In a waiting-list design, people are randomized either to start treatment now or to wait, then get the same treatment later. During the wait, the delayed group serves as the comparison. It is ethically attractive, because everyone eventually receives the treatment.
The 2021 Johns Hopkins depression trial used it: controls were treated after an 8-week wait, and those treated first had much lower clinician-rated scores at the matching time point (Davis et al., 2021). A 2026 Johns Hopkins OCD study, posted as a preprint, compared immediate treatment with a waitlist in an open-label design (Tiwari et al., 2026).
The weakness is plain: a waiting list is not a placebo. People on it know they haven’t been treated, may feel disappointed, and have no reason to expect improvement, so expectancy is not controlled (Davis et al., 2021). Waiting-list trials therefore tend to show larger differences than trials using an active placebo.
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