Scraping a chalkboard with the fingernails produces a sound which most people find unpleasant. The basis of this innate reaction has been studied in the field of psychoacoustics. One explanation for the adverse reaction is that the sound is similar to the warning call of a primate.
A 1986 study used a tape-recording of a three-pronged, metal garden tool being ‘grided’ across a chalkboard, which roughly reproduces the sound of fingernails on chalkboard. The recording was then manipulated, removing pitches at the extremities and the median. The results were then played back. It was determined that the median pitches are in fact the primary cause of the adverse reaction, not the highest pitches as previously thought. The authors hypothesized that it was due to predation early in human evolution; the sound bore some resemblances to the alarm call of macaque monkeys, or it may have been similar to the call of some predator.
The Daily Omnivore 
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