People’s aversion to the sound could be because it sounds like the warning call of a primate ancestor. However, a study using Cottontop Tamarins, a kind of New World Monkey, found that they react similarly to both screeching sounds similar to fingernails on chalkboard, and to amplitude-matched white noise. This is different from how humans react, as humans are less averse to the white noise than to scraping.
A 1986 study attempted to determine why the sound was so unpleasant. It used a tape-recording of a three-pronged garden tool similar to a fork being “grided” across a chalkboard, which reproduces the sound of fingernails on chalkboard. It also used other sounds for comparison. They then manipulated the recording to remove the lowest, highest, and middle pitches. The results were then played back. It was determined that the middle pitches are what make the sound of fingernails scraping chalkboard so bad, not the highest ones as previously thought. The authors hypothesized that it might be 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. This research won one of the authors, Randolph Blake, an Ig Nobel Prize in 2006.