Differences in the intensity of valenced experience across species (Rethink Priorities)

Key Highlights

• Differences in the intensity range of valenced experience across species may affect how we ought to allocate resources to help different types of animals
• Humans and other mammals likely share a roughly similar intensity range
• It is unlikely that any species of animal possesses an intensity range that is exclusively extraordinarily mild
• Some aspects of cognitive sophistication appear to be positively correlated with intensity range; other aspects of cognitive sophistication appear to be negatively correlated with intensity range
• Affective complexity generally appears to be positively correlated with intensity range
• There is as yet no good objective measure of valence intensity, though there is much interesting work ongoing in this area

Executive Summary

This post is the fifth in Rethink Priorities’ moral weight series. In this report, I explain why it would matter if different sorts of animals possessed characteristically different intensity ranges of valenced experience, what factors might affect the intensity range of valenced experience, and how we might begin to measure differences in the intensity of valenced experience.
Animals differ in their perceptual abilities, their physiology and neural architecture, and their cognitive, affective, and social complexity. Given these differences, it would be surprising if the intensity range of valenced experience were uniform across species. To investigate differences in the intensity range of valenced experience across species, I adopted a three-stage methodology:

1. First, I explored what, if anything, theoretical evolutionary biology could tell us about the function of valenced experience.
2. Next, I explored how different aspects of intellectual and emotional complexity might affect the characteristic intensity of valenced experience.
3. Finally, I explored potential neurobiological, behavioral, and physiological markers of the intensity of valenced experience in humans and nonhuman animals.
   Broadly speaking, the evolutionary function of valenced experience is to promote fitness-improving behaviors. It’s plausible that natural selection would not produce animals for whom valenced experiences were always extraordinarily weak because subjective experiences that were so faint as to be almost imperceptible would appear to do a poor job motivating behavior. Conversely, it appears unlikely that evolution would select for animals with a non-contiguous range that was exclusively extraordinarily strong because extremely intense experiences are distracting in a way that appears likely to reduce fitness.
   ‘Cognitive sophistication’ is a nebulous term that may refer to any of a constellation of mostly independent traits. Many of these traits plausibly affect the intensity of valenced experience, but the sign of the effect is often unclear. ‘Affective complexity’ refers to the diversity and depth of emotional sensations an animal can experience. Increased affective complexity may unlock qualitatively unique emotional states—such as fear, depression, or love—that by themselves or in combination with physical states increase the intensity range of experience.
   There are currently no good cross-species measures of the intensity of valenced experience, though there is intriguing recent evidence that neural oscillations in the gamma band may track differences in pain intensity in both humans and nonhuman mammals.
   Humans and nonhuman mammals possess neurologically and behaviorally similar affective systems, suggesting that most mammals are capable of experiencing roughly the same base set of emotions. It’s unclear how stark the differences in cognitive sophistication are across mammalian species and how these differences might affect the intensity of valenced experience. Given these facts, it would appear that we are more justified in thinking that humans possess an intensity range roughly similar to other mammals than we are in thinking that humans possess a much wider range than other mammals. It is unclear how mammals compare to other groups of animals, in part because there is a much sparser scientific literature on the capabilities of non-mammals and in part because it is unclear how increasing phylogenetic distance ought to influence reasoning-by-analogy about subjective experience.