Fingernails and blackboards

Why is the sound of fingernails dragging down a chalk board so horrible? This question just won’t go away. Ever since I ran an experiment into the worst sound in the World (, a lack of a good answer has been bothering me.
I’ve been doing some science shows for sixth formers in London recently, and this included lots about the science of screaming. Alongside the screaming science, a comment in Daniel Gilbert’s excellent book Stumbling on Happiness has got me thinking about the fingernails again. Maybe we find scraping sounds unpleasant because the sound characteristics of a scream are pretty similar to those of a scraping sound? This explanation isn’t that far from Blake’s suggestion from the 80s, but newer stuff coming out of neuroscience about how we react to threatening sensations (such as sound) puts it in a whole new light.
Hearing is an early warning system against danger, especially when something or someone is creeping up from behind. And the first bit of this early warning system uses a part of the brain called the amygdala – in evolutionary terms one of the older parts of the brain. Quick decisions must be made about whether a sound is a threat to give us the best chance of not being eaten by a Sabre-toothed tiger or beaten up by Caveman Dave. The first decisions must be based on some crude features. So although screams and scraping sounds are not identical, maybe they have enough in common to confuse our defence systems.
Have a listen and see what you think:
1. A really unpleasant scream
(thanks to Freqman on
2. A Scraping sound (the middle bit is best)
I think the features we might be listening to are the roughness and the relatively pitch. All I now need is free access to an fMRI scanner and a lot of guinea pigs willing to be tortured by sound to prove this!

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