Unfortunately for millions of sufferers worldwide, scientists haven't nailed down why scratching a chalkboard makes people wince. Prevailing studies have looked at the physiology of the inner ear, pointed to the "roughness" of certain sounds, and even associated chalkboard scratching with wild primate distress calls. Most studies agree on one thing, however: sounds that have a frequency between 2,000 Hz and 4,000 Hz make us cringe. As you can imagine, scratching a chalkboard falls directly within that range.
In 1986, three scientists at Northwestern University broke down the sound of a garden tool scraping a chalkboard. They removed certain high, middle or low frequencies and played them back to test subjects who rated their adverse reactions. To the scientists' surprise, people cringed most at the middle frequency sounds and not at the high frequency sounds as they hypothesized.
One of the key takeaways from this 1986 study was that the middle frequency sounds were very similar to primate distress calls in the wild. The scientists' theory was that this is an evolutionary condition. Other studies seem to support this idea, too.
In 2011, European musicologists Michael Oehler and Christopher Reuters found that the inner ear canal was built to amplify certain sound ranges - specifically those that pertain to communication. They also found a new, psychological twist: test subjects found chalkboard scratching less cringe-worthy when they were told it came from a musical composition.
Finally, in 2012 MIT scientist John McDermott found that sounds can have "roughness," which comes from how rapidly it fluctuates between extreme frequencies. Nails scratching and reverberating on a chalkboard, says McDermott, would produce a very "rough" sound. Of course, we didn't quite need science to know that already.