Humans are an ultrasocial species; this sociality, however, cannot be fully explained by the canonical approaches found in evolutionary biology, psychology, or economics

The Origins and Psychology of Human Cooperation. Joseph Henrich and Michael Muthukrishna. Annual Review of Psychology, Vol. 72:- (Volume publication date January 2021). https://doi.org/10.1146/annurev-psych-081920-042106

Abstract: Humans are an ultrasocial species. This sociality, however, cannot be fully explained by the canonical approaches found in evolutionary biology, psychology, or economics. Understanding our unique social psychology requires accounting not only for the breadth and intensity of human cooperation but also for the variation found across societies, over history, and among behavioral domains. Here, we introduce an expanded evolutionary approach that considers how genetic and cultural evolution, and their interaction, may have shaped both the reliably developing features of our minds and the well-documented differences in cultural psychologies around the globe. We review the major evolutionary mechanisms that have been proposed to explain human cooperation, including kinship, reciprocity, reputation, signaling, and punishment; we discuss key culture–gene coevolutionary hypotheses, such as those surrounding self-domestication and norm psychology; and we consider the role of religions and marriage systems. Empirically, we synthesize experimental and observational evidence from studies of children and adults from diverse societies with research among nonhuman primates.

Patients with focal lesions to the vmPFC were selectively impaired in projecting themselves to the future and in recognizing relative-future events

An asymmetry in past and future mental time travel following vmPFC damage. Elisa Ciaramelli, Filomena Anelli, Francesca Frassinetti. Social Cognitive and Affective Neuroscience, nsaa163, December 31 2020. https://doi.org/10.1093/scan/nsaa163

Abstract: The role of ventromedial prefrontal cortex (vmPFC) in mental time travel toward the past and the future is debated. Here, patients with focal lesions to the vmPFC and brain-damaged and healthy controls mentally projected themselves to a past, present or future moment of subjective time (self-projection) and classified a series of events as past or future relative to the adopted temporal self-location (self-reference). We found that vmPFC patients were selectively impaired in projecting themselves to the future and in recognizing relative-future events. These findings indicate that vmPFC damage hinders the mental processing of and movement toward future events, pointing to a prominent, multifaceted role of vmPFC in future-oriented mental time travel.

Keywords: mental time travel, self-projection, episodic memory, future thinking, vmPFC

Discussion

The present study investigated the effect of vmPFC damage on two component processes of MTT: the ability to project the self in time to assume different temporal perspectives (self-projection), and to determine, for each event in a series, whether it has already happened or is yet to happen relative to the currently assumed time perspective (self-reference). We found a striking asymmetry in the effect of vmPFC damage on both aspects of MTT, which hindered vmPFC patients’ possibility to project the self to the future, but not the past or the present, and to recognize relative-future but not relative-past events.

Before discussing, in turn, each aspect of vmPFC patients’ deficit in future-oriented MTT, we wish to emphasize that this deficit is not a common consequence of brain damage, for example reflective of a shortening of future time perspective following illness and perceived vulnerability (Ciaramelli et al., 2019). Indeed, problems with future-oriented MTT were consistently present in vmPFC patients but not in control patients with brain damage not involving vmPFC. Our results are also unlikely to reflect poor comprehension of self-projection or task instructions on the vmPFC patients’ part. Indeed, all participants, including vmPFC patients, were slower at recognizing events while assuming a past or future (compared to present) time perspective, a robust finding reflecting the cognitive cost of self-projection (Arzy et al., 2008, 2009; Anelli et al., 2016a,b; Gauthier et al., 2019). That this pattern was observed also in vmPFC patients strongly suggests they did indeed try and abandon the present moment to mentally move toward the subjective past and future. The vmPFC patients’ self-projection toward the future, however, went often awry, as indicated by an abnormal number of errors while processing events from that time perspective, as if patients failed at assuming (or maintaining) a future self-location, while they were normally capable to project the self back to the past. This finding aligns with recent neuroimaging evidence that during MTT the medial prefrontal cortex exhibit graded, progressively increasing activation with the succession of past, present and future self-projection (Gauthier et al., 2019).

The selective deficit in future-oriented self-projection we detected in vmPFC patients stands in contrast to previous studies showing a pervasive impairment in remembering the past and imagining the future in vmPFC patients (Bertossi et al., 2016a,b), which also characterizes amnesic patients with medial temporal lobe lesions (Race et al., 2011). These findings, however, are only apparently in conflict. Indeed, previous studies had used MTT tasks requiring both the adoption of past and future temporal perspective and the construction of complex events from those perspectives (Bertossi et al., 2016b), while the task we used here only involves the former. Thus, while impaired construction of both past and future experiences in previous studies may reflect a general problem in assembling detail-rich events, apparent even when vmPFC (as well as medial temporal lobe) patients have to construct atemporal experiences (Hassabis et al., 2007; Bertossi et al., 2016a; see also De Luca et al., 2018), the present findings insulate an additional problem vmPFC patients have in assuming a future temporal perspective, above and beyond their event construction deficit, which affects MTT upstream. In contrast, patients with medial temporal lobe lesions have proven able to project the self in (future) time (Dalla Barba, 2001; Arzy et al., 2009; Kwan et al., 2013; Craver et al., 2014), and their problems in imagining future experiences are likely to be fully explained by impaired event construction (McCormick et al., 2019). Indeed, a single-case study of an amnesic patient with bilateral medial temporal damage using the same paradigm we have used here found no impairment in self-projection (Arzy et al., 2009). Our findings make contact with previous evidence that vmPFC patients have more problems in constructing future compared to atemporal experiences (Bertossi et al., 2016a), an asymmetry not present in medial temporal lobe amnesia (Hassabis  et al., 2007), and that, when asked to enumerate personal future life events, vmPFC patients produce events less far ahead into the future than do brain-damaged controls, suggestive of a short future time perspective (Fellows  and Farah, 2005). Also, vmPFC patients (but, again, not medial temporal lobe amnesiacs; Kwan et al., 2013) show increased delay discounting (Sellitto et al., 2010; Peters and D’Esposito, 2016), in line with the view that they fail to conceive the future, even in purely semantic, abstract terms, hence devalue future rewards.

Why would vmPFC be necessary for future-oriented self-projection? It has been shown that self-projection to one’s personal past/future typically originates from the activation of high-level knowledge structures, such as schematic representations of life time periods and the self (e.g. when I graduated and when I will have a child), to then possibly converge on specific events of one’s personal timeline (Conway and Pleydell-Pearce, 2000; D’Argembeau and Mathy, 2011; D’Argembeau, 2020). Imagining the future relies more on schema-based knowledge than remembering the past, because we have no direct experience of future events (Anderson and Dewhurst, 2009; Berntsen and Bohr, 2010; Rubin, 2014). Knowledge about personal goals (e.g. I want to become a researcher) is especially effective in driving the construction of ones’ personal future (D’Argembeau and Mathy, 2011). The vmPFC is critical for appropriate processing of schema-related information (Burgess and Shallice, 1996; Ghosh et al., 2014), including knowledge about the self (Philippi et al., 2012; Verfaellie et al., 2019) and personal goals (D’Argembeau et al., 2010). We argue, therefore, that vmPFC patients failed to project the self to the future due to an inability in activating schematic knowledge critical to construct a mental representation of their personal future, so as to assume the perspective of their future self. This interpretation aligns with current views of the dynamics of MTT, according to which vmPFC initiates the activation of high-order autobiographical knowledge (e.g. lifetime periods and self schema), from which the hippocampus may then access (McCormick et al., 2019; D’Argembeau, 2020; Dafni-Merom and Arzy, 2020; see also Barry et al., 2019) or process (Schurr et al., 2018) specific experiences.

Orthogonal to their impairment in future self-projection, vmPFC patients additionally showed a deficit in self-referencing future events, that is, in recognizing events lying ahead in the future with respect to their assumed location in subjective time (whether past, present or future), which were misclassified more often than relative-past events. Healthy as well as brain-damaged controls showed a comparable performance in recognizing relative-future and relative-past events, hence the selective deficit evinced by vmPFC patients with relative-future events is unlikely to merely reflect task difficulty. Note, also, that vmPFC patients’ false recognition of future events also emerged in the past self-location condition, that is, when dealing with events that were not actually future (with respect to the present time), and therefore it does not simply denote a problem in distinguishing familiar from novel events, or factual from potential, hypothetical events (see also Anelli et al., 2018).

One is tempted to interpret vmPFC patients’ deficit as reflecting disordered chronology. The vmPFC, together with the basal forebrain, is indeed thought to support the correct assignment of memories to their correct place in time (Moscovitch, 1995; Tranel and Jones, 2006), and confabulation, a consequence of vmPFC damage, consists of memories that are often false in temporal context (Schnider, 2003; Gilboa et al., 2006; Bertossi et al., 2016b). Moreover, the vmPFC is engaged while individuals orient themselves in time (Peer et al., 2015), and determining the temporal distance between the self and an event engages the prefrontal cortex (Gauthier et al., 2019). Yet impaired chronology is again too general an interpretation for vmPFC patients’ performance in this task, as it would have led to an equal distribution of wrong attributions of relative-future events to the past and of relative-past events to the future, while vmPFC patients only showed an increased tendency to falsely recognize relative-future events as past.

According to one prominent view, the timing of one’s memories is not indicated by stable ‘time tags’ marking each of them in succession (Friedman, 1993), but dynamically reconstructed by strategic processes depending on retrieval goals (Burgess and Shallice, 1996). We propose that vmPFC patients’ false recognition of relative-future events reveals a specific deficit in monitoring novelty signals originating from events that, with respect to the current self-position in time, are yet to happen, which felt wrongly familiar. This deficit is reminiscent of other instances of false recognition in vmPFC patients. For example, in recognition memory tasks, vmPFC patients falsely recognize distractors that were targets in previous runs of the experiment (Schnider, 2003), or with which they had pre-experimental experience (Ciaramelli and Ghetti, 2007), as if they failed to appreciate that, in the context of the current run or experiment, they were novel. More generally, vmPFC patients tend to assimilate irrelevant information into activated schemata (Ghosh et al., 2014). One could argue, therefore, that vmPFC patients failed at using a schema of the current reality to identify (future) events that mismatched the schema because they were not previously experienced. This deficit had an impact on recognition of personal as well as general relative-future events, consistent with previous evidence of false recognition in both domains following vmPFC damage (Schnider, 2003; Gilboa et al., 2006; Ciaramelli and Ghetti, 2007), in fact depriving vmPFC patients fully of a view of the future.

We end by noting some limitations and future directions of our work. The sample size is small, and therefore some effects may have gone undetected due to limited statistical power. Future studies involving more vmPFC patients will help confirm the selective deficit we observed in future MTT and link it to specific vmPFC subregions. Also, our results show that, even though control patients did not show a future MTT impairment at the group level, a few of them did, suggesting that other (e.g. temporo-parietal) regions may be causally linked to future self-projection and self-reference. Again, testing this hypothesis will require recruiting larger samples of patients, and with more homogeneous lesion sites than our control patient group. Finally, the task we used can detect whether patients place events correctly in the (relative) past and future, but not whether they would put the events in the correct chronological order with respect to one another within the past or the future. Thus, a future direction of the study is to test whether the disadvantage observed in vmPFC patients in processing future events is limited to the recognition of events, or it would extend to their ordering.

In summary, we have shown that vmPFC patients have a multifaceted impairment of future MTT, being unable to project themselves to the future, and to anticipate the events that await them in the future, pointing to a prominent role of vmPFC in future-oriented cognition. Future self-projection and self-reference have a profound impact on future-oriented choice: taking the perspective of one’s future self reduces discounting of future rewards (Peters and Büchel, 2010), and the anticipation of future events is associated with goal-directed behavior and motivation (Boyer, 2008). Thus, the future-oriented MTT deficit we detected in vmPFC patients is likely to play a role in their steep delay discounting (Sellitto et al., 2010), poor problem-solving (Peters et al., 2017) and apathy (Fellows and Farah, 2005). If so, cueing future thinking (Peters and Büchel, 2010), or the deployment of spatial attention toward future locations of the mental timeline (Anelli et al., 2016b), may be effective in pushing patients’ temporal horizon ahead into the future, reducing their shortsightedness.

Temporal Distancing During the COVID-19 Pandemic: Letter Writing with Future Self Can Mitigate Negative Affect

Chishima, Yuta, I-Ting H. Liu, and Anne E. Wilson. 2021. “Temporal Distancing During the COVID-19 Pandemic: Letter Writing with Future Self Can Mitigate Negative Affect.” PsyArXiv. January 2. doi:10.31234/osf.io/2yw67

Abstract: Novel coronavirus disease (COVID-19) is spreading across the world, threatening not only physical health but also psychological well-being. We reasoned that a broadened temporal perspective may attenuate current mental distress and tested a letter-writing manipulation designed to connect people to their post-COVID-19 future selves. Participants were randomly assigned to either send a letter to their future self (letter-to-future), send a letter to present self from the perspective of future self (letter-from-future), or a control condition. Participants in both letter-writing conditions showed immediate decrease in negative affect and increase in positive affect relative to the control condition. These effects were mediated by temporal distancing from the current situation. These findings suggest that taking a broader temporal perspective can be achieved by letter-writing with a future self and may offer an effective means of regulating negative affect in a stressful present time such as the COVID-19 pandemic.

Seeing others yawn selectively enhances vigilance: An eye-tracking study of snake detection

Seeing others yawn selectively enhances vigilance: an eye-tracking study of snake detection. Andrew C. Gallup & Kaitlyn Meyers. Animal Cognition, Jan 1 2021. https://rd.springer.com/article/10.1007/s10071-020-01462-4

Rolf Degen's take: https://twitter.com/DegenRolf/status/1345601521480855552

Abstract: While the origin of yawning appears to be physiologic, yawns may also hold a derived communicative function in social species. In particular, the arousal reduction hypothesis states that yawning signals to others that the actor is experiencing a down regulation of arousal and vigilance. If true, seeing another individual yawn might enhance the vigilance of observers to compensate for the reduced mental processing of the yawner. This was tested in humans by assessing how exposure to yawning stimuli alters performance on visual search tasks for detecting snakes (a threatening stimulus) and frogs (a neutral stimulus). In a repeated-measures design, 38 participants completed these tasks separately after viewing yawning and control videos. Eye-tracking was used to measure detection latency and distractor fixation frequency. Replicating previous evolutionary-based research, snakes were detected more rapidly than frogs across trials. Moreover, consistent with the view that yawning holds a distinct signaling function, there were significant interactions for both detection latency and distractor fixation frequency showing that vigilance was selectively enhanced following exposure to yawns. That is, after viewing videos of other people yawning, participants detected snakes more rapidly and were less likely to fixate on distractor frogs during trials. These findings provide the first experimental evidence for a social function to yawning in any species, and imply the presence of a previously unidentified psychological adaptation for preserving group vigilance.

Two forms of victimhood (an egocentric one entailing only perceptions of one’s own victimhood, & one focused on blaming “the system”) are confined to neither “actual” victims nor those partisans on the losing side of elections

‘Why Me?’ The Role of Perceived Victimhood in American Politics. Miles T. Armaly & Adam M. Enders. Political Behavior, Jan 2 2021. https://rd.springer.com/article/10.1007/s11109-020-09662-x

Rolf Degen's take: https://twitter.com/DegenRolf/status/1345596994706935810

Abstract: Despite growing recognition among journalists and political pundits, the concept of victimhood has been largely ignored in empirical social science research. In this article, we develop a theory about, and use unique nationally-representative survey data to estimate, two manifestations of victimhood: an egocentric one entailing only perceptions of one’s own victimhood, and one focused on blaming “the system.” We find that these manifestations of victimhood cut across partisan, ideological, and sociodemographic lines, suggesting that feelings of victimhood are confined to neither “actual” victims nor those partisans on the losing side of elections. Moreover, both manifestations of victimhood, while related to candidate support and various racial attitudes, prove to be distinct from related psychological constructs, such as (collective) narcissism, system justification, and relative deprivation. Finally, an experiment based on candidate rhetoric demonstrates that some political messaging can make supporters feel like victims, which has consequences for subsequent attitudes and behavior.

---

Perceiving oneself to be a victim is ubiquitous in American politics. As Horwitz (2018) remarks, “The victim has become among the most important identity positions in American politics” (553). This is no accident. Victimhood is a central theme of modern political messaging. For instance, a Republican strategist observed, “At a Trump rally, central to the show is the idea of shared victimization...Trump revels in it, has consistently portrayed himself as a victim of the media and of his political opponents...” (in Rucker 2019). However, if you consider Trump’s demographic characteristics (white and male) and his successes (in terms of wealth and being president), he is not a victim by any serious societal standard. While Trump’s supporters may, to varying degrees, be victims of certain social and political circumstances, the rallies at which the president is reveling in their shared victimhood are direct consequences of at least their recent political successes.

This narrative of victimization transcends Trump and other political elites. Regular Americans have broadly been considered, or considered themselves to be, victims—of China (Erickson 2018), immigrants (Politi 2015), income inequality (Ye Hee Lee 2015), and much more. It is in the interest of political candidates to cue victimhood, to make their potential supporters feel as though they have been wronged and that she is the best candidate to rectify things. If would-be constituents can be made to feel victimized, regardless of any “truth” of the matter, it may also be possible to demonstrate the relevance of such feelings to immediate political choices, such as voting or issue positions.

We demonstrate that a general sense of victimhood is an important ingredient of various political attitudes, beliefs, and orientations. Specifically, we investigate two manifestations of perceived victimhood—egocentric (i.e., “I am the victim because I deserve more than I get”) and systemic (i.e., “I am the victim because the system is rigged against me”). Much of the existing research on victimhood operates in the critical tradition (e.g., Horwitz 2018), or the concept is measured only very narrowly.Footnote1 We opt for a more general, flexible approach that allows us to record feelings of abstract victimization. Using nationally-representative survey data, we estimate and validate measures of both expressions of perceived victimhood. We find that these measures of victimhood are largely unrelated to political predispositions or sociodemographic characteristics. They are, however, related to, but both conceptually and empirically distinct from, various views of government (e.g., perceived corruption, efficacy, trust), of society and the world (e.g., system justification, conspiratorial thinking, relative deprivation), and personality traits (e.g., agreeableness, emotional stability, collective and trait narcissism, and entitlement). These relationships (or the lack thereof) suggest that perceived victimhood is neither a mere reflection of “true” victim status or previously identified personality traits, nor a post hoc justification for maintaining the status quo. Instead, it cuts across the social and political hierarchy.

More specifically, systemic and egocentric victimhood are also related to 2016 vote choice and candidate support. Those exhibiting higher levels of egocentric victimhood are more likely to have voted for, and continue to support, Donald Trump. However, those who exhibit systemic victimhood are less supportive and were less likely to vote for Trump in 2016. Perceived victimhood also relates to attitudes about a host of racial policies and racial resentment, reflecting the belief that others benefit disproportionately or unjustly at the victim’s expense. Finally, using an experiment with two different types of treatments, we find that both manifestations of victimhood are manipulable, including by elite messages. The sum of our evidence indicates that feelings of victimhood exist in the mass public, can be mobilized by political elites, and can potentially influence support for specific policies and candidates.

Permanently Online—Always Stressed Out? The Effects of Permanent Connectedness on Stress Experiences

Permanently Online—Always Stressed Out? The Effects of Permanent Connectedness on Stress Experiences. Anna Freytag, Katharina Knop-Huelss, Adrian Meier, Leonard Reinecke, Dorothée Hefner, Christoph Klimmt, Peter Vorderer. Human Communication Research, hqaa014, December 30 2020. https://doi.org/10.1093/hcr/hqaa014

Abstract: Concerns have been expressed that permanent online connectedness might negatively affect media user’s stress levels. Most research has focused on negative effects of specific media usage patterns, such as media multitasking or communication load. In contrast, users’ cognitive orientation toward online content and communication has rarely been investigated. Against this backdrop, we examined whether this cognitive orientation (i.e., online vigilance with its three dimensions salience, reactibility, monitoring) is related to perceived stress at different timescales (person, day, and situation level), while accounting for the effects of multitasking and communication load. Results across three studies showed that, in addition to multitasking (but not communication load), especially the cognitive salience of online communication is positively related to stress. Our findings are discussed regarding mental health implications and the origins of stress.

Unfamiliar faces look sicker; familiar ones look healthier; and during a pandemic the ramifications of neither proposition look good

Strangers look sicker (with implications in times of COVID‐19). Paola Bressan. BioEssays, November 20 2020. https://doi.org/10.1002/bies.202000158

Rolf Degen's take: https://twitter.com/DegenRolf/status/1345432495551107072

Abstract: We animals have evolved a variety of mechanisms to avoid conspecifics who might be infected. It is currently unclear whether and why this “behavioral immune system” targets unfamiliar individuals more than familiar ones. Here I answer this question in humans, using publicly available data of a recent study on 1969 participants from India and 1615 from the USA. The apparent health of a male stranger, as estimated from his face, and the comfort with contact with him were a direct function of his similarity to the men in the local community. This held true regardless of whether the face carried overt signs of infection. I conclude that our behavioral immune system is finely tuned to degrees of outgroupness — and that cues of outgroupness are partly processed as cues of infectiousness. These findings, which were consistent across the two cultures, support the notion that the pathogens of strangers are perceived as more dangerous.

Box 1: Why Nonlocal Pathogens Spell More Danger

The findings I report here endorse the principle (nonlocal parasites are treated as though they were more dangerous) but not necessarily the underlying mechanism — coevolution between host populations and their parasites — as originally outlined by Fincher and Thornhill.[19] For example, it has been objected[28] that coevolution might often work in a direction contrary to that presupposed by the theory. Many pathogens are selected to spread best, or do most damage, within their current host population (i.e., to be “locally adapted”;[29] but see[30]) and would cause less, rather than more, trouble in neighboring groups. True, contact between separate groups has led to disastrous epidemics,[20] but such occurrences would have been rare and only relevant to totally isolated populations, rather than to the adjoining communities typical of our evolutionary history.[28] Yet the unfamiliar‐pathogens theory could work even if coevolution entered the picture only occasionally or not at all. We may prove more vulnerable to outgroup parasites not because these are more virulent due to our maladaptation to them, but simply because numerous illnesses leave us with immune cells that respond efficiently to subsequent exposure to the same pathogens as opposed to novel ones (see also[20, 28]). It is indeed telling that, in many species, exposure to new parasites is meticulously regulated. For example, several primates keep a newcomer at the periphery of the group for weeks or months before allowing it in. This admission practice not only makes it likely that any latent infection will reveal itself, but also ensures lengthy low‐level exposure to the alien pathogens — permitting residents to develop immunity to them before the stranger carries them into the group in large numbers.[24, 25]

IT'S A SMALL WORLD

Unfamiliar faces look sicker; familiar ones look healthier; and during a pandemic the ramifications of neither proposition look good. In times of COVID‐19, the only comforting thing that can be said of these findings’ implications is that they are scientifically very interesting. The sicker our fellow humans appear, the more we feel compelled to keep them out of the way — which we do also by becoming unkind, prejudiced, intolerant, and aggressive. There is no escaping the pressure of an infection threat as formidable as COVID‐19 on millions of behavioral immune systems, to the effect that our prospects for world peace and universal harmony are unlikely to take an upturn anytime soon.

Yet the other side of the coin — familiar faces seem healthier — is hardly a compensation when the pathogen we are up against is entirely novel. In unremarkable times the special intimacy we enjoy with our ingroup serves us quite nicely, because we are better adapted to their parasites than to the outgroup's. No human, however, is pre‐adapted to viruses that jump out overnight from bats or camels or pangolins[56] and quickly proceed to invade a globalized and hyperconnected world. Being ingroup or outgroup matters no longer: but not in the sense that one would have hoped for. And of course our ingroup‐loving instincts continue to run as blindly as they always have done. Impressively, comfort with contact with familiar‐looking individuals was higher than the neutral point even in front of a glaring contagion cue (infected ingroup: left panel of Figure 2, rightmost solid symbols). That is, provided they look like community members, unmistakably infectious strangers appear fine to shake hands with and sit close to; strict proximity to them feels more comfortable than uncomfortable. It is not a matter of contagiousness being misconstrued either, because the very same signs keep us well away from those who look different from the locals (infected outgroup: left panel of Figure 2, leftmost solid symbols). With a pandemic ongoing — let alone one that presents with a lack of obvious symptoms[57] rather than with a disgusting facial rash — these findings are disquieting.

The behavioral response of individuals to an epidemic is capable of altering its dynamics with catastrophic consequences.[58, 59] The most effective measure to stop or slow down the spread of airborne diseases like COVID‐19 is to avoid person‐to‐person proximity (in conjunction with wearing masks whenever spatial or temporal separation is less than ideal,[60] which in places shared with others means virtually always[61, 62]). In the face of infection, social distancing is practiced in nature by mostly every species except superlatively social ones, such as bats[63] and mongooses,[64] where group members are connected so tightly that isolation might prove undesirable, and pathogen exposure is inevitable. Of course, spontaneous social distancing is driven by the detection of physical or behavioral signs of infection, which in humans is largely unconscious[5] and not necessarily accurate. For example, we are unable to judge from the sound alone whether a cough comes from someone who is infected or not; disgusting coughs appear more alarming regardless.[65] Here I have shown that identical infection cues can be perceived as more or less threatening, and lead to a stronger or weaker avoidance response, depending on whether they show up on unfamiliar or familiar faces. A weaker avoidance response translates, needless to say, into reduced spontaneous social distancing and reduced compliance with enforced social distancing.

Mathematical models of human epidemics have begun to recognize that not everyone in a population has an equal chance to become infected or infect others. Infections propagate primarily through networks that, being formed by individuals who are habitually in contact, tend to be clustered in space.[66] Yet, unless one is modelling the inhabitants of North Sentinel Island, there also exist rare random links to distant individuals (“small‐world” networks[67]), which permit infection to expand relatively quickly — allowing indeed for multiple epidemics or even pandemics. The findings I have presented hold two implications for disease transmission. First, individuals are more likely to infect and be infected by community members, as opposed to nonmembers, not only because they meet them more often and for a longer time;[68] but also because — on account of perceiving them as healthier and hence safer — they are bound to take fewer precautions upon meeting them. And second, individuals are more likely to infect, and be infected by, strangers who are not even community members but just look similar to them. This bears evident potential repercussions on contagion patterns: predicting as it does, for instance, that at the start of the COVID‐19 outbreak in the USA white Americans might have been less guarded toward (possibly infected) white European tourists than toward healthy fellow Americans of African origin. Thus, perceived familiarity effectively reduces the distance between individuals within a network, changing their probability of both acquiring and transmitting infection. Incorporating properties such as familiarity or outgroupness to alter the weight of the links between individuals may increase models’ realism, with immediate relevance to epidemiology.

I have proposed that facial familiarity decreases infection cues’ perceived threat by serving as a proxy for previous exposure to the same pathogens. Mandrills abstain from grooming contagious mates unless these are close maternal kin (mother, offspring, maternal half‐siblings) and it has been suggested they might be less sensitive to infection cues associated with kin.[69] Note, however, that they treat infected paternal half‐siblings exactly as they treat infected distant kin or nonkin, even though paternal half‐siblings are every bit as related to them as are maternal ones, and can be recognized as kin.[70] Yet of course maternal half‐siblings are exposed to one another a great deal because they are raised together from birth, whilst paternal half‐siblings grow up in entirely different families.[70] I suggest, then, that mandrills’ disregard of contagion when attending to maternal kin might reflect not their genetic relationship, but their larger familiarity with them and hence with their parasites.

CONCLUSIONS: TO THE BEHAVIORAL IMMUNE SYSTEM ALL STRANGERS ARE EQUAL, BUT SOME STRANGERS ARE MORE EQUAL THAN OTHERS

The results described in this paper fit effortlessly what I have called the “unfamiliar‐pathogens” theory — a facet of the general parasite‐stress explanation of human behavior ([19, 53]; see also[16]). Building upon new data, here I have unpacked the basic idea and stretched it slightly in depth and breadth. I have argued that our treacherous cohabitation with parasites has forced on us the compulsion to assess others’ dissimilarity from the people to whom we are usually exposed (our quintessential “ingroup”). Individuals who do not look like them are likely to be coming from elsewhere, carrying pathogens that are novel to us and thus more dangerous. Therefore, our behavioral immune system has specifically evolved to pay “outgroups” the greatest attention, perceiving them as sicker from the start. And because the members of our local, familiar community embody “normality,” and outgroupness is detected as a deviation from that, our aversion generalizes to all deviations from normality. This can only deepen and widen our disinclination to engage with people who are (or we perceive as) malformed, disfigured, disabled, or just “strange” — anomalies that happen to be statistically associated to disease on their own merits.

If the idea laid out here is correct, discomfort with contact should not be confined to actual strangers or atypical individuals but extend to familiar, ordinary‐looking community members we seldom bump into. Indeed, when 30,000 people from 165 countries were asked who was the least likely person they would share a toothbrush with, 2% indicated their spouse, 25% the boss at work, and 60% the postman.[71] Our spouses, bosses, and postmen do not represent increasing degrees of outgroupness in terms of which tribe or village or ethnicity they belong to. They do, however, in terms of how regularly they happen to lavish their own parasites on us.