Nails on a Chalkboard

In the History Channel documentary How the Earth Made Man, they list how various aspects and attributes in human biology and behavior can be explained as remnants of the evolution of humans and the Earth.

One of the qualities they explained was the reason that humans find the sound of nails scraping on a chalkboard to be so grating and causing us to cringe. Unfortunately they got it completely wrong.

Their explanation is that our primate ancestors who lived in trees and avoided predators would use a screeching sound to warn of danger, and so we now find that sound to be disturbing. It sounds like a good explanation but it is specious.

It is true that humans find the sound of nails grating on a chalkboard to be unnerving, but it is not the sound itself that is disturbing, it is the knowledge of what it feels like. To wit, scraping a lenticular with our nails produces a completely different noise, but the same cringe-inducing shudder. It is the physical sensation that repulses us so much.

Next time your hands are slick with oil or soap, scrape your fingernails along the ridges of the fingerprints on your thumb. There is pretty much no noise at all, yet the feeling is just awful. Clearly it is the sensation, that is, the vibration that is so aversive.

Something about the tactile feel of quick, small, sharp, repetitive vibrations is extremely uncomfortable and undesirable to humans, and certain sounds like nails on a chalkboard remind us of that.

Fight or Flight vs. Recognition

I had an interesting experience with my cat. I turned around and saw her standing there, and because I was not expecting her to be there, I was surprised and jumped. It has probably happened plenty of times in different ways for most people, but the long and short of it is that sometimes we get shocked and react to something that is completely innocuous.


The brain has different parts that are responsible for recognizing things (especially faces) and for reacting to danger. One part is responsible for “fight or flight” whereby the body almost entirely automatically reacts to perceived threats. Another part is responsible for analyzing visual input and determining what is being looked at.

This scared-by-the-familiar response is very revealing and to be honest, not surprising. It shows that the part of the brain that detects and reacts to danger functions at a “lower-level”, a more base instinct and thus faster and at a higher priority than the part that recognizes objects which is a somewhat higher-level function (though obviously not exclusive to humans). This is not a surprise because fight-or-flight is a survival instinct and more important (at least more immediate) than object/person recognition.

Chickens Will Not Evolve to Taste Bad

For some time I have mused as to why animals do not evolve to taste bad.

The whole point to natural selection is to promote the survival of a species by simply finding that the members that have certain desirable characteristics which help it to live, go on to have children who are likely to have those characteristics who can then pass it on to their children and so on. Over time, most members of that species will have that characteristic.

Humans have been eating animals for many, many years so I could not understand why the animals that get eaten so often—chickens, cows, pigs—have not yet evolved to taste bad. After all, any animals who taste good are more likely to be killed while the ones that taste bad are more likely to be spared. It makes sense.

A while ago however, the answer dawned on me. Not only do animals not evolve to taste bad but they in fact evolve to taste better. Of course it does not occur because of “natural selection” but rather due to human interference and meddling.

For example, lets say that there are two chickens, one happens to taste great should it be eaten, the other tastes awful. Of course both have been slaughtered already, that is how we know how they taste, however they have already been bred by a chicken farmer. The one who tasted good had children which were more likely to taste good as well. The one who tasted bad had children which were more likely to taste bad as well. Over several generations, the one offspring from the one who tasted good are bred more and more often for obvious reasons and the one offspring of the one that tasted bad are bred less and less often—perhaps only used for eggs, maybe not. After enough time has passed, the ones that tasted bad become extinct—at least on the farm—while the ones that taste good end up becoming ubiquitous.

In nature on the other hand, it is possible for an animal—for example and antelope—to evolve to taste bad because a lion will not breed them, it will only hunt them and in time learn which ones taste good and which ones taste bad. It will leave the bad ones alone and hunt the good ones. Eventually, the antelopes who survive will be the ones who taste bad.

Humans are meddlesome creatures who interfere with everything for their own interests. This is just another example of this albeit a rather major one, after all tampering with the very essence of evolution is not to be taken lightly.

In summary, chickens will not evolve to taste bad because of humans and their artificial selection.