The Evolution of Disgust

Part Of: Affective Neuroscience sequence
Content Summary: 1400 words, 14 min read.

Introduction

Why did disgust evolve? Why does it play a role in morality? Should it?

One of the best ways to understand an emotion is to build a behavioral profile: a list of its responses (outputs) and elicitors (inputs).

Disgust Responses

One of the striking features of disgust is how diverse its set of responses. These include an affect program:

  • Gape face. This is characterized by a nose wrinkle, extension of the tongue, and wrinkle upper brow.
  • Feeling of nausea. In fact, the physiological signature of intense disgust closely matches physical nausea.
  • A withdrawal reflex. This reflex need not be physical retreat, but can also yield motivation to remove the offending object.

But disgust also produces an inferential signature:

  • Sense of oral incorporation. That is, the subjective feeling that the offending object is already in one’s mouth.
  • Offensiveness tagging. Even after the object has been removed, it will continue to be treated as offensive indefinitely.
  • Asymmetric transmission logic. See the law of contagion: a clean object that touches something gross is contaminated, but not vice versa.

Disgust Elicitors

Even more diverse than its outputs, the elicitors of disgust include cultural universals, including:

  • Organic decay.
  • People and objects associated with illness
  • Compromised body envelope. These include: cuts, gashes, lesions, or open sores.
  • Substances that have left the body. These include feces, vomit, spit.  

Swallowing the saliva that is currently in your mouth is innocuous, but even imagining yourself drinking a glass of spit (even if it is (was?) your own, is disgusting. These last two elicitors are body perimeter tracking: they not only police the boundaries of the body in peripersonal space, but also seem to enforce a no re-entry policy: anything that exits or becomes detached triggers it.

There exists another suite of elicitors that are culturally tuned

  • Specific foods.  Some foods are deemed disgusting even when they have never been tried (e.g., liver).
  • Specific living animals. These can include: flies, maggots, worms, rates, lice, tics, slugs, snails, and spiders…
  • Specific sexual practices. These can include: homosexuality, pedophilia, bestiality, necrophilia, …
  • Specific morphological signatures. Deviations from bodily normality, however that is construed in a particular culture. These can include: the elderly, disabled, little people, …

It is worth emphasizes that disgust over sexual practices and morphological signatures varies widely across cultures and across individuals. For example, ancient Greece mostly didn’t find homosexuality disgusting but 20th century Americana mostly did.

Finally, people comprise another category of elicitors.

  • Moral transgressors. These can include: murderers, rapists, …
  • Members of an out-group. These can include: untouchable caste, Jews (in Nazi Germany), …

Neuroscientific data suggest that, when people are deemed sufficiently disgusting, brain areas associated with mindreading become deactivated. This is likely the neural basis of dehumanization.

The Entanglement Thesis

Taken together, here is the behavioral profile of disgust:

disgust_ behavioral profile

Puzzle: Why should the sight of a person with leprosy evoke a gape face and a feeling of nausea? Leprosy has nothing to do with digestion.

Solution: Disgust is a kludge! It is the unholy merger of two separate systems.

Poison monitoring is a faculty of the digestive system. It evolved to regulate food intake and protect the gut against ingested substances that are poisonous or otherwise harmful. It was designed to expel substances entering the gastrointestinal system via the mouth. It also acquires new elicitors very quickly.

Infection avoidance is a faculty of the immune system. It evolved to protect against infection from pathogens and parasites, by avoiding them. Not specific to ingestion, but serves to guard against coming into close physical proximity with infectious agents. This involves avoiding not only visible pathogens and parasites, but also places, substances and other organisms that might be harboring them.

Any theory of disgust should explain the unity of responses to disgust. Here is how entanglement theory does it:

  • Poison monitoring produces the affect program. Gape face, nausea and withdrawal all serve digestive (and not immunological) purposes.
  • Infection avoidance produces (most of) the inferential signature. The tendency to monitor disgusting things even when not immediately exposed, and the asymmetric logic of contamination, make perfect sense when tracking the spread of parasites.

Any theory of disgust should explain the diversity of elicitors of disgust. Here is how entanglement theory does it:

  • Poison monitoring is sensitive to certain foods (namely, those that are associated with toxicity)
  • Infection avoidance explains the aversion to certain living animals (flies are more likely to carry disease than dogs), apparently disease-infected substances, to certain sexual practices (sexual practices can bring increased risk of disease) and morphological deviations (e.g., violates of facial symmetry correlate with parasites). It also explains the general tendency for disgust to monitor the body perimeter: which is, after all, how pathogens can enter the body!

Any theory of disgust should explain cultural variation of the elicitors. Here is how entanglement theory does it:

  • The poison monitoring system is very quick to learn features the Garcia effect: one-shot learning.
  • In women, aversion to deviant sexual practices (and not other forms of disgust) vary with where they are in the ovulation cycle.

disgust_ entanglement thesis

Besides the increase in explanatory power, phylogenetic and ontogenic data also support the independence of these two systems:

  • Researchers disagree whether disgust is unique to humans, or whether homologies exist in the animal kingdom. Both are right: animals show clear signs of the existence of both systems but the systems are expressed separately.
  • Ever wonder why children don’t seem to mind disgusting objects & behaviors? It is because poison monitoring appear very early (within first year of life) but infection avoidance emerges significantly later.

The Evolution of Disgust

Why should the poison avoidance and pathogen monitoring have become entangled in the course of human evolution? Why didn’t poison avoidance become entangled with e.g., FEAR instead?

First, the two systems both care about digestion. Food intake can bring both poison and pathogens into the body, and as such it is monitored by both systems.

Why did entanglement only happen in humans, specifically? Compared to other primates, early hominids adopted a unique lifestyle, that combined scavenging with a nascent ultrasociality. These two characteristics put enormous adaptive pressure on the pathogen avoidance system to innovate.

Perhaps the most important reason for entanglement has to do with signaling. As hominids began to increasingly emphasize social cooperation, there became a need to communicate pathogenic information. Before the emergence of language, the pathogen avoidance module had an inferential signature – but how to communicate this contamination tagging information with others? The functionally-overlapping toxin monitoring system had a clearly visible output: the gape face. Plausibly, the two modules merged such that pathogen monitoring system could co-opt gape face to communicate. We can call this the gape face as signal theory.

My Take on the Theory

The theory I have presented here was developed by Daniel Kelly’s book Yuck! The Nature and Moral Significance of Disgust. The theory strongly complements Mark Schaller’s work on the behavioral immunity system. The overlap between these two researchers will become clear next time, when we turn to the social co-optation of the disgust system.

I personally find the entanglement thesis (the merger of toxin monitoring and pathogen avoidance systems) compelling, given its tremendous explanatory power outline above.

Despite accepting the overall architecture, Kelly’s theory for why the architecture evolved (gape face as signal) strikes me as incomplete.

I also feel like this theory will remain incomplete until we discover how toxin monitoring and parasite avoidance are implemented in dissociable neurobiological structures (i.e., modules).

After the psychological mechanisms are mapped to their physical roots, we could attempt to integrate our knowledge of disgust with other systems:

  • What is the relationship of disgust to the generalized stress response? Stress & the immune systems co-evolved to share the HPA axis, after all.
  • How is disgust implemented in the microbiome-gut-brain axis, which also has links to both the digestive system (enteric nervous system) and the immune system (e.g., leaky gut)?
  • How does the MGB axis differentially produce both disgust and other social phenomena like anxiety?

Open questions are exciting! To me, it suggests a clear research program where we can start integrating our newfound theory of disgust into the broader picture of visceral processes (the hot loop).

Takeaways

The human brain comes equipped with two systems:

  1. Poison monitoring is a faculty of the digestive system. It evolved to regulate food intake and protect the gut against harmful substances.
  2. Infection avoidance is a faculty of the immune system. It evolved to protect against infection from pathogens and parasites, by avoiding them. 

In humans, these two systems were entangled in the emotion of disgust. This explains the otherwise baffling diversity of disgust elicitors & behaviors.

Related Resources

  • Kelly (2013). Yuck! The Nature and Moral Significance of Disgust.
  • Fessler & Haley (2006). Guarding the Perimeter: the inside-outside dichotomy in disgust and bodily experience.
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