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:

  1. 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).

  2. “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[1]

  3. 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.”

  4. 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.

  5. 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.

  6. 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:

  1. 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.[2]

  2. 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).

  1. 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. ↩︎

  2. 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. ↩︎


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[Epistemic status: Thinking out loud; copying my EA Forum comment about these papers from a couple weeks ago]

If the evolutionary logic here is right, I'd naively also expect non-human animals to suffer more to the extent they're (a) more social, and (b) better at communicating specific, achievable needs and desires.

There are reasons the logic might not generalize, though. Humans have fine-grained language that lets us express very complicated propositions about our internal states. That puts a lot of pressure on individual humans to have a totally ironclad, consistent "story" they can express to others. I'd expect there to be a lot more evolutionary pressure to actually experience suffering, since a human will be better at spotting holes in the narratives of a human who fakes it (compared to, e.g., a bonobo trying to detect whether another bonobo is really in that much pain).

It seems like there should be an arms race across many social species to give increasingly costly signals of distress, up until the costs outweigh the amount of help they can hope to get. But if you don't have the language to actually express concrete propositions like "Bob took care of me the last time I got sick, six months ago, and he can attest that I had a hard time walking that time too", then those costly signals might be mostly or entirely things like "shriek louder in response to percept X", rather than things like "internally represent a hard-to-endure pain-state so I can more convincingly stick to a verbal narrative going forward about how hard-to-endure this was".

Wow, this is fascinating speculation, thanks for posting.

The section on pain varying with the social environment was especially interesting. It reminded me of the (common but not uncontroversial) parenting strategy whereby babies are left to cry at night, so as to avoid positively reinforcing crying and instead train them to sleep unaided.

Would it suggest that exhortations to 'stop being a wuss' were actually effective? The nearby people are effectively precommitting to not be moved by visible suffering, which might reduce the incentive for the victim to experience pain.

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.

Physical damage which is self-induced is less painful. They give as examples “runner’s high”, cosmetic procedures like hair removal, and self-harm.

I wonder how certain we are that this is about pain levels, rather than the addition of emotions like uncertainty or alarm to pain that comes from others. If I knowingly cut myself with a knife, I'm ready for the pain; I can predict what will happen and steel myself. If someone else cuts me with a knife, I'm not prepared in the same way, and many other negative emotions are going to be flooding my system.

This makes me wonder whether accidental self-harm is more painful than purposeful self-harm (e.g. I cut myself while cutting up vegetables). If not, evidence for social factors mediating pain; if so, evidence for the role of surprise.

(Warning: Graphic medical descriptions)

I'm reminded of Reflections on pain, from the burn unit:

[...] But the one thing that did seem to dramatically affect my pain level was my belief about what was causing the pain. At one point, I was lying on my side and a nurse was pulling a bandage off of one of my burns; I couldn’t see what she was doing, but it felt like the bandage was sticking to the wound, and it was agonizing. But then she said: “Now, keep in mind, I’m just taking off the edges of the bandage here, so this is all normal skin. It just hurts because it’s like pulling tape off your skin.” And once she said that — once I started picturing tape being pulled off of normal, intact skin rather than an open wound — the pain didn’t bother me nearly as much. It really drove home to me how much of my experience of pain is psychological; if I believe the cause of the pain is something frightening or upsetting, then the pain seems much worse.
And in fact, I’d had a similar thought a few months ago, which I’d then forgotten about until the burn experience called it back to mind. I’d been carrying a heavy shopping bag on my shoulder one day, and the weight of the bag’s straps was cutting into the skin on my shoulder. But I barely noticed it. And then it occurred to me that if I had been experiencing that exact same sensation on my shoulder, in the absence of a shopping bag, it would have seemed quite painful. The fact that I knew the sensation was caused by something mundane and harmless reduced the pain so much it didn’t even register in my mind as a negative experience.
Of course, I probably can’t successfully lie to myself about what’s causing me pain, so there’s a limit to how directly useful this observation can be for managing pain in the future. But it was indirectly useful for me, because it proved to me something I’d heard but never quite believed: that the unpleasantness of pain is substantially (entirely?) psychologically constructed. A bit of subsequent reading led me to some fascinating science that underlines that conclusion – for example, the fact that the physical sensation of pain is processed by one region of the brain while the unpleasantness of that sensation is processed by another region. And the existence of a condition called pain asymbolia, in which people with certain kinds of brain damage say they’re able to feel pain but that they don’t find it the slightest bit unpleasant. [...]

In my opinion, DIY waxing hurts a lot more than waxing done by a professional. I guess they're both 'self-induced' in a way, but in general I'm not sure hair removal hurts less if you choose it - you're just more okay with it happening.

Thanks for writing this.

How would this model explain Cluster Headaches? They are not particularly more incapacitating than migraines, yet they are (possibly literally*) thousands of times more acutely painful than them. What is the role of this X1000 multiplier on phenomenal pain in such cases? As far as I can tell, in the ancestral environment nobody could have done anything to help you if you were having a Cluster Headache, and your chances of reproduction seem to be the same whether that pain was a thousand times less bad (which would still be VERY bad, but not in the level of ultra-Hellish pain). In particular, other species are known the have Cluster Headaches too, such as cats. So perhaps we should cluster pains into two buckets - those that have social significance and those that don't. I worry that this study will make people dismiss extreme suffering in nonhuman animals, but that should only really apply to socially-useful pains. I suspect that there are many species-specific ultra-painful experiences that we will not discover (and prioritize!) unless we look for them.


One possibility, if this theory is correct, is that cluster headaches are a spandrel, i.e. a (very unfortunate) unintended side effect of the pain system being accidentally fired in a case when it isn't beneficial for it to be but doesn't get selected out because it doesn't have much of an impact on differential reproduction rates.

Another is that causality is slightly different, pain is amped up in some cases to elicit altruism, but the mechanisms of pain are "lower in the stack" and so can be triggered by things other than those considered here, making cluster headaches something outside the bounds of what this model would need to explain since there can be many things accentuating pain and the considered cases are just one and the situation with cluster headaches is another.

I think it's worth qualifying these statements with "all else equal".

But note that all else is not equal. Humans, having greater capacity to reason and make predictions, can probably anticipate pain (and its end) and negative outcomes better. This could cause anticipatory anxiety, but it could also help us avoid pain in the first place, or rationalize our pain. Nonhuman animals have to rely much more directly on their pain signal reinforcement. So is the pain signal more useful to us or to them? I don't think the answer is clear either way.

Also, don't forget that we're attempting interpersonal utility comparisons. If we scaled up the intensity of all experiences without affecting how an animal behaves, we wouldn't be able to tell the difference.

This is a fascinating idea! I have a question though. I'm not exactly sure why (2) (more women have chronic pain, less pain tolerance etc) is evidence for this. Is the idea that women in the ancestral environment were more in need of assistance (eg because they were physically weaker, or made more vulnerable by bearing/raising children), and therefore evolved more capacity to feel (and thus express) pain?