Preamble
Differences in the intensity of valenced experience across species may affect the proportion of resources we ought to allocate to helping different types of animals. I recently wrote a long report on this topic. (You can view a PDF of the report here.) In this summary, I attempt to more succinctly convey my main conclusions.
Main Text
We all know that some experiences feel good, while other experiences feel bad. In the jargon, we can say that experiences often take on a valence: an overall positive or negative flavor. We also know that some good experiences feel better than others, and some bad experiences feel worse than others. We can call these degrees the intensity of an experience.
Human lives are full of an assortment of valenced experiences of varying intensities. The difference between a person’s most positive experience and most negative experience is the person’s realized intensity range of valenced experience. But some individuals realize wider intensity ranges than others. Zooming out, we can describe the characteristic intensity range of a representative human being.
Are there differences in the characteristic intensity ranges of valenced experience across species? This question is important because the answer to it could affect the way we wish to spend our scarce resources helping different types of animals, including humans. For example, if the pains and pleasures of trout and salmon are but pale reflections of the pains and pleasures of cows and pigs, then in many instances we may be able to do more good helping mammals rather than fish.
I identified a three-stage approach to tackling this question:
- Start with theoretical evolutionary biology. Reflecting on the evolutionary function of valenced experience could help us discern rough limits on what the intensity range of a species might look like.
- Next, think about how cognitive and emotional traits contribute to the intensity of valenced experience. If we can get a handle on the role that cognition and emotion play, then we may be able to use differences in cognitive and emotional capacities as rough guides to differences in intensity range.
- Finally, examine the potential neurobiological, behavioral, and physiological markers of the intensity of valenced experience. Researchers have been working for decades to validate objective metrics of pain intensity in humans. If successful, we could perhaps extend these metrics to nonhuman animals, thus offering a chance to measure the intensity of valenced experience directly.
My results are full of uncertainty. I combed through hundreds of studies and papers but struggled to piece together a coherent picture of the state of our knowledge. At times, it felt like I needed to be an expert in half a dozen distinct disciplines just to make sense of all the data I had collected. In the end, I arrived at a handful of tentative conclusions, two of which may be relevant to the way we allocate resources to interventions targeting different types of animals.
Tentative Conclusion #1: Humans and other mammals likely share a similar intensity range
The neurobiological machinery that controls human pain and pleasure sensitivity, as well as our basic emotional responses, is evolutionarily ancient, and therefore likely conserved across all mammals. From a behavioral, physiological, and neurological perspective, there is nothing uniquely human about bodily pains and pleasures. What’s more surprising is that from the same behavioral, physiological, and neurological perspective, there appears to be nothing uniquely human about our basic emotional responses. Other mammals probably feel fear, anger, anxiety, and sadness. Positive emotions appear equally widespread, with pretty robust analogues of (maternal) love and even friendship seemingly common among mammals. When the relevant behavior is similar, the physiological changes associated with the behavior are similar, and the neurobiology that presumably governs the behavior is similar, the most reasonable conclusion is that the emotional lives of nonhuman mammals are, in most respects, approximately as intense as the emotional lives of humans.
Although it’s plausible that there are big differences in intensity range across species, I’m doubtful there exist any animals with the capacity for valenced experience whose intensity range exclusively vacillates so close to neutral as to be undeserving of our moral concern.
Sometimes people suggest that even if insects or crustaceans are sentient, these animals don’t merit much moral attention because even their most intense pains and pleasures are so faint as to be hardly noticeable. My research suggests that there is currently little evidence to support this claim, and at least provisional reason to reject it. For one, we aren’t in a position to conclusively rule out the possibility that insects and crustaceans have characteristically more intense experiences than more typical targets of our moral concern, such as pigs or chickens. More to the point, the idea that the experiences of some animals never rise above the palest hint of a feeling sits in tension with our best understanding of the evolutionary role of valenced experience. One of the most important functions of pains and pleasures is to encourage animals to pursue fitness-improving behaviors and to discourage animals from engaging in fitness-reducing behaviors. The felt goodness or badness of an action provides a motivational oomph that tells an animal how to behave. Under normal conditions, motivational force generally co-varies with the intensity of the felt goodness or badness. But by definition, extraordinarily mild experiences feel almost indistinguishable from neutral experiences. Subjective experiences so faint as to barely register would probably do a poor job motivating an animal.
Of course, this is a complicated issue, and there are replies to the above argument that are worth considering. It’s certainly possible that insects and crustaceans have characteristically less intense experiences than, say, pigs and chickens. Nonetheless, anyone who grants that insects and crustaceans are sentient but denies they merit attention on the grounds that their experiences are so faint owes us an evolutionary story about the connection between intensity and motivation.
Why Read the Whole Report?
If you read the report in full, you’ll discover the reasons I believe cognitive sophistication is a poor indicator of intensity range, why I’m excited about examining neural oscillations in nonhuman animals, and what questions I think are worth investigating further. And if you slog through the 123 footnotes, you’ll be rewarded with a smattering of interesting ideas, including the claim that humans may have the best orgasms (both male and female) in the animal kingdom and the claim that human childbirth may be so painful not because of our large heads (marmosets actually have greater head-to-pelvis disproportion) but because painful cries elicit assistance from other humans.
This area of research is admittedly quite speculative, and I regret that, due to a confluence of complexity, I was unable to provide a more quantitative accounting of my credences in various propositions. Nonetheless, if we’re serious about comparing interventions that target different species, we inevitably must confront difficult questions. I hope to have taken an initial step in helping map the terrain that we will have to traverse if we are ever to be confident that we are distributing resources across species efficiently.
Credits
This research is a project of Rethink Priorities. It was written by Jason Schukraft. Thanks to Michael Aird, Janique Behman, Kim Cuddington, Marcus Davis, and Derek Foster for helpful feedback on earlier drafts. If you like our work, please consider subscribing to our newsletter. You can see more of our work here.
Notes
Hello Jason. Thanks for doing all this work! I haven't kept up with all of it, so apologies if you've covered this elsewhere, but I had a nascent thought that links and challenges your two tentative conclusions.
Okay, so the idea is that valenced states - colloquially, pleasure and pain - provide "oomph" to get creatures to do things. That seems fine. But it's unclear what this tells us about the intensities of experiences. Imagine we have two creatures that are the same, except A has 10x valence intensity than B. Why should there be any difference about how the two of them behave and thus, their evolutionary fitness? Couldn't they just act in the same way? And supposing more oomph is better, how much oomph should we expect, given there are, e.g. energy costs to producing sensations?
From the armchair, what matters for behaviour the relative intensity of different things for a given creature: if the deer loves eating berries and doesn't fear pain enough to run away from wolves, it will get eaten. But that doesn't tell us about inter-creature cardinal intensities.
My thought is something like this. Creatures need a range of cardinal intensities large enough to allow them to choose between all the different behaviours they need to undertake to survive and reproduce. As a toy example, if you only have 3 levels of pleasure - zero, 1, and 2 - but you have very many different choices to make - eat, mate, run away, sleep, etc. - then that's not enough resolution to make decisions. An entity that needs to make more decisions needs a greater range of sensations.
This takes us back, crude, to something about brain size a proxy for valenced states. And the possibility that 'simple' creatures, i.e. those that don't have lots of decisions to make, don't feel very much. I'm not sure where that leaves us in practice.
Hey Michael,
Thanks for your comment! The point you raise is a good one. I’ve thought about related issues over the last few months, but my views still aren’t fully settled. And I’ll just reiterate for readers that my tentative conclusions are just that: tentative. More than anything, I want everyone to appreciate how much uncertainty we face here.
We can crudely ask whether motivation is tied to the relative intensity of valenced experience or the absolute intensity of valenced experience. (‘Crudely’ because the actual connection between motivation and valenced experience is likely to be a bit messy and complicated.) If it’s the relative intensity, then, all else equal, a pain at the top end of an animal’s range is going to be very motivating, even if the pain has a phenomenal feel comparable to a human experiencing a very mild muscle spasm. If it’s absolute intensity, then, all else equal, a pain like that won’t be very motivating. I’m not sure what the right view is here, but the relative view that you endorse in the comment is certainly a live option, so let’s go with that.
If it’s relative intensity that matters for motivation, then natural selection needs a reason to generate big differences in absolute intensity. (Setting aside the fact that evolution sometimes goes kinda haywire.) You suggest the fitness benefit of a fine-grained valence scale, especially for animals that face many competing pulls on their attention. I agree that the resolution of an animal’s valence scale probably matters. But it’s unclear to me how much this tells us about differences in absolute intensity.
It seems possible to be better or worse at distinguishing gradations of valenced experience. It might be the case that animals with similar intensity ranges can differ in the number of intensity levels they can distinguish. (It might also be the case that animals with different intensity ranges have a similar number of intensity levels they can distinguish.) So if there were a fitness benefit to having 100 distinguishable gradations rather than 10, evolution could either select for animals with wider ranges or select for animals with better resolutions. (Or some combination thereof.) Considerations like the Weber-Fechner law incline me toward thinking an increase in resolution would be more efficient than an increase in range (though of course there are limits to how much resolution can be increased). But at this point I’m just speculating; there’s a lot more basic research that needs to be done to get a handle on these sorts of questions.
As far as I know, it's plausible to me that different neuron firing rates can represent different degrees of valence pretty close to continuously, so extra neurons might not be necessary to increase the resolution.
There are practical limitations about the resolution with which neurons can increase resolution (noise would be limiting factor, maybe other considerations). A common 'design scheme' that gets around this is range fractionation: If the receptors are endowed with distinct transfer functions in such a way that the points of highest sensitivity are scattered along the axis of the quality being measured, the precision of the sense organ as a whole can be increased.
This example of mechanosensory neural encoding in hawkmoths is a good example of range fractionation (and where I first heard about it).
Range fractionation is one common example where extra neurons increase resolution. There may be other ways that neural resolution can be increased without extra neurons. Also note that this has mostly been studied in peripheral sensory systems - I'm not sure if similar encoding schemes have been considered to represent the resolution of subjective experiences that are solely represented in the CNS.