This post is an attempt to summarize The neuroscience of vision and pain: evolution of two disciplines, The pain of altruism, and some key papers they cite. From the abstract of the second one:
We suggest that both the human experience of pain and the expression of distress may result from many causes not experienced as painful in our close primate relatives, because human ancestors motivated to ask for help survived in greater numbers than either the thick-skinned or the stoic.
The basic idea, as well as its supporting evidence, seems relatively easy to understand, and could have relevant implications for EA: Firstly, it would imply that species which are not social would feel less pain, and secondly, it would imply that animals without "visible" signals of pain might not experience as much pain (since pain is less useful if it's not visible).
Case study: pain of childbirth
A large part of The pain of altruism is a case study into the pain of childbirth.
The pain of childbirth is religiously, medically, and culturally enshrined in Western culture, but even with cultural modifications it has pervasive cross-cultural universals. Surprisingly, the presence of assistants rather than essential difficulty distinguishes human birth from birth in great apes and monkeys…
A common assumption is that the exceptional pain of human birth is the result of its unusual physical difficulty, given humans’ large head and the modification of the pelvis for walking, and damage often occurs at the point of birth, where mortality of both infant and mother is quite high.
Curiously, however, the labor pains associated with the dilation of the cervix occurring for the multiple hours anticipating delivery produce no particular tissue damage…
Furthermore, great difficulty in parturition is not unique to humans among primates, nor mammals. For example, due to head and body scaling, it is the smallest monkeys – marmosets – that have the greatest cephalopelvic disproportion and mortality associated with parturition [10]. Large ungulates give birth to exceptionally large-headed and -hooved offspring with little if any announcement of distress and both mother and offspring are developmentally and physiologically prepared to move off immediately, presumably due to the dangers of predation…
In summary, an essentially neutral physiological event (cervical dilation) predicts a dangerous event (birth). We suggest that cervical dilation has become rivetingly painful to induce help seeking and all of its subsequent cultural elaborations in our social species. The offspring of those who sought help are more likely to be (with) us.
Other pieces of evidence
The authors mention other pieces of evidence:
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Humans have a highly visible response to pain (tears). If an image of a person crying is manipulated to remove the tears, the resulting expression is often interpreted as awe, concern or puzzlement, implying that tears provide significant benefits in signaling sadness/pain. This seems like evidence that there is some significant evolutionary value in signaling pain (or else we would not have evolved this adaptation).
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“A clear majority of patients with chronic pain are women; however, it has been surprisingly difficult to determine whether this sex bias corresponds to actual sex differences in pain sensitivity. A survey of the currently available epidemiological and laboratory data indicates that the evidence for clinical and experimental sex differences in pain is overwhelming… What has struck many researchers, however, is the fact that when differences are observed, they almost unanimously show that women have a higher sensitivity and lower tolerance to pain than men, report higher pain ratings and have a greater ability to discriminate among varying levels of pain.” - Sex differences in pain and pain inhibition: multiple explanations of a controversial phenomenon
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Pain responses differ in complicated ways based on who is in the room with the person in pain. For example: “Participants reporting higher levels of everyday social support and higher attachment avoidance, as well as participants with a solicitous spouse, had worse pain outcomes when a social partner was present than when they were alone, while participants with low levels of everyday social support showed the opposite effects.”
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Physical damage which is self-induced is less painful. They give as examples “runner’s high”, cosmetic procedures like hair removal, and self-harm. Tickling is an extreme example of this: many people find it impossible to tickle themselves, even though the physical stimulus is the same whether they are doing it or someone else is. Presumably, one is less likely to need altruistic help in preventing self-induced harm, so it would make sense that self -induced harms are less painful.
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Predictive coding seems to be a generally successful paradigm in neuroscience, particularly in vision: your brain doesn’t merely passively receive information from your eyes, but also predicts what it will see next, and these predictions are “seen” by you. This is analogous to how cervical dilation could be felt as pain simply because it is a prediction of future tissue damage, even if no real damage is currently occurring.
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From an evolutionary perspective, “internal theater is not the goal of sensory systems” – animals feel things because it is, in some sense, useful for them to feel those things. Our default assumption should be that sensations (including pain) are only felt because there is some evolutionary purpose to them being felt, and we should be unsurprised if it turns out that the sensation of pain is disconnected from things like physical damage.
Implications
The following are a few implications which come to my mind. They are not in the original papers, and I am not sure if the authors would endorse them:
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A continual challenge in improving the welfare of nonhuman animals is understanding what things are painful for them. A particular challenge is if an animal is suffering but is not “visibly” in pain. This theory would predict that, to some extent, an animal not being visibly in pain is evidence that they are not actually in pain (because part of the purpose of pain is to be visible).
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We would predict that animals can feel pain to the extent that they have allies who are capable of helping with that pain. Animals who are either solitary or have allies who are not capable of helping would not feel pain. Humans and domesticated animals might be hypothesized to be the most pain-sensitive (because we have capable allies, i.e. other humans).
To me this is interesting evidence suggesting that one purpose of pain in humans is to be visible (attracting help). It doesn't go very far to suggest that this is pain's only purpose, which I think is what would be needed for me to hypothesize that solitary animals feel little or no pain.
I'm currently in school for physical therapy assisting; as you might imagine, pain is a big topic for us! The standard hypothesis we've been taught is that pain is most typically experienced as a signal to the person experiencing it that something needs to be done. (This is the model where the pain you feel on burning your hand is what causes you to take it off the hot stove—or more accurately, what causes you not to put your hand right back on the stove, because you've probably had a reflex reaction that removed your hand from the hot stove before the signal reached your conscious awareness.) It's easy to see how in humans "something needs to be done" can include a much broader range of behaviors than simply moving away from something unpleasant, avoiding immediate weight-bearing on a sprained ankle, etc. But the pain is already useful even if the reaction to it is smaller. If you've sprained your ankle, it's a good idea to be a bit more sedentary than usual the next day even if you never cry and try hard not to limp. It seems to me like a lot of animals that we think of as showing less pain may show it in subtle behavioral ways that would not be noticeable to their typical predators. (These may also be harder to observe in situations that don't allow for normal behavior, such as a laboratory.)
But you're right that pain is very complicated, and it doesn't always correspond to tissue damage, especially in the case of chronic pain. It may be that some situations in which humans feel pain have evolved specifically in tandem with our complex social behavior. I would just be very wary of extending that to acute pain in general, which does seem to be strongly associated with tissue damage in humans in a broad variety of cases—and for which there is often a direct physical response the pain seems adapted to provoke.