Science Explains Why Fingernails On A Chalkboard Feel Unbearable

Ever wonder why a small action like fingernails dragging across a chalkboard pierces our ears and causes our skin to crawl? Well, there are actually physiological and psychological explanations for this strange abhorrence. As one of the six human senses, hearing and the way we respond to sound is both important and mysterious.  

NPR reported the results of a study that examined the effects on human subjects of sounds produced by dragging different objects across a chalkboard. Dr. Michael Oehler and colleagues tested sounds spanning a range of frequencies, with the highest frequencies reaching more than 12,000 Hertz (a common measure of frequency). Interestingly, the most piercing sounds (that is, the ones at the highest frequencies) weren't the most unbearable for people. 

Dr. Oehler and team discovered that it was actually sounds in the mid-frequency range between 2,000 and 4,000 Hertz that produced the strongest reactions in terms of changes in skin conductivity (which causes the sensation of your skin crawling) and feelings of revulsion. Once these frequencies were removed, the study subjects perceived the sounds as more pleasant. What is the frequency of the sound of fingernails on a chalkboard? You guessed it — somewhere between 2,000 and 4,000 Hz. Interestingly, the ear canal amplifies frequencies in this range, making them sound louder and more irritating. 

There are several theories why we respond with such visceral repulsion to the sound of fingernails scratching a chalkboard. One theory is based on Blake's vestigial reflex hypothesis and proposes that certain sounds provoked a protective response in long-ago mothers that helped them protect their children. To lend credence to this theory, the sound of fingernails on a chalkboard was rated as slightly worse by females than males in a study and also rated as worse by females of reproductive age, with scores decreasing in older women. If this is really what's happening, this vestigial reflex is an example of evolutionary adaptation that may have been useful in the past but isn't anymore. 

Another theory is that we perceive this sound negatively because it is in the same frequency as human speech and may interfere with our ability to hear important sounds. This theory needs to be tested more to determine if it's a feasible one. 

Part of the strong negative reaction to the sound is the knowledge that it's caused by fingernails scratching a chalkboard. Going back to Dr. Oehler's study, people responded with more extreme negative reactions when they were told that the sound was caused by fingernails on a chalkboard. On the other hand, when the subjects were told that the sounds were part of a contemporary piece of music, they perceived the sounds as less irksome. This susceptibility to visual cues could be similar to the bizarre "nocebo" effect that can be easily explained by science.

Although the sound of fingernails on a chalkboard is undoubtedly icky, it fortunately doesn't happen very often, especially with the scarcity of chalkboards nowadays. Imagine having that same adverse reaction to common, everyday sounds like breathing, chewing, slurping, and coughing. According to the Anxiety and Depression Association of America, this condition is called misophonia, and it can invoke very extreme reactions in some people. For example, people with misophonia may feel rage, panic, and distress bubbling up at the sound of someone crunching on an apple nearby.  

Much is still unknown about misophonia, but a study of 20 people with the condition found that the anterior insular cortex (AIC), an area of the brain that links senses with emotions, is activated to a greater degree by trigger sounds than in people without misophonia. In addition, the AIC in these individuals had abnormal connections with other parts of the brain. The study subjects who had misophonia tended to run away from the triggering sounds or felt extremely angry and anxious if they couldn't escape, which the researchers attributed to a fight or flight response. 

A randomized clinical trial showed that cognitive behavioral therapy (CBT) improved symptoms of misophonia in 37% of patients compared to 0% who did not receive CBT. The effect was maintained at one year. Although more such trials need to be conducted to better understand the role of CBT, there is hope for people who find certain sounds unbearable.