That’s disgusting! The behavioral immune system

Disgust, from The Expression of the Emotions in Man and Animals, by Charles Darwin.

According to Charles Darwin, “extreme disgust is expressed by movements round the mouth identical with those preparatory to the act of vomiting.”

Recently disgust and its associated behaviors gained attention as an unappreciated arm of the immune system. This idea was recently explored in a paper by Steve Gangestad and Nick Grebe at the University of New Mexico: Pathogen Avoidance Within an Integrated Immune System: Multiple Components With Distinct Costs and Benefits.

Most of us take our immune systems for granted. Having an immune system allows us to interact with other individuals, the environment, and their respective microbiomes and not get sick, at least most of the time. If we encounter a dangerous microbe, the innate and adaptive arms of the immune system generally prevent invasion and disease.

Though effective, non-specific innate immune defenses and more targeted antibody-directed immunity have significant costs. These defenses are expensive both in terms of energy, friendly-fire damage to our own cells, and premature aging. It might be better for the organism to avoid pathogens in the first place.

Gangestad and Grebe are among the scientists who have shown that organisms demonstrate behaviors that decrease their contact with pathogens. These behaviors and the physiology responsible for them collectively comprise the behavioral immune system.

Behavioral adaptations involving pathogen defense, like every trait, involves both costs and benefits. These trade-offs were recently cataloged in Gangestad and Grebe’s recently published paper in Evolutionary Behavioral Sciences.

Gangestad and Grebe posited that sensitivity to disgust might be heightened in people who live dangerously, so to speak. These would be socially capable extroverts who have more interaction with other human beings. On the other hand, introverts with limited interaction with other humans might be less sensitive to disgusting stimuli, things like a bleeding cut, body odor, etc.

In line with these expectations, Gangestad and Grebe found that pathogen disgust was associated with social potency and extroversion. The authors conclude that “these findings offer provisional evidence that pathogen disgust elicited by human contaminants is more potent in individuals likely to experience higher, not lower, encounter rates with novel others.”

From the paper:

“The ideas about the behavioral immune system recently put forward have created much excitement, and for good reason. It makes perfect sense that a fully functional, adaptive immune system would not be designed to passively wait for pathogens to arrive, only to prepare for that arrival. Rather, a major function expected of an adaptive immune system is avoiding contact with potentially harmful pathogens in the first place. Specific ideas about the nature of behavioral pathogen avoidance tactics (e.g., sensitivity to disgust, withdrawal from specific social situations, ethnocentrism) have been empirically highly generative and productive.

It seems to us that the behavioral immune system should not be thought of as an entity distinct from the “classical” immune system. Neither should it be thought of in terms of a unitary trait defined in terms of “strength” of this system (cf. Terrizzi, Shook, & McDaniel, 2013). Rather, in our view, it makes sense that, just as specific tactics of and allocations to immunoresistance are multiple and regulated by sets of costs and benefits particular to each, so too might different tactics of avoidance. Allocations to specific avoidance tactics may well be adaptively regulated in concert with resistance functions (in many cases via shared physiological mechanisms) as part of an integrated immune system.”

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