Thursday, January 26, 2012

That's disgusting! Make up your own Just-So story about the evolution of an emotion

The evolution of disgust
Everyone seems to be talking about disgust these days, from why it evolved to what parts of our brains light up when we feel it (it's the anterior insular cortex).  There was a story in the NYT about it on Tuesday ("Survival's Ick Factor"), and a review of a new book (one of many) about it in the Sunday NYT Book Review, a conference in Germany, and an issue of the Philosophical Transactions of the Royal Society devoted to the subjectDarwin included disgust in his list of the 6 basic human emotions, and wrote of seeing it on the faces of his infant children. 

Indeed, it seems that disgust now explains many human characteristics from tribalism, to disease avoidance, to poison critter avoidance, and mate choice.  And, disgust gone haywire explains psychological pathologies from obsessive compulsive disorder to excessive anxiety.

A paper in Perspectives in Biology and Medicine in 2001 lists the basic disgust elicitors. 
We suggest that the objects or events which elicit disgust can be placed in the following five broad categories:
1. Bodily excretions and body parts
2. Decay and spoiled food
3. Particular living creatures
4. Certain categories of "other people"
5. Violations of morality or social norms
Bodily secretions are the most widely reported elicitors of the disgust emotion. Feces appear on all of the lists, while vomit, sweat, spittle, blood, pus, and sexual fluids appear frequently. Body parts, such as nail clippings, cut hair, intestines, and wounds, evoke disgust, as do dead bodies. Certain animals are repeatedly mentioned, in particular pigs, dogs, cats, fish, rats, snakes and worms, lice, cockroaches, maggots, and flies. Spoiled food, especially meat and fish, and other decaying substances, such as rubbish, are disgusting to many respondents. Certain categories of other people are also found disgusting, notably those who are perceived as being either in poor health, of lower social status, contaminated by contact with a disgusting substance, or immoral in their behavior.
And then there are sensory cues, smells, feel, sounds.  A number of writers explain that all these things are disgusting because they remind us of our animal -- unhealthy? -- origins.  Others say it evolved to defend body and soul from pollution (as apparently being reminded of our animal origins pollutes the soul).  
In their exploration of Darwinian medicine, Nesse and Williams (Evolution and Healing, 1995) suggest that an instinctive disgust may motivate the avoidance of feces, vomit, and people who may be contagious, and that disgust is one of the mechanisms crafted by natural selection to help us keep our distance from contagion. Pinker (How the mind works,1998) proposes that disgust is "intuitive microbiology," and that this explains our aversion to objects that have been in contact with disgusting substances: "Since germs are transmissible by contact, it is no surprise that something that touches a yucky substance is itself forever yucky." 
It's nice that this emotion is finally getting the attention that it clearly deserves.

But wait a second!
Except -- there had to be an except! -- except that a lot of this starts to sound suspiciously like just another elaborate evolutionary Just-So story.  New parents, nurses, physicians all quickly lose any disgust at bodily excretions, and one person's spoiled food is another's delicacy.  Just think of the rich array of foods that people on this planet eat.  Not to mention dogs, who'll eat just about anything.  Dogs share many of our emotions, and, if essentially all humans feel disgust, our sense of disgust had to have evolved earlier than we did, so shouldn't other lineages who share our disgust-feeling common ancestor, such as dogs, also share our supposedly instinctual disgust with eating, say, rotten meat, or vomit?

Dead Zambian shrew, not Holly's shrew
Which may explain why Holly reports holding up a dead shrew to her two dogs and finding that they wouldn't touch it.  She says her dogs would happily tear apart a dead squirrel, but not the shrew.  She thinks maybe it died of pesticide poisoning, though she couldn't smell anything.  Were they disgusted (by at least this one thing!), thus saving themselves from pesticide poisoning?  Or is it that they have learned to tear apart squirrels and not shrews?  Who knows?

But then, why is it disgusting to some people to eat insects, while others thrive on them (roasted, chocolate covered, etc.)?  Or why did Americans once disdain disgusting lobster....and now drop big bucks for a nice, juicy claw?  European Americans recoil at the thought of eating horse meat, while to many of their Old World brethren it's a delicacy. Or what about latakia pipe tobacco and lapsong suchong tea, 'cured' as one might say, over dung fires?  The list could go on and on and on, but what it means is that there's an obvious learned component.

But, let's agree for the sake of argument that disgust as an emotional reaction in fact evolved as a specific trait.  And even that disgust might have its uses (though, too much of it can be a problem).  All this means is that, as other successful traits that have stood the test of evolutionary time, disgust itself is adaptable.  That is, yes, we may all feel disgust, but what disgusts us at any given time is culturally determined, not innate.  Otherwise, how could we learn that Twinkies were disgusting?  (Or not.  It turns out that if you search in Google Images for Twinkies, you'll find a photo of Twinkies Fondue; Twinkies, circus peanuts, caramel Ho-Hos, marshmallows, and candied orange slices on a skewer, waiting to be dipped into molten chocolate.  Or Scottish deep-fried Mars bars!  Who thinks these things up?)

Our better idea!
And any of us can think of alternative hypotheses as to what disgust is 'for'.  Here's ours -- how about that it's part of our repertoire of communication, rather than an innate ability to save ourselves from decaying meat?  Why would we need a facial expression that communicates disgust if the emotion itself were the survival tactic, alerting us not to eat that rotting wildebeest?  Surely we could teach our children that even bunny rabbits were disgusting, if we started them young enough.  So, in adaptive terms, it's communicating that we're disgusted that's important, not what we're disgusted by.  Why?  Because it elicits caretaking, a survival tactic if there ever was one. And of course survival is very directly tied to evolutionary fitness.

But all this hoopla about disgust is a bit disgusting itself.  Are we really desperate to have specialties so that someone can be called by the NY Times "a pioneer of modern disgust research"? It's one thing to specialize, even to this extent, and perfectly legitimate to identify 'disgust' and try to understand its neurophysiology and physiological triggers -- if there really is an 'it'.  But it's quite another big step to attempt to Darwinize something so vague, and the fact that Darwin mentioned it doesn't change that.  Evolutionary scenarios are hard to pin down, even with well-defined traits.  The evidence by and large suggests that most of the human versions of this emotion, if it is a particular emotion, are learned and experiential and culture-specific -- adaptable. 

Obviously the inherent aspects, the 'adaptive' aspects of our disgusting behavior are unclear, hard to identify, harder to prove, and in any case it is not obvious that we have any such adaptations that were not in place eons before a human ever stepped on a wildebeest patty (barefoot--UGH!).


Erick R. said...

One example, cullled from the annals of parenthood, that would refute the disgust as instinct - toddlers & poop. Any parent will tell you that toddlers have no instinctive disgust of it - they want to touch it, see it, smear it on things, etc. Just a thought.

Anne Buchanan said...

Good point! Is it to late to teach her that bunny rabbits are disgusting? Just in the interests of science?

Erick R. said...

Too late for that I'm afraid - especially given the fact that we've had to read Curious George and the Bunny, and Sesame Street Bunnies for Easter constantly in the last few weeks.

Holly Dunsworth said...

This post is so awesome Anne!

(p.s. shrew saliva is toxic... another thing my dogs may have been disgusted by, but do not have the primate facial muscles to express it in our language)

Holly Dunsworth said...

(... or the evolved propensity to communicate it with others...)

Anne Buchanan said...

I think these are just very smart dogs. They've heard you talking about shrews and their toxic saliva.

Though, if that's not the explanation, and somehow they can pick up a toxic scent, they don't even need to filter it through disgust. They can simply register that it's toxic and so not eat it.

edward hessler said...

I can't resist but should the way in which Steven Pinker introduced his consideration of digust in "How the Mind Works." Alas, I thought of it immediately upon scanning Dr. Buchanan's post. He used the "fondly remembered camp song, sung to the tune of 'The Old Gray Mare'. I recall singing it.

Great green gobs of greasy, queasy gopher guts,
Mutilated monkey meat,
Concentrated chicken feet,
Jars and jars of petrified porpoise pus,
And me without a spoon.

Obviously, I've significantly (in the colloquial sense not statistical) lowered the level of discourse but it resonates with Dr. Buchanan's closing word" "UGH!".

Anne Buchanan said...

That was a great song! And it was so much fun to get grossed out singing it! (And, this brings up another possible purpose for disgust -- the bonding kids do over grossness!)

edward hessler said...

Thanks so much for asking/helping me to think about bonding in a new way.

Holly Dunsworth said...

And they obviously are heartless and care little whether I eat it (and die), or else they'd show disgust!

Anne Buchanan said...

That, or they simply take for granted that you are wise enough not to eat a toxic shrew!

James Goetz said...

I suppose that the notion of human disgust is an undebatable reality. But the objects of disgust are another story. Both beauty and disgust are in the eye and nose of the beholder. : -)

Holly Dunsworth said...

Thanks for the song resurection!

James Goetz said...

Hmm, perhaps some cultures appreciate/appreciated tiptoeing through wildebeest patties.

John R. Vokey said...

Watson and Rayner, 1920, and the Little Albert experiments.

Anne Buchanan said...

Fascinating. (Though, rather like Nazi experiments -- awful that they were done, but everyone's happy to have the data?)

So, tabula rasa?

Ken Weiss said...

I didn't know of this, and the 1920 date is interesting. I intercepted psychology in the '60s, the tabula rasa operant conditioning (BFn Skinner) era, that largely denied inate aspects of behavior, if I can generalize casually in this way.

I've assumed that was in part a reaction (or revulsion, speaking of disgust!) to the Nazi and eugenic abuses that were based on inherited inherency of individuals' traits.

There was also the Freudian first-moment theories flying around. They, too, were environmental or experiential rather than genetic.

As memory faded, we again became complacent about the potential societal devils latent within some views of strongly deterministic Darwinism. We are now blase, naive, or (hopefully) correct about that being behind us. So now everything can be avidly claimed to be 'genetic'.

If personalized medicine works, then for exactly the same reasons, personalized prediction of other things like personality and the like will automatically be assumed to be just as predictable.

Since NIH funds this stuff, and social scientists may not be solving social science problems very effectively, the turn to genetics and its grants and cachet seems a natural trend.

But perhaps I've misunderstood the pre-genetic, post-eugenic Skinnerian period?

Anne Buchanan said...

The experiments are described here. I particularly enjoyed Watson and Rayner's comment, after describing how they conditioned Albert to fear rats and rabbits:

"The Freudians twenty years from now, unless their hypotheses change, when they come to analyze Albert's fear of a seal skin coat - assuming that he comes to analysis at that age - will probably tease from him the recital of a dream which upon their analysis will show that Albert at three years of age attempted to play with the pubic hair of the mother and was scolded violently for it. (We are by no means denying that this might in some other case condition it). If the analyst has sufficiently prepared Albert to accept such a dream when found as an explanation of his avoiding tendencies, and if the analyst has the authority and personality to put it over, Albert may be fully convinced that the dream was a true revealer of the factors which brought about the fear."

Holly Dunsworth said...

Just saw this!

A very recent article describing some major problems.

Anne Buchanan said...

Very major problems. It begins to sound even more like Nazi medicine.

But then one has to wonder -- does discrediting these experiments make it safe for genetic determinism again?

Erick R. said...

Apparently, someone already made an infant afraid of fuzzy creatures in the name of science:

Holly Dunsworth said...

See thread at top of thread

Anonymous said...

Ha! Totally missed that. Sorry.

Psi Wavefunction said...

I applaud you for putting a bit of a brake on the rabid adaptationist storytelling that seems to correlate strongly negatively with the amount of actual facts and data available to work with...

The stark cultural differences really pile up against specific instinctive disgusts -- as an immigrant growing up in two cultures, it was interesting to see the types of things that disgusted my friends at school and my parents at home, and they were very different. My parents (and most other Russians, it seems) recoil at the thought of sitting on the ground/floor/pavement without something lining it; in American elementary schools, sitting on the floor for some sort of storytime was a routine activity that phased no one. Conversely, I'm perfectly content with eating horse, rabbit and odd body parts like tongues (delicious!), while my North American friends find that absolutely horrifying. The list can go on, and on. I wonder if people with multicultural experience tend to find less things disgusting, although that could perhaps also result in more disgust being learned. Must be someone out there who's looked into comparing that, would be interesting data...

But then there's odd things that seem to be learned past any logical level. I don't find much disgusting, but can't stand mould on food. Fun fact is that I'm a biologist and absolutely adore moulds on agar plates, micrographs, out in nature... fungi are gorgeous! I've taken a mycology course even... but still, the sight of mouldy food is disturbing, even though some part of me also thinks it's really cool. It's as if there was already some 'system' for this irrational response already in place, and it somehow got set up (through learning? maybe?) to be triggered by a particular stimulus. Perhaps aversion to mould itself is a little too removed from direct genetic and epigenetic regulation to be molded by selection directly, but there may well be some deeper, more general mechanism that is then 'primed' during the life of an organism. Of course, finding that requires a better understanding of neuropsychology first, but overall would be a more rational pursuit than unrestrained storytelling!

Ken Weiss said...

Adaptation has become an axiom, and that means that every trait is _assumed_ not just to be viable, which it manifestly is if people live with it, but must have an adaptative explanation. So it is only logical to suggest it.

The problem is that one should be questioning the axiom. Our current series on Darwinian method deals a bit with that, and it was debated in Darwin's own time.

Selection is viewed by some as a tautology (equivalent to 'survival of the survivors', and therefore without any scientific content, since it is true by definition. But I think more to the point is that it is an axiom: truth accepted rather than possibility to be tested.

Of course, there is resistance, largely from the 'neutralist' school of thought, that doesn't deny the obvious possibility of selective adaptation, but in a given situation asks when, where, and whether something is due to specific adaptation.