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.