3.1. Individuals adopt superstitions to influence significant outcomes that are random and uncontrollable

Humans choose solutions to deal with problems, such as denying a pregnant woman smelly meat to protect her child or rubbing a rock to win a baseball game. The cognitive mechanisms for choosing among solutions must contend with the uncertainty of whether a strategy actually works. For example, denying a pregnant woman smelly meat may seem to guarantee a healthy child sometimes but not always; similarly, rubbing a rock may appear to ensure victory in a baseball game sometimes but not always. Individuals therefore rely on psychological heuristics to select among strategies and beliefs. Systematic errors by these heuristics lead individuals to adopt interventions that have no causal relationship to their intended outcome (Foster & Kokko Reference Foster and Kokko2009; Johnson et al. Reference Johnson, Blumstein, Fowler and Haselton2013; McKay & Dennett Reference McKay and Dennett2009; Vyse Reference Vyse2014); following previous authors, I refer to these causally innocuous actions as superstitions Footnote ³ (Foster & Kokko Reference Foster and Kokko2009; Ono Reference Ono1987; Skinner Reference Skinner1948; Vyse Reference Vyse2014). That similar behaviors can be induced in other species (e.g., Skinner Reference Skinner1948) suggests that these cognitive mechanisms have deep evolutionary histories.

Individuals are most likely to adopt superstitions when (1) the potential benefit of that intervention working is high, and (2) the intervention is followed by a successful outcome some proportion of the time (Beck & Forstmeier Reference Beck and Forstmeier2007; Foster & Kokko Reference Foster and Kokko2009; Johnson Reference Johnson2015; Ono Reference Ono1987; Vyse Reference Vyse2014). Thus, superstitions can be thought of as bet-hedging strategies: As long as the outcome sometimes occurs after the intervention (e.g., the baseball team wins sometimes after rubbing the rock), and the cost of the intervention is sufficiently low compared with the potential benefit, an individual will benefit on average from adopting those strategies (see error management: Johnson et al. Reference Johnson, Blumstein, Fowler and Haselton2013; McKay & Efferson Reference McKay and Efferson2010).Footnote ⁴ Consequently, contexts in which people are not fully able to control fitness-relevant outcomes and in which success occurs randomly are most amenable to superstitious thinking (Greenaway et al. Reference Greenaway, Louis and Hornsey2013; Keinan Reference Keinan1994; Legare & Souza Reference Legare and Souza2014; Whitson & Galinsky Reference Whitson and Galinsky2008). This accounts for, among other things, (1) the use of magic for catching wild game and inviting rain; (2) the profusion of magic in the wake of deadly epidemics (e.g., Ashforth Reference Ashforth2011); and (3) the prevalence of superstitions in Western societies, such as those surrounding gambling and sports (Burger & Lynn Reference Burger and Lynn2005; Henslin Reference Henslin1967). Moreover, within these domains, those activities with the most uncertainty and the largest benefit invite the most superstition. For example, Trobriand Islanders use magic for open-sea fishing, where “the yield varies greatly,” but not for inner lagoon fishing, which promises “abundant results without danger and uncertainty” (Malinowski Reference Malinowski1948, p. 30). Because it sustains beliefs in ineffective interventions, the psychology of superstition provides the basis for shamanic traditions.

Hereafter, I refer to those outcomes most susceptible to superstition (uncontrollable, fitness relevant, and random) as uncertain outcomes (see Fig. 1). Examples of uncertain outcomes include illness healing, crops failing, and famine ending. These differ from outcomes that are controllable (e.g., the production of fire), outcomes that are uncontrollable but unimportant (e.g., a butterfly landing on one's arm), and outcomes that are uncontrollable and important, yet never occur (e.g., winter never coming).

Figure 1.

Kinds of outcomes. Individuals will develop superstitions to influence or learn about the timing of uncertain outcomes (uncontrollable, important, and random). The importance of the outcome to the individual's perceived welfare and the frequency with which it occurs after an event are both continuous variables, but they are presented in this diagram as binary for simplification.

3.2. People's adoption strategies drive the selective retention of effective-seeming interventions

Cultural evolution occurs as some cultural variants are adopted and transmitted more than others (Blackmore Reference Blackmore1999; Boyd & Richerson Reference Boyd and Richerson1985; Sperber Reference Sperber1996a).Footnote ⁵ The direction of cultural evolution, and whether its products are adaptive, thus are often consequences of individuals' adoption strategies (Henrich Reference Henrich2015; Richerson & Boyd Reference Richerson and Boyd2008). When outcomes are observable, immediate, and predictable, individuals easily can choose innovations based on their effectiveness (Rogers Reference Rogers2003). For example, an individual can gauge whether some tweak to a spear allows it to fly farther, deciding to retain the successful tweaks. As people repeat this adoption strategy over time, technology will become more effective at ensuring outcomes. In short, cultural evolution can produce efficient technologies, such as spears, when the outcomes of innovations are immediate, observable, and predictable.

In the case of superstitions, cultural evolution cannot produce efficient technologies because the practices are, by definition, ineffective. But some innovations will still seem more effective because of innate or cultural intuitions about causality. Consequently, individuals will retain and transmit those practices that seem most plausible, driving the cultural evolution of practices adapted to our intuitions about efficacy. For example, Legare and Souza (Reference Legare and Souza2012) showed that individuals regard magical procedures with more steps and specific times as more efficacious, likely biasing which magical practices spread. Similarly, innate intuitions about how substances spread through contact perpetuate beliefs in contagious magic (Nemeroff & Rozin Reference Nemeroff, Rozin, Rosengren, Johnson and Harris2000; Rozin et al. Reference Rozin, Millman and Nemeroff1986).

Note that the cultural evolution of plausible-seeming practices should be accelerated once shamans exist and compete for clients. Some shamans find techniques that better convince onlookers of their superior efficacy; these techniques are expected to be maintained over time as other shamans copy them (Boyd & Richerson Reference Boyd and Richerson1985; Henrich & Gil-White Reference Henrich and Gil-White2001; Schlag Reference Schlag1998, Reference Schlag1999) or as successful practitioners train more initiates. Many ethnographers have observed intense competition among shamans that should drive this cultural selection. Among the Azande, practitioners amassed prestige as effective doctors and were “envious of the encroachment” of rivals who threatened their wealth and reputation (Evans-Pritchard Reference Evans-Pritchard1937, p. 245). Ona shamans of Tierra del Fuego faced off in public: Practitioners attained states of magical trance, and observers judged them according to the intensity and duration of trance, among other characteristics (Chapman Reference Chapman1982). Similar competitive displays occurred among the Tlingit and the Netsilik (Balikci Reference Balikci1963; De Laguna Reference De Laguna1972). In postsocialist Mongolia, Buyandelgeriyn (Reference Buyandelgeriyn2007, p. 132) observed that “clients [chose] a shaman on the basis of a careful search with diviners, lamas, and clients about an individual's power, disposition, and the level of satisfaction with that individual's service.” In all of these instances, clients choose practitioners according to their credibility and perceived ability, selecting for practices that promote these metrics.

The autobiography of the Kwakwaka'wakw shaman Quesalid (see Boas Reference Boas1930; Lévi-Strauss Reference Lévi-Strauss1963a) provided a telling example of how competition among shamans could drive the cultural selection of plausible-seeming interventions. Quesalid described learning how to duplicitously produce a bloody tuft of down as a purported physical embodiment of illness. He then discovered, however, that the nearby Koskimo shamans lacked this method, and in performing it before them and their audiences, attracted much more credibility while disgracing his rivals. Although Quesalid did not describe the subsequent diffusion of the technique, he recounted how other shamans implored him to share his secrets; one presumably even used his virgin daughter to tempt Quesalid.

In summary, individuals' preferences for effective superstitions fuel the cultural evolution of plausible-seeming magical practices. Once shamans exist, the competition among them should accelerate this process.

3.3. Violating notions of humanness to apparently influence uncontrollable outcomes

In this section, I argue that the cultural selection for plausible superstitions leads to people claiming to engage with the invisible entities (e.g., deities, spirits) who control unpredictable events. To convince potential clients that they have these special powers, these practitioners must ostensibly transform into entities distinct from normal humans.

3.4. Synthesis: The cultural evolutionary theory of shamanism

Some highly important outcomes are uncontrollable and random (Fig. 1). Cognitive mechanisms for decision-making and developing explanations predispose people to (1) adopt causally innocuous interventions (superstitions) to influence these events, and (2) believe in unseen causal forces that control these uncertain outcomes.

Individuals prefer to use those interventions that appear the most effective, selecting for practices that are adapted to intuitions of causality and efficacy (Fig. 2). The most successful-appearing practices involve practitioners claiming to recognize and interact with the invisible causal agents that determine whether these outcomes occur. These practitioners must convince group mates of their non-normal powers while warding off constant invasion from others aiming to invade their jurisdiction. This selects for transformation- practices and mythologies (e.g., asceticism, claims of death and rebirth), which convince observers that the practitioner is different and capable of performing feats otherwise impossible for normal human bodies. Practitioners also need to indicate to observers their supernatural powers while administering their interventions, driving the cultural evolution of displays of behavioral strangeness, or trance.

Figure 2.

Individuals' desire to find treatments selects for interventions that are the most plausible. When outcomes of uncertainty are controlled by invisible forces, cultural selection favors individuals who claim special abilities of interacting with those forces. To increase their credibility, those individuals must violate folk intuitions of humanness, resulting in trance, transformation practices and mythologies, and the predisposition for peculiar individuals to be shamans.

This theory is agnostic as to whether shamans provide benefits to clients or the group. It posits only that the practices surrounding shamanism develop to promote observer perception of efficacy. Although these might provide benefits through the placebo effectFootnote ⁷ (Kaptchuk Reference Kaptchuk2002; Reference Kaptchuk2011; Kaptchuk & Miller Reference Kaptchuk and Miller2015; Kleinman & Sung Reference Kleinman and Sung1979), those added benefits are not necessary for the emergence or maintenance of the practice.

4.1. The shaman's jurisdiction focuses on controlling uncertainty

According to the cultural evolutionary theory, shamans should have jurisdiction over influencing and delivering information about outcomes that are unpredictable, fitness relevant, and uncontrollable (uncertain outcomes).

Comparative ethnographic evidence supports this claim. Winkelman and White (Reference Winkelman and White1987) coded 115 magico-religious practitioners across 45 societies, encompassing priests, trance practitioners, and cultural notions of witches and sorcerers. Seventy-five practitioners representing 43 societies used trance to provide services, qualifying them as shamans. Using their dataset, I calculated how frequently shamans had jurisdictions over uncertain outcomes as compared to how often they offered services in other domains.Footnote ⁸ Figure 3 presents the findings.

Figure 3.

Percentage of societies in which shamans provide a given service or role, according to coding by Winkelman and White (Reference Winkelman and White1987), organized by all societies (A) and politically simple societies (B). Filled black bars signify a jurisdiction that involves influencing or providing information about uncertain outcomes; striped bars indicate assisting or ceremonially overseeing life cycle activities (funerals, birth, initiations); white bars denote decision-making in other domains of social life. Judiciary power is noted with an asterisk because it includes overcoming uncertainty (divination of guilt). See main text for details and note 8 for a discussion of methods.

Shamans overwhelmingly offered control over important, uncontrollable outcomes. The most pervasive and important uncertain outcome among humans is recovering from serious illness: Trance practitioners treated it in every society examined. Successes and failures in food acquisition, including helping to find game and reap successful crop yields, represent another class of important, uncontrollable events: Shamans assisted in promoting these activities in 91% of societies. Weather is similarly random and uncontrollable and bears dramatically on people's lives: Trance practitioners were coded as controlling weather patterns in 65% of societies. Lastly, shamans provided otherwise inaccessible information (divination) in all societies surveyed. This included information about the cause of illnesses, guilt, future events, and the locations of lost or stolen objects.

Figure 3 also illustrates that shamans irregularly contributed to other aspects of social life. Although they assist in various life cycle activities, including birth, funerals, and initiations, and they enjoy leadership roles, such as in military affairs or coordinating economic activities, shamans serve these roles much less frequently than they help overcome uncertainty. This difference is more pronounced when removing chiefdoms and states and examining small-scale societies specifically (see Fig. 3). Notably, those two domains that appear most commonly outside of uncertainty seem the consequence of shamans' magical interventions. First, they were coded as mediating or settled disputes in 60% of the societies – but this is likely inflated because the coders included divining guilt as reflective of judiciary influence. Second, shamans' pronounced role in funerary activities, corresponding with some researchers' observations of shamans as psychopomps (Bäckman & Hultkrantz Reference Bäckman and Hultkrantz1978; Eliade Reference Eliade1964; Hultkrantz Reference Hultkrantz and Å1993), seems a result of their unique relationship with the spirit world. Still, that shamans have roles outside of uncertainty suggests that their liminal status affords them a special prestige that translates into leadership and ceremonial positions.

Ethnographies reveal that, when shamans provide services outside of those in Figure 3, these also involve controlling uncertain outcomes. For example, in addition to calling wild game, healing illness, and controlling weather, Netsilik shamans stopped the cracking of ice, which endangered camps and was uncontrollable but resolved randomly (Balikci Reference Balikci1963). Majangir shamans (southwestern Ethiopia) not only divine, treat illness, and influence natural events, but also more generally “confer protection and good luck” (Stauder Reference Stauder1972, p. 156). Haida shamans cured illness, foretold events, and divined the locations of stranded whales, but they also accompanied war parties to warn of unpredictable ambushes and point out auspicious places to attack (Corlett Reference Corlett1935).

Note that these jurisdictions and abilities apply to the practitioners of Abrahamic religions as well. For example, Thomas (Reference Thomas1971a, p. 26) wrote about medieval Christian saints:

Pentecostal and charismatic priests habitually engage in healing and divination (Csordas Reference Csordas2007). Some scholars even accredit the recent proliferation of these movements, which now comprise more than a quarter of all Christians, to the centrality and allure of divine healing (Brown Reference Brown and Brown2011).

A final prediction regarding jurisdiction is that shamans will exist as long as uncertain outcomes exist – that is, as long as there are important, uncontrollable events that individuals want to influence or about which they want information. I review this prediction in section 6.2.

4.2. Trance is a drama of strangeness

Trance (or ecstasy) is a famously ambiguous and ill-defined concept (Rouget Reference Rouget1985). Contemporary researchers often implicitly or explicitly regard it as an altered state of consciousness that shares a common neurophysiological basis across societies and stimulates similar experiences (Achterberg Reference Achterberg1985; Harner Reference Harner1990; Hove et al. Reference Hove, Stelzer, Nierhaus, Thiel, Gundlach, Margulies, Van Dijk, Turner, Keller and Merker2016; Winkelman Reference Winkelman1986a; Reference Winkelman1986b; Reference Winkelman2004). For example, Harner (Reference Harner1990) argued for the existence of a Shamanic State of Consciousness (SSC), while Winkelman (Reference Winkelman2000) hypothesized the “integrative mode of consciousness.”Footnote ⁹ In contrast to these claims, however, growing research in altered states of consciousness has shown that states brought on by sensory deprivation, drumming, starvation, meditation, hallucinogens, relaxation, and other inducements produce profoundly different physiological and psychological effects (Vaitl et al. Reference Vaitl, Birbaumer, Gruzelier, Jamieson, Kotchoubey, Kübler, Lehmann, Miltner, Ott, Pütz, Sammer, Strauch, Strehl, Wackermann and Weiss2005).

The proposed cultural evolutionary theory of shamanism hypothesizes trance to be a drama of strangeness. By violating folk intuitions for how a human should behave, practitioners convince onlookers of their heightened supernatural powers or experiences. This hypothesis does not necessitate that trance states have the same neurological and physiological correlates across societies, although similar triggers (e.g., hallucinogens and music) likely produce analogous experiences.

The theatrical nature of trance does not mean that an individual engaged in it is faking. In fact, cultural selection should favor interventions that convince both the client and the shaman of the shaman's ability, as long as the client's perception of successful treatment is influenced by the shaman's faith (see the benefits of self-deception: Hartwig & Bond, Jr. Reference Hartwig and Bond2014; ten Brinke et al. Reference ten Brinke, Porter and Baker2012; Trivers Reference Trivers2000, Reference Trivers2011). Medical researchers have found that physician expectations increase the magnitude of the placebo effect, suggesting that the shaman's confidence does indeed influence client belief (Crow et al. Reference Crow, Gage, Hampson, Hart, Kimber and Thomas1999; Gracely et al. Reference Gracely, Dubner, Deeter and Wolskee1985; Gryll & Katahn Reference Gryll and Katahn1978). Consequently, cultural selection should develop practices that change the felt experience while making the shaman's behavior seem as non-human as possible. For example, the healing songs that so frequently characterize shamanic ceremonies may culturally evolve to engage this genuine-seeming superhuman state.Footnote ¹⁰ Future research will reveal whether songs involved in shamanic ceremonies exhibit convergent features around the world that serve this function.

This hypothesis of trance makes several unique predictions that diverge from the accounts of Harner (Reference Harner1990) and Winkelman (Reference Winkelman2000). Here, I consider three: (1) Trance states should include other spectacles to promote perceptions of supernatural powers, aside from non-human behaviors; (2) Trance practices including more humanlike behaviors are perceived of as less genuine; and (3) Reframing trance behaviors as natural parts of human behavior undermines them as indications of supernatural powers or connection.

4.2.1. Trance states include other displays that promote perceptions of supernatural powers

If trance states are culturally selected to assure observers that the practitioner has special powers, these performances should incorporate other spectacles that promote this perception, aside from displays of strangeness. Ethnographic accounts of trance frequently document exhibitions of purported superhuman abilities, supporting this prediction. Table 1 presents a selection of examples.

Table 1.

Some displays of superhuman abilities that occur during trance performances

4.3. Initiation practices and self-denial serve to transform the shaman

According to the cultural evolutionary theory of shamanism, the practices involved in becoming a shaman convince an individual's group mates that the initiate has transformed, making one's claims of supernatural powers more credible. Those individuals with biological or psychological peculiarities more easily become shamans because they pay lower costs to convince their group mates of their liminal nature.

4.3.1. Initiation practices indicate biological or psychological change or strangeness

Shamanic initiation practices frequently involve an individual acting in foreign ways or otherwise transforming, famously reviewed by Eliade (Reference Eliade1964); some examples are presented in Table 2. According to a Nomlaki person (northern California), “When a person starts to become a doctor, he gets out of his head; he won't talk but just stands around as though he doesn't know you, like a man who is unconscious. His eyes, ears, and nose may be bloody, and instead of tears, blood is in his eyes. He gets and worse, just like a dog with running fits, and finally he runs off” (Goldschmidt Reference Goldschmidt1951, p. 358).

Table 2.

Purported shamanic initiation practices involve a biological or psychological transformation of the initiate.

Yaghan initiates had to painfully rub their faces until a new layer of skin supposedly appeared (Gusinde Reference Gusinde1961). Among the Andaman Islanders, one could become an oko-jumu by dying and coming back to life (Radcliffe-Brown Reference Radcliffe-Brown1964). Such individuals were thought capable of communing with the dead, granting them insight into “curing illness and in preventing bad weather” (Radcliffe-Brown Reference Radcliffe-Brown1964, p. 178). Claims of initiates dying and then enjoying new powers are common in shamanic initiations. Winkelman and White (Reference Winkelman and White1987) reported that trance practitioners in nine societies of 43 were said to experience death and rebirth during initiation or trance.

Other frequently occurring shamanic initiations similarly involve the candidate's body or physiology changing. These include, for example, magical treatment of an initiate's eyes or ears (Fig. 4), claims of surgery or dismemberment by spirits or other medicine men, debilitating illness, and bodily insertions of crystals, magical shells, or dust (Table 2). Wilbert (Reference Wilbert1987a) observed how heavy tobacco consumption among South American shamans helped transform initiates' eyes and voices, making credible claims of otherworldly senses. Citing the shamans' “acuteness of vision, night vision, wakefulness, a caraña-masked raspy voice, a furred or rough tongue, and a pungent body odor” (Wilbert Reference Wilbert1987a, p. 195), the author further hypothesized that the physiological effects of tobacco also legitimated the shamans' supposed affinity with jaguars. Among the Igbo, men staged a performance in which they killed a dog, removed its eyes, and then allegedly replaced a healer-initiate's eyes with the animal's (McCall Reference McCall2000). The ethnographer quoted an informant who said, “Now he will be able to see spirits just as dogs are able to see spirits” (McCall Reference McCall2000, p. 27).

Figure 4.

A Mentawai shaman (sikerei) treats an initiate's eyes with turmeric and other potent, magical substances so that he can see otherwise invisible spirits.

5.1. Shamanism professionalizes because individuals must transform to claim the jurisdiction

According to the sociology of professions, individuals want to claim jurisdiction over the treatment or diagnosis of some problem, such as healing illnesses or mediating conflict, because doing so carries material and social benefits. By jurisdiction, I mean that a party has a claim over providing some service (Abbott Reference Abbott1988). The benefits of holding a jurisdiction come from having a unique skill or ability that others prize, which ensures compensation for services, social prestige, and valued social partners (Tooby & Cosmides Reference Tooby and Cosmides1996). Shamans across societies receive payment in exchange for their services (Rogers Reference Rogers1982), including pigs and coconuts (Mentawai: Loeb Reference Loeb1929), tobacco (Ojibwa: Ritzenthaler Reference Ritzenthaler and Galdston1963), yams and sponges (Pohnpei: Riesenberg Reference Riesenberg1948), freshly killed deer (Miwok: Powers Reference Powers1877), sexual partners (Guyana: Roth Reference Roth1915; Inuit: Stefánsson Reference Stefánsson1914), and slaves (Haida: Corlett Reference Corlett1935). Becoming a shaman also provided a way for low-status individuals to attain prestige, such as in some hierarchical societies of the Pacific Northwest (Gunn Reference Gunn1966), while in other instances, shamans were regarded as attractive sexual partners. For example, Katz (Reference Katz1982, p. 186) quoted the !Kung shaman Toma Zho: “The women really did like the healers. Whenever I see one who is getting num [healing energy], I say, ‘Think of the sex the guy's going to get!’”

Sociologists studying professions in industrial societies have identified several conditions under which parties maintain jurisdiction over problems (Abbott Reference Abbott1988). First, parties should have unique skills or expertise that others believe are necessary for diagnosing the problem, administering the treatment, or inferring the treatment from diagnosis (Coy Reference Coy and Coy1989; Freidson Reference Freidson1970). For example, physicians are considered the chief healers in many industrial societies because of a conception that their university training grants them the exclusive know-how in diagnosing and treating ailment (Freidson Reference Freidson1970). Likewise, shamans should maintain jurisdiction if people believe that only shamans have the skills or abilities necessary for controlling uncertain outcomes. Second, a party maintains its jurisdiction when competitors purporting to possess alternative interventions are unable to invade and subsequently dominate the jurisdiction (Abbott Reference Abbott1988). Mechanisms preventing such invasion include the perceived inferior efficacy of alternate interventions, as well as the current party enforcing barriers to competition, as was the case with medieval guilds (Ogilvie Reference Ogilvie2014).

To this point in the paper, I have argued that an individual aiming to invade the shaman's jurisdiction has to observe the existing transformative practices or devise novel ways of garnering supernatural credibility. Some hopeful shamans may convince their group mates of their competence because of lucky early successes or inherent strangeness (e.g., having an extra finger), but according to the proposed theory, most individuals need to transform in the eyes of the community. These transformative practices, including long bouts of asceticism or ceremonies of supposed eye replacement and surgery, represent entry requirements for holding the jurisdiction. They in turn create a separate class of individuals (those who have transformed) uniquely capable of influencing uncertain outcomes. Consequently, shamanism professionalizes: A class of individuals with entry requirements develops with near-exclusive jurisdiction over some services.

Shamanism professionalizes because individuals typically must invest in transformative practices to be considered capable of influencing uncertain outcomes. In contrast, jurisdictions involving technical knowledge (such as making canoes or cooking sago) do not professionalize because individuals are capable of producing technical outcomes without having to observe entry requirements. The professionalization of technical knowledge requires that only those individuals who have observed some entry requirements are capable of providing the service. This in turn requires that either (1) some individuals enjoy a monopoly over technical knowledge, or (2) technical knowledge is sufficiently complicated such that it requires prolonged, intensive training. Neither of these typically holds true in small-scale societies. High levels of interaction and limited personal privacy make technical knowledge difficult to conceal, even when actors explicitly try to control it (e.g., Lindstrom Reference Lindstrom1984). Moreover, the simplicity of physical technology means that “imitators can often replicate new techniques after only minimal observations” (Suchman Reference Suchman1989, p. 1272). The difficulty of controlling technical knowledge is illustrated by the ease and rapidity with which technologies such as the domesticated sweet potato (Wiessner Reference Wiessner2002), the horse (Haines Reference Haines1938), and the bow and arrow (Bettinger & Eerkens Reference Bettinger and Eerkens1999) diffused through societies. The horse is notable: Not only did adoption require special knowledge for use and care, but also societies actively opposed its diffusion, as demonstrated by the Spaniards' laws against Native Americans riding horses (Haines Reference Haines1938).

In summary, entry requirements are maintained for jurisdictions over uncertainty: Individuals must observe transformative practices to convince onlookers of their competency. In contrast, in small-scale societies, entry requirements are difficult to maintain for technical jurisdictions: Technical knowledge easily spreads, and technology is simple enough so individuals outside of the jurisdiction can produce the outcome.

5.2. Self-serving barriers to entry do not explain shamanism's professionalization

Prohibitions on novices sometimes advantage existing practitioners, suggesting that shamans use entry requirements for self-serving ends. For example, Canela initiates were forbidden from having sex with young girls, although they were permitted to enjoy sexual relationships with postmenopausal women (Crocker Reference Crocker1990), removing a source of sexual competition for adult shamans. The !Kung healer Kau Dwa explained that novices must first eat food specifically prepared by and shared with an existing practitioner; otherwise, they risk death (Katz Reference Katz1982).

Despite evidence of selfish rule-making, it cannot by itself explain the existence of entry requirements for becoming a shaman. Self-interested rule enforcement necessitates that these individuals already have supernatural credibility (Singh et al. Reference Singh, Wrangham and Glowacki2017) – but it fails to explain the source of this credibility. The cultural evolutionary theory, which posits that gaining supernatural credibility involves transformative displays, addresses this gap while also explaining entry requirements. Thus, self-interested enforcement can exacerbate entry requirements, but the need for practitioners to transform to gain credibility seems to explain their existence in the first place.

6.1. Competition and the benefits of shamanhood mediate entry requirements

I have established that initiation practices and requirements convince observers of a practitioner's transformation, justifying claims of supernatural powers. According to the cultural evolutionary theory, these practices can then intensify or relax, depending on the competition for the jurisdiction.

Figure 5 illustrates a hypothetical scenario in which a shaman's investment in credibility-building practices (e.g., asceticism, death and rebirth, trance) carries costs, while increasing clients' willingness to pay the practitioner (benefits). This willingness reflects the clients' increasing faith in the practitioner's abilities, although it asymptotes because clients can only pay so much. Note that the practitioner prefers to invest some amount (i₀), but that competition from other practitioners should push this investment up until the benefits of shamanhood are equivalent to the costs of investment (i₁). This trend resembles a more general phenomenon observed in economics wherein competition for clients drives up the (perceived) quality of the service (Domberger & Sherr Reference Domberger and Sherr1989; Matsa Reference Matsa2011; Mazzeo Reference Mazzeo2003; Olivares & Cachon Reference Olivares and Cachon2009).

Figure 5.

A hypothesized relationship between a practitioner's investment in credibility-building practices and payoffs. (A) A self-interested practitioner should maximize the difference between the benefits from clients (b) and the costs of investing in credibility (c), denoting an investment of i₀. (B) Competition among practitioners will push the level of investment as far as (but not passing) the point at which costs equal benefits (i₁).

Ethnographic observations support the prediction that greater competition promotes practitioner investment in credibility-building practices, while reductions in competition co-occur with lower investments. For example, I presented earlier the anecdote of an unenergetic possession performance in a Japanese community. This reduced investment seemed a consequence of a decline in competition: Blacker (Reference Blacker1975, p. 263) observed that the community “had to make do with the present creature, despite her feebleness, because she was the only miko left in the entire area.”

Some hopeful shamans will pay a lower cost to convince observers of their non-normality (see Fig. 6). These individuals, who might possess an extra finger (Bernstein Reference Bernstein2008), ambiguous sexual identity (Coleman et al. Reference Coleman, Colgan and Gooren1992; Peletz Reference Peletz2006), or epilepsy (Loeb Reference Loeb1924), thus can garner more credibility and more easily attract a clientele.

Figure 6.

Some practitioners will pay lower costs (c₂ compared with c₁) for the same level of credibility because of inherent violations of humanness, early random successes, and so forth. These individuals can attract credibility past the point that normal practitioners can (up to i₂, as compared with the normal investment limit of i₁), more easily invading jurisdictions.

Formalizing the intuitions reveals other pathways by which social dynamics impact shamanic practice. For example, if clients' willingness to pay goes up, practitioners should invest more in credibility-building practices, providing a hypothesized mechanism for how asceticism elaborates with increasing societal wealth (Baumard et al. Reference Baumard, Hyafil, Morris and Boyer2015). Relatedly, the entry requirements should become less demanding when clients' willingness to pay decreases, such as if shamans become less valued. This seems to explain the loosening of taboos involved in becoming a Comanche medicine person. As the jurisdiction of traditional doctors gave way to Western medicine, a Comanche medicine woman observed that the taboos discouraged an increasingly acculturated youth from becoming shamans. Thus, she “[deleted] certain tabus from her Medicine way in order to make the acquisition of her powers more palatable to a twentieth-century Comanche” (Jones Reference Jones1972, p. 43).

If clients become less willing to pay but the entry requirements for becoming a shaman cannot change, fewer initiates should enter, contributing to the collapse of the institution. This seems to have occurred in with vegetalista traditions among Peruvian mestizos (Luna Reference Luna1984). Luna (Reference Luna1984) remarked, “None of the four informants I worked with has a successor. They all complain that young people are not interested or are not able to endure the diet and continence necessary for learning from the plants.”

I have considered how competition for the jurisdiction should maintain and mediate entry requirements for becoming a shaman. But as stated in section 5.1, as long as observers afford special expertise to shamans, existing practitioners can capitalize on their influence to selfishly erect barriers to entry. For example, fearing encroaching competition, shamans might ratchet up the intensity of entry requirements, justifying the barriers in their esoteric knowledge or singular communication with the supernatural. Evans-Pritchard (Reference Evans-Pritchard1937) observed a failed attempt at this among the Azande, when the foreign witch doctor Bögwözu tried to impose taboos on novices to control the flood of new shamans.

Whether entry requirements develop emergently or intentionally, the outcome is the same. The intensity of these requirements is partly determined by the competition for this jurisdiction and the benefit that controlling the jurisdiction provides. Entry requirements that are too high discourage initiates from becoming shamans.

6.2. Conditions for collapse

Shamanism is widespread, but it is not ubiquitous. Although shamans in various forms have continued to populate Western industrialized societies (e.g., Ferguson Reference Ferguson1928; Hughes Reference Hughes1991; Lindquist Reference Lindquist2004), people patronize them much less frequently than they did in other cultural contexts. Similarly striking is their variability among small-scale societies and among hunter-gatherers, in particular. Shamanic traditions existed in North and South America, Europe, Asia, Africa, and Oceania (Corlett Reference Corlett1935; Eliade Reference Eliade1964; Peters & Price-Williams Reference Peters and Price-Williams1980), yet ethnographers reported their absence among hunter-gatherers such as the Tiwi, the Siriono, the Mbuti, and the Hadza (Peoples et al. Reference Peoples, Duda and Marlowe2016).

What explains variation in shamanic traditions among hunter-gatherers and the practice's more recent collapse in Western industrialized societies? The cultural evolutionary theory of shamanism identifies at least seven conditions under which shamanism should dissolve:

1. 1. The benefits of shamanism decrease, but entry requirements are unchanging. I discussed above that the benefits of shamanism can change when clients' willingness to pay decreases. If the entry requirements do not change in concert, fewer or even no new initiates should join, weakening or destroying the institution. Nevertheless, this does not explain why clients would be less willing to patronize shamans.

2. 2. Cultural complexity declines. Shamanism is a complex set of practices, often involving ceremonies, initiations, mythologies, and music. Therefore, like other forms of complex culture, it can disappear following demographic fragmentation, isolation, and bottlenecks (Henrich Reference Henrich2004; Shennan Reference Shennan2001). This likely explains the absence of shamans among the Siriono (Peoples et al. Reference Peoples, Duda and Marlowe2016), who also lacked musical instruments, games, poison, watercraft, and firemaking knowledge (Holmberg Reference Holmberg1969; Isaac Reference Isaac1977). Cultural phylogenetics reveal that the ancestors of the Siriono almost certainly had shamans, but that this practice was lost along with canoes, corporate structure, and other cultural traits (Walker et al. Reference Walker, Wichmann, Mailund and Atkisson2012), possibly driven by the demographic destruction following European colonization.

3. 3. Powerful parties control jurisdiction. Parties competing for the jurisdiction of controlling uncertainty will campaign against shamanism if they possess sufficient power. This is best exemplified in the actions taken by various Christian churches towards local magicians and prophets. For instance, Catholic churches in late medieval England approved of supernatural rituals “prescribed by God and the Church,” such as those pertaining to the Mass, holy water, and the healing activities of saints – “the remainder was diabolical and to be abhorred” (Thomas Reference Thomas1971a, p. 255). Following the Reformation, the Spanish Catholic Church controlled access to the supernatural by using science to delegitimize trance: “Inquisitors often pointed out how physical infirmities and natural forces at work within the human body could simulate supernaturally infused spiritual gifts” (Keitt Reference Keitt2005b, p. 77). The final four conditions correspond with basic components of the theory as presented in Figure 2. Modifying any of these will affect shamanism:

4. 4. A society experiences less uncertainty. According to this theory, and the writings of many other researchers (Malinowski Reference Malinowski1948), magical practices result from individuals trying to control important, uncontrollable events. As individuals are better able to deal with this uncertainty – or if they confront less of it – they will depend less on shamans. Although a world completely devoid of uncertain outcomes seems implausible, Hart and Pilling (Reference Hart and Pilling1960) offered this hypothesis to explain the absence of shamanism among the Tiwi. Citing the adequate rainfall, the consistent food supply, a dearth of dangerous animals, the rarity of tropical diseases, climatic docility, and the absence of antagonistic neighbors, the ethnographers concluded, “[The Tiwi] never invented magic to control their environment because their physical environment was on the whole a satisfactory and not a hostile universe” (Hart & Pilling Reference Hart and Pilling1960, p. 88).

5. 5. People do not think that invisible forces influence uncertain outcomes. People across societies believe that invisible forces influence uncertain outcomes. Murdock (Reference Murdock1980) reported that every society in the SCCS believed that some illness was caused by supernatural forces, while psychologists have found that even atheists believe that anomalous events are driven by purposeful forces (Banerjee & Bloom Reference Banerjee and Bloom2015). Nevertheless, the cultural evolutionary theory predicts that ceasing to attribute causality to invisible forces should drive the dissolution of shamanism.

6. 6. People do not think that humans are capable of interacting with invisible forces. In some instances, people will believe that it is impossible to engage with invisible forces through practitioners, despite accepting that these forces exist. Shamanism should not exist under these circumstances.

7. 7. Trance behaviors no longer indicate non-humanness (or supernaturalness). The theory hypothesized that trance behaviors are dramas of strangeness, used to convince onlookers of a person's non-human abilities or interactions. I have discussed how the naturalization of trance led people to turn to other means of communicating their otherness.

Thomas (Reference Thomas1971a) implicated these changes, especially points 4, 5, and 6, in England's waning reliance on magical interventions, although some changes were more impactful and longer lasting than others. For example, he reviewed evidence paralleling the decline of magic with an improved capacity for people to protect themselves against capriciousness (point 4 above), such as through insurance and firefighting technologies. But he also argued that “magic lost its appeal before the appropriate technical solutions had been devised to take its place” (p. 656). Supposedly more important in the decline of magic were the changes discussed in points 5 and 6: a tendency toward believing that events occur according to natural laws, and in the cases of an interfering God, believing that human actions are devoid of supernatural effect (Thomas Reference Thomas1971a).

The origins, timing, and pervasiveness of these intellectual changes are highly debated among historians (Bailey Reference Bailey2006; Scribner Reference Scribner1993; Walsham Reference Walsham2008), and addressing them exceeds the scope of this paper. Still, the substitution of mystical explanations with naturalistic ones has been neither ubiquitous nor permanent (Saler Reference Saler2006). Humans everywhere are endowed with the same sociocognitive biases that so routinely steer us into believing in unseeable, purposeful forces and the powers of supernaturalized humans. Many Westerners, although familiar with scientific epistemologies, continue to subscribe to an enchanted worldview, one populated with out-of-body experiences, chains of reincarnation, transmissible healing energy, and suspicions of witchcraft (Partridge Reference Partridge2005). This cosmology in turn supports a gamut of contemporary trance practitioners – the channelers (Hughes Reference Hughes1991), neo-shamanic journeyers (Lindquist Reference Lindquist2004), charismatic healers (Robbins Reference Robbins2004), and other specialists who, in inspired and altered states, resemble the shamans of ancient and recent human history.