Few sounds cause humans to cringe more than nails across a chalkboard, a fork scraped on a plate, or a heavy metal chair dragged across a tiled floor.  But what exactly is it about this sort of scraping noise that is so offensive to our brains that many even describe it as painful?

Enter researchers D Halpern, James Hillenbrand, and Randolph Blake. In 1986 the trio of scientists conducted a study trying to figure out what exactly in these sounds humans the world over hate so viscerally. In the study, published in the journal Perception & Psychophysics, the researchers were hypothesizing that it was the high pitched sounds that were causing the issue, and thus isolated the sounds between low, middle, and high frequencies. After playing these recordings to subjects, remarkably, they found they were wrong. As Dr. Blake noted, “To our surprise, the removal of the high frequencies didn’t reduce the aversive qualities of the sound, but removing the middle frequencies of the sound did.”

From there, when looking at these middle frequencies, they hypothesized that this must resemble either that of a sound produced by a predator or a warning cry of another primate. They ultimately found, indeed, as stated by Blake, “It turns out the sound waves associated with primate warning cries, particularly chimpanzee warning cries, are remarkably similar in appearance to the aversive, middle frequency sound waves produced by fingernails on a chalkboard…”

Potentially worth noting is that a chimpanzees-human primate was the last common ancestor between humans and our hairier brethren, thought to have split around 13 million years ago or so, though hypothesized by some to still be interbreeding up to around 4 million years ago.

In any event, Blake then sums up, “Our speculation was that the reason the sound of fingernails on a chalkboard have an almost universal aversive quality is that it triggers in us an unconscious, automatic reflex that we’re hearing a warning cry.”

That said, it is widely reported that this hypothesis has been debunked by a study, Are Consonant Intervals Music to Their Ears? Spontaneous Acoustic Preferences in a Nonhuman Primate, looking at how tamarin monkeys responded to certain sounds, including nails on a chalkboard-like screeching and white noise. However, if you actually go read the study, it didn’t definitively debunk anything with regards to the former study, nor was that the point of the new study at all. Most pertinent to the topic at hand, all it showed was that tamarin monkeys didn’t seem to show any more or less aversion to the screeching noise than white noise, unlike humans who are averse to the screeching sound.

As to the debunking angle, this appears to be yet another case of something a lot of sources say because some source somewhere said it once, both clearly not bothering to read the study and acting like a single study was a lot more definitive than it actually was- a favorite practice of media outlets the world over pretty much as long as there have been scientific studies and media outlets reporting on their work. From there, nobody else seems to have been bothered to go actually read the study either, despite it being their job when talking about this stuff.

On this one, it turns out humans and monkeys are not exactly the same (shocker, we know) and have slightly different evolutionary paths and in some cases range of vocals. As an example of how this can come into effect with sounds and tamarin monkeys, we have a 2010 study by Professor Charles Snowdon of the University of Wisconsin-Madison looking at frequencies of sound that tamarin monkeys do respond to. Important to note here is that the tamarin monkeys’ resting heart rate is about twice that of humans and their typical vocalizations are about three octaves higher.

In the study, they found these monkeys responded most strongly to music centered around their vocal frequencies and tempo of their heart rate, both in the sounds’ ability to agitate the monkeys and to calm them. Further, the tamarin monkeys had no such responses to human music played for them. This is a common theme several studies show is true across various animals with different resting heart rates and vocal ranges than humans. For more on this, see our article, Do Dogs and Cats Like It if You Leave the Radio or TV on When You Leave the House?

In any event, unsatisfied with this incomplete and fairly speculative answer of the 1986 study, researchers Michael Oehler and Christoph Reuter decided to conduct a study ultimately titled Psychoacoustics of chalkboard squeaking, published in the Journal of the Acoustical Society of America in 2011. This more or less repeated the previous 1986 experiment, but using modern technologies and collecting a lot more data, particularly looking at heart rate, blood pressure, and skin electrical resistance of the subjects as they listened to the sounds of nails on a chalkboard, squeaky Styrofoam, forks scraping on a dinner plate, and chalk making a high pitched squeaking sound against slate.

What they found is that if they filtered out the sounds from 2000-4000 Hz, subjects actually sometimes even described the sounds as pleasant. In contrast, when the offending isolated range of sound was played for people, they hated it, confirming it is somewhere in the ballpark of these frequencies that are the offending bits. Noteworthy here for reasons we’ll get into shortly, typical human speech tends to average around 300-3000 Hz, though for reference humans can hear up to somewhere in the vicinity of 16 kHz- 20 kHz.

Going back to the study, while the researchers further found if they, for example, told the participants the sounds they were hearing were from a musical arrangement, people found them much less offensive than if they told them what they actually were, regardless, there was an observed change in heart rate, blood pressure and most noticeably skin conductivity when the unpleasant range of frequencies were being played.

While fundamentally this research didn’t really add too much new to the original study, the researchers here hypothesized that the reason we react so viscerally to the sounds in this range is because they are already really loud in many cases and the human ear canal actually amplifies sounds in that range. This is obviously helpful for perceiving human speech, but if too loud a source can become unpleasant and even perceived as painful.

As acoustic engineer Trevor Cox of the University of Salford sums up, “It hits a range of frequency where your ear is particularly sensitive, and therefore you would expect a stronger response.”

Adding another significant piece to the puzzle, Sukhbinder Kumar et al in 2012 published the results of their study Features versus Feelings: Dissociable Representations of the Acoustic Features and Valence of Aversive Sounds. In it, they took fMRI’s of a group of people listening to these types of sounds, among other more pleasant sounds like babbling water- in total 74 different types of sounds were looked at.

The results? They found that when these sorts of nails on a chalkboard sounds were perceived, not only did people rate these sounds as unpleasant, but the level at which they rated a given sound unpleasant was directly proportional to the activity of the amygdala and the auditory cortext. They also found that the activation of the amygdala in turn heightened the activation of the auditory processing centers of the brain, further making your brain pay even more attention to these sounds and, in this case, not in a pleasant way because of the activation of the amygdala.

For those unfamiliar, the amygdala is, among many other interesting things we’ll get into in the Bonus Fact later, involved with emotional responses, including things like fear and, in turn, anxiety. On the note of fear, patients who have certain types of damage to the amygdala tend to fail to exhibit fear where others would, and animals who have the amygdala completely removed are drastically less likely to exhibit signs of fear and in turn anxiety. The amygdala also is thought to be a part of the brain that triggers the fight or flight response.

Going back to sounds, the worst rated sounds, and the ones that likewise showed the most activity between the amygdala and the auditory cortex were a knife scraped on a bottle, a fork on a glass plate, and the screeching version of chalk on a chalkboard. And if you’re wondering, one of the most pleasantly rated sounds was that of a laughing baby.

In the study, they further concurred that the troublesome parts of the sounds appear to be between around 2,000 Hz and 5,000 Hz. Dr. Kumar notes of this, “This is the frequency range where our ears are most sensitive. Although there’s still much debate as to why our ears are most sensitive in this range, it does include sounds of screams which we find intrinsically unpleasant.”

Speaking of screams, there is a certain dissonance to the sound of this sort of nails on a chalkboard scraping, which is also similar to the type of vibrations created when a baby is crying loudly or a non-parasite human screams.

This, perhaps, all lends credence back to the original hypotheses in the 1986 study that there is something from our long past that has taught us to fear these sounds to the extent that our brains are apparently literally wired to induce anxiety and partially trigger the fight or flight response when they are perceived. As summed up by Dr. Kumar, “It appears there is something very primitive kicking in… It’s a possible distress signal from the amygdala to the auditory cortex.”

Thus, to sum up the answer to the question of why people hate the sound of nails on a chalkboard, for whatever reason this triggers the amygdala to activate and interact with the auditory cortext of the brain, increasing its activation and creating an overall unpleasant sensation between the two.

As to why, the working hypothesis to date is either because the sounds are being amplified in the ear canal to unpleasant levels, that the specific sounds in the offending range mimic that of a human screaming or a mini-human crying loudly, or possibly all of this combined.

And speaking of mini-humans, while a baby crying is likewise one of the most unpleasant sounds, apparently in part from this amygdala activation, a baby laughing, as noted in the aforementioned 2012 study, is rated as one of the most pleasant to our brains- perhaps all giving at least one evolutionary benefit to why this is so encoded into our brains on both ends.

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

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• Why Do Other People’s Farts Smell Worse?

Bonus Fact:

While you might think all of this research on why humans hate the sound of nails on a chalkboard is kind of trivial and a waste of research dollars and the time of some of the world’s finest minds, Dr. Kumar takes the opposite line of thinking- that it has interesting implications to other research. For example, this may help shed some light into why, for example, autistic individuals are so much more sensitive to many sounds others don’t find offensive, with some such sounds causing extreme panic responses at times.

This is even more noteworthy when connected to the fact that autistic people also often exhibit a certain level of facial blindness and lack of ability to read emotional states of others by their facial expressions and the like. Why is this connection potentially important?  It turns out, not just inducing certain anxiety responses from sounds, the amygdala also is integral in your ability to recognize faces of people and their emotional state from this.

Interestingly, the amygdala also plays a strong role in social interactions. For example, it’s been found that the larger your amygdala, the more likely you are to have a larger and more diverse social sphere, with many more connections than those with smaller amygdalas. The amygdala also plays a critical role in storing memories that occurred during emotional times, which is why you can remember them better.

On a similar note, the amygdala also seems to play a role in learned aversion of certain things. So, for example, should you burn your hand in a fire, it becomes drilled into your brain to fear fire. For autistic people, this sort of learned anxiety seems to be turned on overdrive at times, with a minor incident sometimes producing long term anxiety about the situation in future, even sometimes long after the original incident is forgotten.

From all of this, there seems to be an awful lot of traits of autistic people that are potentially connected to the amygdala in some way. As one 2009 study from the University of Washington, Heightened Level of Amygdala Activity May Cause Social Deficits in Autism notes, “An increased pattern of brain activity in the amygdalas of adults with autism… may be linked to the social deficits that typically are associated with the disorder. Previous research has shown that abnormal growth patterns in the amygdala are commonly found among young children diagnosed with autism.”

The researchers further note of the amygdala of autistic people, “What we are seeing is hyperexcitability or overarousal of the amygdala, which suggests that neurons in the amygdala are firing more than expected… If you consider that habituation reflects learning in as simple a task as looking at a face, slowness to habituate in people with autism may contribute even more markedly to difficulty with more complex social interactions and social cognition. If the brain is not reacting typically to a static face with a neutral expression, you can imagine how difficult it may be for someone with autism to pick up more subtle social cues.” They also found those autistic individuals with the highest over-arousal of amygdala also exhibited the greatest social impairment.

Going back to sounds, in the end, the results of this seemingly silly research on why humans are averse to the sound of nails on a chalkboard is just another research log to throw on the fire of the growing body of evidence that there is something weird going on with autistic people’s amygdalas and may well be a key contributor to many aspects of the condition. Most pertinent to the topic at hand, this potentially may point to why the nails on a chalkboard aversion extends to many other types of sounds, or even just too much sound from a variety of sources, in autistic people.

Something similar might also be occurring with visual stimulus. For example, vocal autistic people often describe looking people directly in the eyes when you talk to them as giving the same sensation as listening to the sound of nails on a chalkboard.

And by the way, for those unfamiliar, it turns out the Spanish language has a word for this rather unique feeling- grima, which is often defined as “disgust” in English, but for whatever it’s worth, one study done at the Complutense University of Madrid looking into the matter actually showed people’s skin conductivity was noticeably different when feeling disgust at certain sounds vs. the sensation when listening to nails on a chalkboard-like sounds, more or less concluding that this is an inaccurate translation and grima is a distinct unpleasant sensation from disgust.

That poor Grima Wormtongue in Lord of the Rings never had a chance with that first name… Though apparently on this one it is widely reported that the first name was actually from the Old English for “mask,” though we couldn’t find a primary source for this and the Spanish word would make some sense here as well, unless anyone knows of Tolkein explaining the name origin or some other significant evidence either way?