Pigeons can helps us model gambling behavior; they too choose suboptimally, especially when they are hungry or isolated

An Animal Model of Human Gambling Behavior. Thomas R. Zentall. Current Research in Behavioral Sciences, February 26 2023, 100101. https://doi.org/10.1016/j.crbeha.2023.100101

Abstract: Human gambling almost always results in a loss. Thus, it is generally assumed that humans gamble for enjoyment and the enticement of winning. Although animals are purported to engage in optimal foraging behavior and should be sensitive to the probability of reinforcement, similar suboptimal behavior can be found in pigeons and other animals. They show a preference for an alternative that is associated with a signal for a low probability of a large reward (e.g., 20% probability of 10 pellets – a mean of 2 pellets) over an alternative that is associated with a signal for a high probability of a smaller reward (100% probability of 3 pellets). This effect may result from the strong conditioned reinforcement associated with a stimulus that is always followed by a reinforcer, but surprisingly, little conditioned inhibition associated with the signal for the absence of a reinforcer. A similar mechanism appears to be responsible for human gambling (gamblers tend to overvalue wins and undervalue losses). We have also found that for pigeons (and perhaps humans as well), the probability of the conditioned reinforcer is relatively unimportant, it is primarily the value of the reinforcer when it does occur (e.g., 10 pellets vs. 3 pellets) that is important. Interestingly, pigeons show several other parallels to human gambling behavior. For example, hungrier pigeons show a greater tendency to choose suboptimally. Also, pigeons that have had enrichment in the form of social experience in a larger cage show a reduced tendency to choose suboptimally. This animal model may provide a useful analog to human gambling behavior, one that is free from the influence of human culture, language, social reinforcement, and other experiential biases that may encourage human gambling.

Keywords: gamblingsuboptimal choiceimpulsivitypigeonshumans

Is Human gambling different from animal suboptimal choice?

The suboptimal choice procedures with pigeons differ somewhat from typical human gambling conditions. It could be argued, for example, that humans choose to bet with money that they already have, whereas pigeons choose between two alternatives, in either case the outcome of which is a gain. This means that for humans, a loss is a real loss, whereas for pigeons, it is a lost opportunity or an opportunity cost. In the case of human gambling, the choice is between a probable loss (the gamble) and no loss (refraining from the gamble), whereas for pigeons the suboptimal choice it is between less gain (suboptimal choice) and more gain (optimal choice). Thus, for the pigeon there is no actual cost. For this reason, however, one would think that for humans who gamble, an actual loss would be more aversive than a lost opportunity, and therefore, humans should be less likely to choose to gamble. In fact, as noted earlier, proportionally, humans are generally less likely to gamble (Molet et al., 2012) than pigeons are to choose suboptimally.

Second, in the pigeon task, there is typically a 10-s delay between the choice and the outcome of that choice (a reinforcer or its absence). However, for pigeons, it is not primarily the delay to the primary reinforcer that is critical, rather it is the immediacy of the appearance of the conditioned stimulus, and typically the conditioned stimulus appears immediately following the choice - if the conditioned stimulus does not appear immediately following choice, suboptimal choice is greatly reduced (McDevitt et al., 1997).

Third, the gambling task can be thought of as a go/no-go task (to gamble or not gamble), whereas the pigeon suboptimal choice task involves a simultaneous two-alternative choice. In fact, however, the human choice can be thought of as between the act of gambling and engaging in some other activity (e.g., going out to dinner or seeing a film). Thus, a clear distinction between the go/no-go task and the two-alternative choice task is not always easy to make.

One might suggest that another difference between the pigeon suboptimal choice task and human gambling is the importance of the post choice conditioned stimulus for the pigeon. If the signal for reinforcement is missing, pigeons generally choose optimally (Spetch et al., 1990; Spetch et al., 1994). Rats too chose risky options significantly more when win-associated audiovisual cues are added to the task (Barrus & Winstanley, 2016)

Although perhaps not as obvious as it is with pigeons, conditioned stimuli also play an important role in human gambling. For example, there are symbols that show up on the reels of a slot machine, numbers on the lottery ticket, and the ball falling into a winning number (or color) in roulette. If the only feedback gamblers received when they win or lose is receiving money or not, (imagine covering the symbols on a slot machine), it is unlikely that humans would be as inclined to gamble as they are. Evidence that conditioned stimuli play an important role for humans has been found by Cherkasova et al. (2018). Furthermore, Spetch et al. (2020) found that humans chose more risky slot machines when casino-related cues were associated with payouts.

Another difference between human gambling and pigeon suboptimal choice is that humans often describe gambling as an enjoyable activity (entertainment) rather than as a chance to make money. This difference is difficult to address directly because the emotion of a pleasurable activity is hard to measure (especially in pigeons). The question is, do humans gamble for the enjoyment of gambling rather than for the money? When it comes to games of chance, would humans be as likely to gamble if it were not possible to win money (or a prize)? The possibility of winning money (or the anticipation of the possibility of winning money) certainly contributes to what makes the game enjoyable. People may sometimes gamble for social rewards (being the best, earning the most points), but even with many social games (e.g., poker), humans often prefer to play for money rather than points alone.

I have suggested that the mechanism responsible for the pigeons’ suboptimal choice is the value of the outcome predicted by the conditioned stimulus, together with the difference between reward expected and reward obtained. An important component of this theory is the absence of inhibition-associated signals for the absence of reward. That is, the pigeons’ suboptimal choice is not inhibited by the frequent signals for the absence of reward. Consider the procedure used by Vasconcelos et al. (2015) in which starlings preferred the signaled alternative over the 50% unsignaled alternative. The signaled reinforcement was selected even when the signal for the absence of a reinforcer appeared 19 out of 20 times that the suboptimal alternative was selected, whereas reinforcement followed the unsignaled alternative half of the time. Thus, an important similarity between human gambling and pigeon suboptimal choice is the fact that, based on traditional reinforcement theory, losses contribute much less to conditioned inhibition than one might expect that they should, (Holst et al., 2010).

The cost of the relative absence of conditioned inhibition in the suboptimal choice procedure can be easily calculated. For pigeons, in the Case and Zentall (2018) experiment, choice of the optimal alternative would have resulted in twice as much food (50% vs. 100% reinforcement), in the Stagner and Zentall (2010) experiment, choice of the optimal alternative would have resulted in 2.5 times as much food (20 % vs. 50% reinforcement), and in the Vasconcelos et al, (2015) experiment, in the extreme condition, choice of the optimal alternative would have resulted in 10 times as much food (5% vs. 50% reinforcement). In these experiments, choice of the suboptimal alternative represents a considerably greater opportunity cost than the near-even chance of winning in many casino games (e.g., roulette or blackjack), although perhaps not as costly as the return on buying a lottery ticket.

Although the research with animals may appear to be different from human gambling, as noted earlier, not unlike pigeons, human gamblers are rarely aware of the probabilities of winning, but they are more likely to be aware of amount of money they would obtain if they won (billboards often announce the winning amount).

The research with pigeons suggests as well that there is an additional factor that may contribute to human gambling, positive contrast (or relative delay reduction), the positive feeling that gamblers may get when they expect a loss, but they experience a win. The results of the research on suboptimal choice with pigeons suggest that these tasks may provide a viable analog of unskilled gambling by humans. They may be useful in understanding the mechanisms responsible for human gambling behavior, as well as suggesting a possible treatment of problem gambling, the substitution of other pleasurable activities that do not have the negative social or economic implications that often accompany pathological gambling.

The belief in repressed memories of trauma (and multiple personality syndrome) has become a zombie idea, so seductive that it lives on despite long since being discredited

The Memory Wars Then and Now: The Contributions of Scott O. Lilienfeld. Steven Jay Lynn, Richard J. McNally, and Elizabeth F. Loftus. Clinical Psychological Science, February 24, 2023. https://doi.org/10.1177/21677026221133034

Abstract: In this review, honoring Scott O. Lilienfeld, we reflect on key conflicts, controversies, and flash points in the so-called memory wars that have captured headlines, affected legislative action, and influenced civil suits and criminal trials. We trace the memory wars, beginning in the 1990s to the present. From the outset, the memory wars featured debates regarding repressed memories, recollections of trauma, and the hazards of memory recovery therapy, and these disagreements persist today in controversies concerning dissociative amnesia, beliefs about memory, suggestive psychotherapies, and the genesis of dissociative identity disorder (DID). We acknowledge Lilienfeld’s contributions, particularly to the sociocognitive model of DID, reviewed in the second half of the article, and to a recent transtheoretical framework that contrasts sharply with the posttraumatic view of DID. The memory wars greatly enhanced scientific understanding of memory, trauma, iatrogenic psychotherapies, and dissociative disorders. We conclude with suggestions for future research to deepen understanding of issues stimulated by the memory wars.

Black-White Differences in Parental Happiness

Black-White Differences in Parental Happiness. Jennifer Augustine, Mia Brantley. Socius, February 23, 2023. https://doi.org/10.1177/23780231231153617

Abstract: Lower levels of happiness among Blacks compared with Whites are well documented, as are lower levels of happiness among parents compared with nonparents. Yet it remains unclear whether the parenting happiness gap is larger among Blacks compared with Whites. Drawing on the General Social Survey (2010–2018), the authors investigate this question. The authors find that White mothers reported less happiness compared with their White female nonparent counterparts, but contrary to research highlighting the profound challenges of parenting for Black women, a parental happiness gap among Black women was not observed. Among Black men, parents reported a much higher probability of being very happy than their nonparent counterparts, whereas White fathers’ happiness was no different from that of their male counterparts without children. These findings are discussed in view of stereotypes about Black mothers and fathers, their resilience to stressors such as racism and discrimination, and emerging research on the salience of fatherhood for Black men.

Discussion

Race differences in happiness among Americans are well documented, as are differences in happiness between parents and nonparents (Herbst and Ifcher 2016; Iceland and Ludwig-Dehm 2019). Yet few studies have aimed to bridge these literatures by examining whether differences in parental happiness are the same across racial groups. This oversight is noteworthy, given the various strands of research highlighting the ways that parenting creates additional challenges for Black parents compared with White parents. It is also surprising, given the broader literature on parental happiness and its emphasis on how demographic factors moderate the “costs” of parenthood (Nomaguchi and Milkie 2020), as well as the profundity of race in stratifying other forms of well-being. Thus, in this study, we take steps to fill this important gap in knowledge by drawing on a seminal source of research on happiness, the GSS, to investigate racial disparities in the parental happiness gap for men and women. Our findings provide several fresh insights, several of which ran counter to our expectations.

To begin, we found that the happiness gap among parents compared with nonparents was observed for White women, but not Black women. These results suggest that despite the challenges that Black mothers face—such as stereotyping around motherhood, fears around their children’s well-being, and the need to contend with hostile environments (Dow 2019; Elliott and Aseltine 2013)—they may be exceptionally resilient in terms of their happiness. This pattern is consistent with other research on resiliency among Black Americans. For example, numerous studies find that Blacks suffer no worse mental health disorders than Whites, a phenomenon sometimes called the Black-White mental health paradox (Erving, Thomas, and Frazier 2019). Although the reasons for this phenomenon remain unclear, some have argued that Blacks promote a type of racial socialization that celebrates overcoming adversity and a strong sense of group identity, both of which may enhance the positive emotional aspects of parenting, despite its challenges. In further support of this view, studies have suggested that Blacks are more likely to view themselves than Whites as role models to their children (Hart et al. 2001). Doing so in the face of challenges may thus enhance meaning and the positive emotional experience Black mothers derive from parenting in ways that help offset some (albeit not all, given that Black women in general reported less happiness than White women) of the factors that would potentially undermine Black mothers’ well-being. Other explanations for why we did not observe lower levels of happiness among Black mothers compared with Black nonmothers include that Black mothers often experience high levels of community support-in contrast to mothers in White communities, which embrace a more individualist approach to family life (Collins 2002; Dow 2019)-which may promote greater subjective well-being. Black mothers also spend more time in the company of their children (Nomaguchi et al. 2022) than White mothers, which has been associated with greater happiness among parents (Negraia and Augustine 2020). More broadly, these findings also indicate that parental status does not play a role in Black-White differences in women’s happiness.

For men, we found that parental status also did not differentiate the happiness of Whites, perhaps because White fathers share less responsibility for more time-intensive and stressful caregiving than their partners (Musick et al. 2016). An alternative set of explanations, which are informed by and evoked in studies showing that the parental happiness gap has grown smaller (Herbst and Ifcher 2016; Preisner et al. 2020), are that the relative happiness advantage of men without children may have declined in the context of growing social disconnectedness, which parenthood helps buffer. At the same time, the happiness of men with children has grown as gendered norms around fathers’ caregiving have allowed fathers to experience more time in play, leisure, and other enjoyable family activities (Negraia et al. 2018).

For Black men, those with children were substantially more likely to report being very happy, and far less likely to report being not very happy, than their nonparent male counterparts. These results suggest that fathering is a far more salient experience for Black men than prior research has recognized. Results also extend a handful of ethnographic studies on lower income, generally noncustodial Black fathers (who are of note far more involved with their children than commonly assumed; Abdill 2018), which indicate that Black men may subscribe to a different view of fatherhood borne out of structural barriers that have historically hindered Black fathers from providing financially in the same way as White fathers (Bloome 2014). Specifically, Black fathers reject traditional notions of the package deal, in which satisfaction from fathering is derived from one’s ability to financially provide (Townsend 2002) and instead endorse a model of “relational fathering” or “new package deal” that celebrates the joys of fathering (see Edin and Nelson 2013). For example, as many of the men in the study by Edin and Nelson (2013) recounted, fatherhood “made life worth living,” children were viewed as the ultimate gift, and many of the banal aspects of basic care in which White fathers engage less frequently than White mothers—such as teaching children and helping them dress—were described as “priceless and a treasure any man would want to claim” (p. 221). In this way, these findings also serve to contradict stereotypical notions of Black fathers as being uninterested in fathering. More broadly, these findings also indicate that parental status does not explain Black men’s lower levels of happiness compared with White men’s, as observed in prior studies (e.g., Cummings 2020; Iceland and Ludwig-Dehm 2019).

Of course, at this time, many of the inferences based on these patterns of results are conjecture. It remains unclear how Black mothers blunt the potentially negative impact that “mothering while black” (Dow 2019) has on their subjective well-being or why fathering has such a positive impact on Black men. However, this study underscores the importance of exploring such questions in future research. At the same time, this study has several other limitations that should be noted. First, the validity and reliability of self-assessed generalized measures of happiness, including the happiness measure the GSS, continue to be debated. Some have argued that a more nuanced scale of happiness is preferable (Lyubomirsky and Lepper 1999). Others argue in support of a conceptually distinct way of measuring well-being through momentary measures (Negraia and Augustine 2020). Future studies should therefore also replicate the present study using other measures of happiness. At the same time, the use of the GSS measures provide a strong connection to past research on happiness, and our within-race estimation procedure accounts for race differences in the interpretation and conceptualization of happiness that challenge reliability.

Second, we also must acknowledge that it is likely that the patterns we observed are further differentiated by adults’ education, income, and marital status, as well as characteristics of parents’ children (e.g., ages, gender). Although an exploration of such factors is beyond the scope of the present study, and in many cases is limited by small sample sizes, particularly among Black fathers, future studies based on other data should consider these sources of variability as well. Last, consistent with prior research, we focused on parents who were coresidential and caring for minor children. Yet the experiences of nonresidential parents, and particularly of fathers, are also important to recognize, as highlighted in recent ethnographic accounts of minority fathers, and should be considered more carefully in future research as well, as should the experiences of other parents, such as step and social parents as well as parents who are empty nesters or caring for household adult children. Doing so, however, would also require other data.

In sum, the aim of this study was to address an important question that had yet to be answered: whether parenting (vs. not caring for minor household children) is negatively associated with the happiness of Blacks more so than that of Whites. Our results indicated a surprising pattern of results. Among women, White mothers were less happy than female nonmothers, but this was not the case for Black women. These results suggest that Black mothers’ levels of happiness were resilient to the numerous well-documented challenges they face protecting and promoting the welfare of their children. For men, Black fathers were far more likely to be happy than their nonparent counterparts, although this was not true for White men, for whom we did not observe any differences in happiness by parental status. These results highlight the profundity of the father role for Black men and controvert much conventional thinking and stereotyping and Black men’s experiences of fatherhood.

Miscitation in Psychology: About 19% of citing claims either failed to include important nuances of results (9.3%) or completely mischaracterized findings from prior research altogether (9.5%)

Cobb, C. L., Crumly, B., Montero-Zamora, P., Schwartz, S. J., & Martínez, C. R., Jr. (2023). The problem of miscitation in psychological science: Righting the ship. American Psychologist, Feb 2023. https://doi.org/10.1037/amp0001138

Abstract: Scholarly citation represents one of the most common and essential elements of psychological science, from publishing research, to writing grant proposals, to presenting research at academic conferences. However, when authors mischaracterize prior research findings in their studies, such instances of miscitation call into question the reliability and credibility of scholarship within psychological science and can harm theory development, evidence-based practices, knowledge growth, and public trust in psychology as a legitimate science. Despite these implications, almost no research has considered the prevalence of miscitation in the psychological literature. In the largest study to date, we compared the accuracy of 3,347 citing claims to original findings across 89 articles in eight of top psychology journals. Results indicated that, although most (81.2%) citations were accurate, roughly 19% of citing claims either failed to include important nuances of results (9.3%) or completely mischaracterized findings from prior research altogether (9.5%). Moreover, the degree of miscitation did not depend on the number of authors on an article or the seniority of the first authors. Overall, results indicate that approximately one in every 10 citations completely mischaracterizes prior research in leading psychology journals. We offer five recommendations to help authors ensure that they cite prior research accurately.

Impact Statement: This article suggests that approximately one in every 10 citations across leading psychology journals is inaccurate. Such instances of miscitation may call into question the reliability and credibility of scholarship within psychological science. Scholars in psychology should be careful to ensure that they cite and characterize findings from prior research accurately in their studies.

The human appetite for recreational drugs could be a heritage of the Stone Age switch to a meat-based diet and the need to protect an ever-expanding brain from zoonotic infections

Homo medicus: The transition to meat eating increased pathogen pressure and the use of pharmacological plants in Homo. Edward H. Hagen, Aaron D. Blackwell, Aaron D. Lightner, Roger J. Sullivan. American Journal of Biological Anthropology, February 23 2023. https://doi.org/10.1002/ajpa.24718

Abstract: The human lineage transitioned to a more carnivorous niche 2.6 mya and evolved a large body size and slower life history, which likely increased zoonotic pathogen pressure. Evidence for this increase includes increased zoonotic infections in modern hunter-gatherers and bushmeat hunters, exceptionally low stomach pH compared to other primates, and divergence in immune-related genes. These all point to change, and probably intensification, in the infectious disease environment of Homo compared to earlier hominins and other apes. At the same time, the brain, an organ in which immune responses are constrained, began to triple in size. We propose that the combination of increased zoonotic pathogen pressure and the challenges of defending a large brain and body from pathogens in a long-lived mammal, selected for intensification of the plant-based self-medication strategies already in place in apes and other primates. In support, there is evidence of medicinal plant use by hominins in the middle Paleolithic, and all cultures today have sophisticated, plant-based medical systems, add spices to food, and regularly consume psychoactive plant substances that are harmful to helminths and other pathogens. We propose that the computational challenges of discovering effective plant-based treatments, the consequent ability to consume more energy-rich animal foods, and the reduced reliance on energetically-costly immune responses helped select for increased cognitive abilities and unique exchange relationships in Homo. In the story of human evolution, which has long emphasized hunting skills, medical skills had an equal role to play.

8 DEFENDING THE BRAIN FROM PATHOGENS WITH PSYCHOACTIVE DRUGS

The final category of pharmacological plant use that requires an evolutionary explanation is the widespread, habitual use of “recreational” drugs like caffeine, nicotine, and THC, which we also conceptualize as (mostly) a constitutive defense. Previously, two of the authors (EHH and RJS) and their colleagues proposed that use of these substances might have evolved as constitutive and inducible defenses against pathogens (Hagen et al., 2009; Hagen et al., 2013; Roulette et al., 2014; Sullivan et al., 2008). Here we extend this hypothesis to the behavioral defense of the CNS specifically, which tripled in size in the Pleistocene and might have been subject to increased virulent infections, as described earlier. This extension is based on the substantial differences in immune defenses of the brain vs. other tissues and organs.

Most tissues have mechanisms to restore functionality when damaged or infected, which typically involves the destruction and removal of injured or infected cells (D'Arcy, 2019; Deretic et al., 2013), and the generation of new cells (Clevers & Watt, 2018; Xia et al., 2018). Herpes simplex virus infection of skin cells, for example, results in massive immune- and virus-mediated cell death, followed by rapid replacement of the cells. Most human neurons, however, cannot be replaced in adulthood because loss of neurons entails the loss of functionality and often irreplaceable information, such as in Alzheimer's disease where neuronal cell death causes permanent loss of memory and other cognitive dysfunctions (Arendt et al., 2015). Although adult neurogenesis has been reported in a wide range of vertebrates, including birds, rodents, and primates, in humans it is very limited and perhaps non-existent (Denoth-Lippuner & Jessberger, 2021; Franjic et al., 2022; Gage, 2019; Lucassen et al., 2020; Moreno-Jiménez et al., 2019; Oppenheim, 2019; Sorrells et al., 2018). The unique value of neurons presents a conundrum to the immune system: how to defend the brain from pathogens if destroying infected neurons would cause permanent loss of critical learned information or other functionality (Miller et al., 2016; Solomos & Rall, 2016)? Moreover, CNS inflammatory responses interfere with CNS functions, sometimes permanently, even without neuronal death (Klein et al., 2017). Constitutive defenses are one solution (Paludan et al., 2021).

8.1 The blood-brain barrier, a constitutive defense

The brain is defended by a physical blood brain barrier (BBB). The BBB prevents most blood-borne pathogens from infecting the brain. It also prevents most plant toxins and other xenobiotics from entering the brain (Banks, 2016; Iadecola, 2017; Villabona-Rueda et al., 2019), including most pharmaceuticals, which often chemically resemble plant toxins (Agúndez et al., 2014). These properties pose a considerable challenge to drug treatment of pathogens that do manage to infect the CNS (Pardridge, 2012; Terstappen et al., 2021). Certain small molecules can cross the BBB via lipid-mediated free diffusion, however, including widely used “recreational drugs” like nicotine and caffeine.

8.2 CNS immune privilege and defense

For much of the last century, knowledge that the BBB prevented most pathogens from reaching the CNS and that tissue grafts implanted in the CNS parenchyma (functional tissue) did not provoke rejection, supported the view that the CNS was an “immune privileged” site. Recent discoveries that the brain parenchyma is connected to the peripheral immune system via meningeal lymphatic vessels have stimulated debate over the nature of immunity in the brain.

One mainstream view is that barriers establish compartments in the CNS that differ functionally in their access to the immune system and some are immune privileged and others are not (Engelhardt et al., 2017). The meninges surrounding the CNS parenchyma, for instance, contain a wide repertoire of immune cells, including monocytes and B cells from special skull and vertebral bone marrow reservoirs, that provide immune surveillance of the CNS (Alves de Lima et al., 2020; Brioschi et al., 2021; Cugurra et al., 2021). Although the CNS parenchyma can mount an inflammatory response to infection via resident microglia (brain-specific macrophages) and other cells, as well as cells migrating from the meninges, it is characterized by a dearth of adaptive and innate immune responses relative to peripheral tissues (Engelhardt et al., 2017).

Immune privilege is a double-edged sword, however. Despite formidable CNS defenses such as the BBB, pathogens do manage to infect the CNS. The protection immune privilege provides to neurons also creates a niche in which pathogens that manage to infect the CNS can evade destruction by the immune system (Cain et al., 2019; Forrester et al., 2018). In fact, to maintain neuronal integrity, immune responses in the CNS might favor controlling pathogens rather than eliminating them (Matta et al., 2021; Miller et al., 2016).

8.3 Habitual recreational drug use as a constitutive pathogen defense

Humans have evolved to be exceptionally reliant on learned information and other CNS functions across a lifespan that exceeds that of most other mammals, and they occupied a dietary niche with high exposure to potentially zoonotic pathogens, including those that infect the CNS. Yet immune defense of the CNS is constrained. Chemoprophylaxis and chemotherapy with compounds that are harmful to CNS pathogens but well-tolerated by the CNS would complement the immune system. Such an evolved chemoprotective strategy for the CNS requires antipathogenic compounds that can cross the BBB.

Most common recreational drugs, including caffeine, nicotine, THC, and arecoline in betel nut, are plant defensive neurotoxins (ethanol, a yeast fermentation product, is the major exception). Sullivan, Hagen, and colleagues argued that the prevailing evolutionary “hijack hypothesis” of recreational drug use, in which evolutionarily novel substances incidentally activate dopamine reward circuits (Kelley & Berridge, 2002; Wise, 1998), was implausible because similar compounds have been part of primate diets for millions of years (Hagen et al., 2009; Hagen et al., 2013; Sullivan et al., 2008; Sullivan & Hagen, 2002).

Psychoactive substance seeking might instead be an evolved self-medication strategy to defend against intestinal helminths and other pathogens (Hagen et al., 2009; Hagen et al., 2013; Sullivan et al., 2008; Sullivan & Hagen, 2002). All globally popular recreational drugs are toxic to helminths, as are some hallucinogenic plants used by Amazonian peoples (Rodríguez et al., 1982); nicotine was widely used to deworm livestock prior to the development of modern anthelmintics, and has the same mechanism of action as some commercial anthelmintics; an aqueous solution of tobacco is still used to deworm livestock in some low-income settings (efficacy quantitatively verified); tobacco is widely mentioned as an anthelmintic in ethnomedical texts; treatment of intestinal helminths in hunter-gatherers transiently reduces tobacco use, and tobacco and cannabis use is negatively associated with worm burden and reinfection; and there is a switch-like transition by virtually all humans to regular use of one or more of these pharmacologically potent substances in adolescence once teratogenic risks to the developing brain have dropped (Hagen et al., 2009; Hagen & Sullivan, 2018; Roulette et al., 2014; Roulette, Kazanji, et al., 2016; Sullivan et al., 2008; Sullivan & Hagen, 2002). In this model, females avoid culturally identified teratogenic substances such as tobacco during pregnancy and their reproductive years, increasing use postmenopause (Hagen et al., 2016, 2013; Hagen & Tushingham, 2019; Placek et al., 2017). See Figures 7 and 8.

FIGURE 7

(a–d) The universal transition to psychoactive druge use in adolescence. Cumulative distribution of self-reported age of first use of alcohol, tobacco, cannabis, and cocaine in a large (N = 85,052) cross-national sample of users of these substances. Figure from Degenhardt et al. (2016). (e) Prevalence of tobacco and cannabis use among Aka forager children, adolescents, and adults, by sex (no children reported use). Data from Roulette, Hagen, et al. (2016). (f) Urinary caffeine metabolite (AAMU: 5-acetylamino-6-amino-3-methyluracil) excretion rate in a nationally representative US sample (N = 2714); 97.5% had detectable AAMU. Self-reported caffeine intake in this sample exhibited the same age dependence, as did concentrations of urinary caffeine and other caffeine metabolites. Figure and data from Rybak et al. (2015). These patterns suggest the existence of a developmental “switch” to psychoactive drug use during adolescence.

FIGURE 8

Theoretical model of recreational drug use as an evolved constitutive pathogen defense that varies by age, sex, reproductive status, total fertility rate (TFR), and cultural information about teratogenic substances. For details, see Hagen et al. (2016) and Hagen and Tushingham (2019).

Helminths are an important class of CNS parasites, and of course all recreational drugs cross the BBB. Here we extend the antiparasite hypothesis of recreational drug use to pathogens that infect the CNS, focusing on the helminth T. solium as a key example. Humans, dogs, and other animals infected with Taenia and other tapeworm species have often been treated with arecoline hydrobromide (Gemmell, 1958; Li et al., 2012). Arecoline is an agonist of muscarinic acetylcholine receptors, which have numerous roles including in neuromuscular junctions. Arecoline's mechanism of action against cestodes is probably to induce paralysis (Liu et al., 2016). Arecoline readily crosses the BBB and is the primary psychoactive alkaloid in the seed of Areca catechu palm, which is typically chewed with the leaf of the Piper betle and slaked lime, a concoction termed betel quid or paan (Volgin et al., 2019). Betel quid is widely consumed in Asia and the Pacific and is probably the fourth most widely used psychoactive substance after caffeine, alcohol, and tobacco (Arora & Squier, 2019; Gupta & Warnakulasuriya, 2002; Mehrtash et al., 2017). Areca seeds, often combined with pumpkin seeds, were one of several frequently mentioned treatments of Taenia infections in Chinese medical texts dating back about 2000 years (Zou & Ye, 2014). In a controlled study in humans this combination was found to be close to 90% effective at expelling Taenia tapeworms (Li et al., 2012). Whether arecoline also kills Taenia larvae in the brain is unknown, and killing larvae in the brain induces inflammation, potentially creating more problems than it solves. However, most of the medical community has accepted that the benefits of antiparasitic treatment of neurocysticercosis outweigh the risks (García et al., 2003). In an observational study of individuals suffering epileptic seizures, many of whom probably had neurocysticercosis based on the local prevalence of this disease, chewers of Areca catechu (1/3 of the sample) had 59% fewer seizures in the month prior, compared to non-chewers (a mean of 1.4 vs. 3.3 seizures, respectively, Mateen et al., 2017).

It is intriguing that a tapeworm that humans acquired from carnivores in the Pleistocene, and which infects the CNS and other tissues, is potentially treatable with the active compound in one of the world's most popular “recreational” drugs, used on a daily basis by a sizable fraction of the world's population, and that among those with seizures, use of the drug is negatively associated with seizure frequency. It is also intriguing that caffeine, the world's most popular drug, inhibits growth of T. gondii (Munera López et al., 2019), another common neurotropic pathogen. Consumption of ethanol, like consumption of pharmacological plant substances, could also be a self-medication strategy: it is a potent antimicrobial compound, and there is evidence that it mitigates infections of H. pylori in vitro and in vivo (Liu et al., 2016; Xia et al., 2020).

Extending previous work (Hagen et al., 2009; Hagen et al., 2013; Sullivan et al., 2008), we propose that when the benefits exceed the costs, humans, and perhaps other animals, have an evolved propensity to seek out and regularly consume psychoactive plant defensive chemicals, that is, those that cross the BBB and interfere with neural signaling, so as to deter, control, and eliminate pathogen invasions of the immune privileged CNS parenchyma.