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 . 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:
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).
“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
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.”
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.
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 would have been seen if you were looking elsewhere, 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.
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.
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:
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 (or they are in less pain), because part of the purpose of pain is to be visible.
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 (as much) pain. Humans and domesticated animals might be hypothesized to be the most pain-sensitive (because we have capable allies, i.e. other humans).
The authors don’t explicitly state why this fact is relevant, but I presume it is because they believe women to be somehow better at leveraging social connections. ↩︎
By "visible" I mean "is a credible signal to whatever entities the animal is trying to signal", which of course can include sounds, smells, etc. We still have the problem that things which might signal pain to a chimpanzee or mouse wouldn't necessarily signal pain to a human. ↩︎