A spot on your arm is sending signals to your brain. There is an upset area of your epidermis wanting attention. You reach for it without thinking, and you scratch, nails digging into skin repeatedly. Why? It feels better, sometimes; sometimes it doesn’t. It’s addictive, somehow. Why?
This Newsweek interview with a dermatologist is far too folksy to give any specifics, but it does contain a drive-by explanation:
Animals scratch, and humans scratch. It helps remove irritants. It’s an evolutionary defense mechanism, a reflex.
Another report, by Medill, puts it this way:
Itching is a defense mechanism, Chen said. “It gives you a warning. Something you have on you is not part of you.”
I’m sure there’s more behind these soundbites, but I was unable to find it online. So for now, here’s my take: I’m not buying the idea that scratching is an evolutionary adaptation meant to remove irritants.
What does it mean to look for an evolutionary explanation? Doesn’t scratching work “because it feels good”? This kind of answer only postpones the real question: why does it feel good? (Similarly, it is often said that music evolved “because it brings the community together.” But why is it able do that?)
Generally speaking, things feel good because they’re good for us. Those who had a reason to eat, procreate, rest, and create, did better in the biological race. An old joke says that only a biologist would ask why sex feels good, but that is a question that needs answering, and the complications in the answer help us understand our existence a little better.
I’m skeptical of the explanation above, about scratching evolving to “remove irritants”, because I can think of only a few irritants which we want to remove in this way (bugs, maybe thorns). On the other hand, there are loads of itch causes which we in fact exacerbate by scratching. Bug bites, for one, and most rashes. We turn them into scabs, which we then scratch some more, causing further infection. Also, wouldn’t pesky bugs make us evolve slapping instead of scratching?
My idea is that it’s maybe more like this: scratching piggybacks on our basic fondness for touch. Many animal behaviors became common because similar behaviors are beneficial, and in a different environment and context, we approximate them: we chew on straws and pencils because of our food-inspired oral fixation. Similarly, we enjoy poking our skin by massaging, caressing, and feeling with our fingers, all of which have their own evolutionary benefits. Perhaps a sort of shorthand rule develops which says “playing with skin is good.” Combined with the focusing power of an itch (which “feels bad” for somewhat obvious reasons - it’s the same thing as otherwise-induced pain) this may create a very strong interaction bias: if a small spot is aching, give it your attention. And since this attention, in the form of scratching, may not help and may in fact make the pain worse, you’ll rinse and repeat.
If this is the basic reason, the question remains why scratching has not been selected against, which in biology means, why have those with a predilection for scratching not been kicked out of the pool? My intuition is that scratching causes more problems than it fixes, so much so that it may hamper evolutionary odds. But I admit that this is all just five minutes of armchair thinking by a complete amateur.
What do you think - why do we scratch?
