Is pain just a signal to enlist altruists?

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. ↩︎