In a masterful and thorough review, Manvir Singh considers the cultural and cognitive origins of shamanism. He makes a compelling case: Shamans occupy a unique space in culture, and they adopt a unique state in the physical and metaphysical worlds to explain the inexplicable to their subjects and to render the world more predictable for them. We agree. Singh is quick, however, to dissociate the experiences and practices of shamans from the psychotic experiences (such as hallucinations, percepts without stimulus; and delusions, unreasonable beliefs) that are present in (but not exclusive to) serious mental illnesses such as schizophrenia. Here, we disagree.

In our own work, we have identified – in contemporary Western society – a group of people who experience daily auditory verbal hallucinations, who interpret those experiences metaphysically, and who share their experiences with non–voice-hearers as relevant messages from deceased relatives or other spirits (Powers et al. Reference Powers, Kelley and Corlett2017a). Thus, these self-described clairaudient psychics appear to inhabit shaman-like roles. Their voice-hearing experiences, as assessed using scales developed to detect malingering of hallucinations, are largely indistinguishable from those of people with diagnosed psychotic illnesses who hear voices. The psychics, however, reported successful volitional control of their voice-hearing and experienced fewer negative and more positive consequences of these phenomena compared with patients with psychosis who hear voices (Powers et al. Reference Powers, Kelley and Corlett2017a). We assert that these observations highlight potential commonalities between shamanism and psychosis, and we argue for a continuum of severity from odd and unusual experience, through shamanism, to psychosis (Powers et al. Reference Powers, Kelley and Corlett2017a).

We interpret our findings in terms of the increasingly influential predictive processing view of mind and brain function, which holds that the brain reduces free energy by inferring the causes of our sensory inputs (Friston Reference Friston2005). Perception, in this view, is an inference of the best explanation for those inputs based on our prior experiences and the current sensory evidence (Clark Reference Clark2013). Hallucinations, then, may represent a bias towards those top-down prior experiences (Friston Reference Friston2005; Powers et al. Reference Powers, Kelley and Corlett2016; Reference Powers, Kelley and Corlett2017a), which may be under volitional control in the psychics.

From this vantage point, we may make several connections to the review at hand. First, unusual experiences may involve occupying two separate sets of mental books – one for the real world and one for the psychotic reality. Such double bookkeeping often is discussed in the context of delusions, about which patients are sometimes curiously ambivalent (Sass Reference Sass1994). This is one of the key aspects of shamanism that Singh highlights, and again, we find evidence for it in psychosis.

Second, Singh highlights the self-deception that must be required to convince oneself of one's shamanic abilities to successfully convince others. We find a similar proposal from Ed Hagen, the evolutionary biologist, when he considers delusional disorder: Such psychotic beliefs often involve grand conspiracies and significant inside knowledge on behalf of the patient that can be shared with others, sometimes for considerable secondary gain (Hagen Reference Hagen, Elton and O'Higgins2008).

Finally, the predictive processing mechanism we favor may be the mechanism driving selection of shamans, as well as the experiences of those who seek their help. The dyadic interactions between shaman and client (Friston & Frith Reference Friston and Frith2015), the psychotomimetic experiences driven by the ingestion of sacred entheogens (Corlett et al. Reference Corlett, Frith and Fletcher2009), and perhaps the non-pharmacological aspects of the shaman's routine increase free energy (and, therefore, uncertainty) for the client in a tolerable manner such that novel solutions can be created and adopted. Under this view, the shaman resolves impasses for the client by broadening their explanatory model space, and the degree to which this is accomplished may underlie success or failure as a shaman.

This latter view finds recent support in studies of expectancy-related phenomena, tying perceptual, social, and medication-related experiences into a common neurobehavioral framework (Schwarz et al. Reference Schwarz, Pfister and Buchel2016). Viewing shamanism through the lens of hierarchy-spanning expectancy may reveal that some of the same factors driving the psychotic-like perceptual experiences of shamans also may be driving their placebo-like healing effects. Underlying this commonality of experience may be activity in common brain regions, including the nucleus accumbens, anterior cingulate cortex, and insula (Powers et al. Reference Powers, Mathys and Corlett2017b; Schwarz et al. Reference Schwarz, Pfister and Buchel2016).

Before broadly dismissing the relevance of psychotic experiences to shamanism, Singh may find much to support his thesis in considering the phenomenology and neurobiology of psychosis. He may also find a mechanism – free energy minimization through predictive processing – that may serve as a unit of selection for the occupational survival of shamans, as well as for the experiences that may have first driven them toward shamanism.