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The hazards of congenital insensitivity to pain (CIP) are well-known.

This question is about the obverse: what selective advantage, if any, does the normal sensation of pain confer? I'm thinking of severe pain such as that associated with back injury, burns, and some intestinal problems in which the pain seems to serve no function other than to incapacitate.

A related question is whether there is a spectrum of sensitivity to pain, ranging from CIP to heightened sensitivity, what the distribution looks like, and whether this is a genetically plastic trait (like color perception)? If the distribution were sensitive to selective pressure then naively I would guess it serves our needs well, but we spend a lot of our collective time trying to dial it back, hence my question.

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Interesting fact : A person having diseases which disable the pain sensation can not live beyond their 20s or 30s. –  biogirl Mar 30 at 11:46

2 Answers 2

Yes a range of pain exists. This can be seen probably in your friends who all probably have a completely different threshold for pain. When doctors ask patients to rate their pain it's usually to establish a baseline so after treatment we can see if there's a decrease. The different thresholds have so many factors from general health to receptors to brain activity to emotional state. You name it! Often just focussing on a pain makes it more painful.

It's definitely an unpleasant sensation pain but it's our way of our body saying "hey, you need to pay attention". Usually the response required is rest or to identify what might be there causing the pain. For example pain means we will withdraw very quickly from a hot surface. Doesn't even require the brain as it is a reflex arc which minimises the time we spend burning that part of our body. And as it continues to hurt we use the arm less letting it recover and protecting it from infection by doing so. As these hazards of being cut, burnt, bit, knocked, fractured are very common there is a survival advantage to be the individual that is alerted to these hazards and take the necessary actions. Particularly in children where pain is necessary as it makes the baby know it needs to eat (or the mother) or that something isn't a good idea (putting hands on something hot).

However as you say there are times when the pain itself is more crippling than anything else and in that situation we have a problem. Childbirth, backache and in cancer or in tension headaches are just some examples. As being able to feel pain is pretty much required for a long life, the disadvantage of having pains like the above is less and doesn't outweigh the benefits. Pain is always going to be necessary. They also usually don't interfere with reproduction and so genes are passed on.

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+1 but we could imagine a world in which a much higher threshold (lower sensitivity) was an advantage. You agree that there is a a range--do we know anything about the distribution? Has it been quantized? Does it change in response to selective pressures? –  daniel Apr 5 at 0:34
    
Don't think of the whole world's threshold changing, think smaller. Boxers and fighters are likely a population which represent those with a higher threshold as a selective pressure exists. I've seen a distribution curve somewhere, I don't know where though. –  AndroidPenguin Apr 5 at 7:33

In terms of inflammation, the purpose of pain is to force the organism to rest the area that is inflamed. Thus promoting healing. In terms of chronic pain, this is the same thing. However I think that if the pain will not cease regardless of protection of the affected area, and say if the underlying cause of the pain is ultimately fatal, then the pain is subordinate. The phenotype is already being selected out.

I think the concept shows a lot of parallel to the underlying concept of hypersensitivity (allergy).

Remember that within a population, over time as all these different things that can cause and prevent pain in our bodies are evolving. So too is the pain mechanism itself evolving.

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