab study of 34 people found microdosing effects only in those who correctly identified their condition (Cavanna et al., 2022).
Researchers have asked for expectancy to be measured, not just assumed. A 2024 BMJ review of psilocybin for depression called for more research on expectancy effects (Metaxa and Clarke, 2024). Whether expectancy explains part, most or little of the benefit seen in trials is still unknown.
Sources
- Szigeti et al., eLife, 2021
- Muthukumaraswamy et al., 2021
- Cavanna et al., Translational Psychiatry, 2022
- Metaxa and Clarke, BMJ, 2024
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