Fingernails on a blackboard, or cat claws scraped across an old chalky car hood! Or... a steel garden hoe hitting a smooth dry rock. Or tear a ball of cotton between your fingers. Write with a nearly-dry magic marker. Trying to saw through a brick with a steak knife. Rub a block styrofoam across cardboard. EEEEEEEURRRRRRRRT!!!

How can a simple sound make you feel so awful? Why do humans seem programmed to avoid it? We want to make it stop right freakin' now.

The answer came to me like a blinding flash. I was eating something at a picnic and I dropped it on the ground. I wiped it off and continued eating. (Oh, you DO SO do it too!)

As I was chewing, suddenly I heard SKKKKEEEEEERRRCH!!!!! ...as I bit down hard on a tiny stone. I think every single hair on my body stood on end, and my jaws froze instantly.

THAT'S IT! Fingernails-on-blackboard: it sounds exactly like the destruction of tooth enamel. It's not learned. Instead, we're instinctively programmed to respond instantly. Of course! It's so sensible and obvious. Every little kid knows it. I remember many incidents from my own childhood. Why didn't we adults ever realize? The scraping of fingers on a blackboard is the classic, high-frequency violin-like waveform of hard dry surfaces moving with chaotic stick/slip motion. And that could very well be why our instincts are programmed to repond to it so strongly, as strongly as a pain signal.

It's the sound of body damage; but it's a particular type of body damage for which there is no pain ...yet no healing.

We get no second chance with teeth. If we bite down on rocks, we wreck our enamel, and that could be why fingernails-on-blackboards makes everyone around us take drastic action to halt that noise. Why didn't anyone realize this origin? It's because we're too damned civilized, and we rarely have rocks in our food anymore. But whenever we bite down on something which is far harder than tooth-enamel, our inborn pain-avoidance instantly informs us about the problem in no uncertain terms. Sensible? Flesh can heal, but tooth surfaces do not.

I had rather hear a brazen canstick turn'd,
Or a dry wheel grate on the axle-tree;
And that would set my teeth nothing on edge...
- Shakespeare, Henry IV

Is the above idea actually correct? Who knows. Well, I think mine's a much better theory than the recent paper about the Macaque warning calls. And I haven't seen any papers about teeth-on-edge written elsewhere. Perhaps scraping our fingernails on blackboards isn't making fellow primates stop and look for danger. Perhaps it's worse than that, since if the sound is a form of pseudo-pain, then it's forcibly informing all the reptillian brains of all the nearby organisms to stop biting that rock! NOW!!

Predictions

If I'm right, then the type of audio waveforms which trigger our revulsion will have major similarity to the audio waveforms heard via tissue/bone conduction when our teeth suffer damage by scraping across a hard object.

This might not necessarily be a particular sort of spectrum shape. If phase between spectral components can be sensed by brains, then time-domain features in the waveform itself might be more important for detecting tooth-damage than features in it's frequency spectrum. Or, perhaps the internal bone-conduction signal is the avoidance signature, and if sounds in air gets filtered so they resemble the tooth-damage signal inside tissues, that triggers our avoidance response.

Ooo! Idea! Maybe this auditory sensing of tooth-damage is why mammal/reptile/etc. ears evolved to be near their jawbones in the first place? If ears tended to move much farther away with time, then the bone-conduction sound wouldn't work very well, and that horrible noise of squealing tooth-enamel wouldn't be so... informative.

Going even farther: what if ears originally evolved to be tooth-damage sensors?

This would explain where the sense of hearing came from in the first place. Preventing constant bodily damage is an important advancement in the evolutionary track. The ability to detect the sounds of approaching underwater predators might have just been a fringe benefit, a spinoff from the vibrational tooth-damage detector in the jawbone. This leads to another prediction: the atmospheric sound channel wouldn't necessarily have the same evolutionary reinforcement as the through-flesh internal or 'bone conduction' channel. Someone should check to see if the frequency and phase detection of the internal signal channel has any vast differences from ears. Evolution would be expected to redesign ears to become excellent at the two very separate tasks (at least in toothed animals, not in all of them.) In particular, ultrasound is suppressed by air, so we might expect that the internal through-tissue channel might evolve to include the extreme shortwave ultrasound signals of enamel-destruction far about 20KHz, while the gas-vibration channel might have relatively little need for such wideband detection.

Subjectively to its victims, fingernails/blackboards seem very similar to a pain signal. Instant inbuilt avoidance! If you want to give yourself the willies, imagine biting down hard upon a dirty iron bar or on a smooth, dry pebble. Imagine grinding your teeth back and forth on the hard rough object so it squeaks... and CRUNCHES. (Almost as bad as imagining thorns under fingernails, or sharp branches poking towards your eyes.) The surface of the chewed pebble is just like chalkboard slate, but this time it's not your fingernails making the noise, it's your teeth. Yeesh. For me all this is like like visualizing stroking my fingertips along a knife edge, or watching Buñuel's sliced-eyeball film (Heh. Another "instinctive avoidance algorithm" programmed deeply. I call it by the name Triggered Creepout Effect: the everyday evidence of mammal instict remaining in humans.)

Now that I think about it, this might also explain the reason for the existence of "baby teeth." If pain doesn't work very well in stopping certain destructive behaviors, and if our instincts and our biology need to somehow *teach* us not to bite down hard on rocks... then it will take some time for us to get the hang of it. We could have the pseudo-pain avoidance response built in, but we don't instinctively know that bone is OK to knaw, but pebbles are not. But with teeth, we only get one chance. First, the baby arrives pre-programmed with chalkboard-noise sensitivity. Then baby bites on rocks a few times, experiences revulsion-sound, and learns not to do this any more. But by then it's too late, and Baby's teeth have suffered significant damage. No matter. Baby can shed teeth once. Finally no big need to shed teeth in adulthood, since the acoustic pseudo-pain already had taught Baby the lessons earlier... don't knaw rocks or mommy's socket set. Hmmm. In the same vein, I wonder if our sensitivity to fingernails/blackboard sound decreases in adulthood? As with childhood pain extrasensitivity, the pseudo-pain could wither away without major consequence. Baby animals would exhibit far more need to be sensitive to the signal. If we ever test whether non-primate animals are sensitive to the teeth-on-edge effect, we should also test across age, check out puppies or kittens for a human-like response childhood extreme response.

Suppose an animal ignores the tooth-damage sound. Why didn't we just evolve in such a way that teeth are rapidly replaced, like rats and sharks? But this constantly repeated teeth-shedding would cause problems for mammals. A fighting animal can't afford to be missing some teeth unnecessarily. Over tens of thousands of years, the few humans who have ongoing "multiple dentitions" might get into trouble during the times their old teeth are falling out and the new ones haven't yet grown back. Unless their teeth were almost certainly going to get ruined over time, there would be no good reason for new teeth to grow. Better just learn to avoid biting stone adzes and flint scrapers and obsidian arrowpoints and socket sets, and instead evolve the system to where teeth aren't being replaced.

Yet as above, there might be one significant event where a new set of teeth would give an overall payback: a one-time repair for the months of trial and error during the babyhood knawing stage. Hey, nature gives us a cheap set of "training-teeth" to destroy, before the long-term serious ones appear.

All speculation, of course. But it's like a missing puzzle-piece evolutionarily. Man o man, look at the explanatory power.

If I'm right, then someday the trivia-gam experts will know that there's a clear connection between fingernails on blackboard, the position of human ears and jawbone, and the need for our "baby teeth."

And when people say that the sound of fingernails on blackboards always "sets their teeth on edge"... or if they call it "teeth-gnashingly annoying"... we should leap up and shout "eureka That's IT!!!"

PS
Fingernails-on-blackboard is pseudo-pain. If I'm right, then any toothed animal is programmed to respond to it. That means we can build pain guns! Pain guns that repel barracudas, and make mollusks crawl away from us as fast as they can. Or actually "pain" guns. If you're being attacked by a predator, just take out your patented Bill Beaty Tooth-Annoyatron&tm; with Evolutionary Optimized Waveform&tm; and press the button. The agonizing high-power recordings of garden hoes scraping across dusty shale sidewalks will send those sharks or cougars racing away... while shaking their heads and holding their jaws carefully apart!

:)

LINKS

• Tooth grinding video (arrrg. Send to your friends!)
• Trigger warning Styrofoam
• FMRI of glass scraping w/fork 2012 Kumar
• WP: chalkboard scraping
• misophonia.com
• Scraping sounds and disgusting noises Applied Acoustics v69 #12 2008
• SCIENCE Oct 2011
• mp3 archive: school, chalkboard
• Ace Ventura 2: knife on old dinnerplate
• World's worst sound
• Why fingernails/blackboard is so annoying (The Straight Dope)
• Sci Canada: fingernails on chalkboard
• Sharks hate that sound?!! (Oooo! product idea!)

• Auditory preferences and aesthetics JH McDermott 2012 MIT
• Acoustic features aversive soundsS. Kumar et al 2012 J. Neuroscience
• Psychoacoustics of chalkboard squeaking Oehler 2011
• Psychoacoustics of a chilling sound
• google scholar: "chilling sound"
• google scholar: "unpleasant sounds"
• Google: Halpern et al paper
• auditory.org, message thread
• ABCnet AU, forum discussion