There are few natural sounds more irritating that fingernails on a chalkboard, with the possible exception of foam cups rubbing together, a squeaky door hinge, or a dentist's drill. What seems to set the sound of fingernails apart from other irritants is the apparent universality of it. Few people living on the planet can avoid cringing in agony as the high-pitched scraping noise begins, and everyone is relieved when the event is over. No one can say with any scientific certainty why the sound is so incredibly irritating, but there are a number of interesting theories.
Some believe the sound is similar to an animal's high-pitched screech indicating danger to the rest of its group. Macaque monkeys, for example, have been known to emit a warning noise of relatively the same pitch and duration of the dreaded fingernails on a chalkboard sound. It has been suggested that humans react to the sound instinctively, creating a "fight or flight" response.
Another theory holds that the sound of fingernails on a chalkboard is akin to the scraping sound of rocks against teeth. Scraping noises are especially irritating because they remind us of painful dental experiences or the unpleasant sensation of biting down on a piece of rock or metal. The scraping sound is created by the fingernails alternatively slipping against and gripping the rough surface of the chalkboard. Experiments conducted by scraping a garden tool against a chalkboard demonstrated that the lower frequencies of the noise were especially bothersome.
In a similar vein, some have theorized that it's not the sound, it's the action itself. Anyone who has ever scraped his or her fingernails against a chalkboard is not likely to forget the experience. As the fingertips drag across the rough surface of the chalkboard, a number of nerve endings are stimulated, and not necessarily in a good way. When people hear the sound, it is highly likely they will have a sympathetic response and identify with the unpleasant sensations accompanying the sound.
Sometimes it's a matter of harmony versus disharmony, auditorily speaking. Many people are very sensitive to changes in pitch or frequency. A guitar with one string out of tune can disrupt an entire performance, for example. The attention signals generated by local television stations are often irritating for a reason. The emergency signal has been detuned in order to stand out from normal background noises. The ultimate in irritating and detuned harmonics is most likely the sound generated by fingernails on a chalkboard. There is simply nothing remotely harmonious about that screeching, abrasive sound, so we react strongly whenever we're forced to hear it.