Opinion: Estimating Invertebrate Sentience


Between May 2018 and June 2019 Rethink Priorities completed a large project on the subject of invertebrate sentience.[1] We investigated the best methodology to approach the question, outlined some philosophical difficulties inherent in the project, described the features most relevant to invertebrate sentience, compiled the extant scientific literature on the topic, summarized our results, and ultimately produced an invertebrate welfare cause profile. We are currently in the process of identifying concrete interventions to improve invertebrate wellbeing, with a report on the welfare of managed honey bees due out in mid-November and a report on the welfare of farmed snails nearing completion.

One thing we did not do was publish explicit numerical estimates of the probability that various groups of invertebrates are sentient.

Our team discussed publishing such estimates many times, but these discussions generated considerable internal disagreement. Two members of the (four person) team believed that publishing explicit sentience estimates was a bad idea. The other two members felt that it was a good idea. In the end, we settled on the following compromise: several months after the completion of the invertebrate sentience project, we would publish an unofficial opinion piece in which each of us could share her/​his own reasoning on the subject and, if so desired, her/​his own estimates.[2]

This post fulfills that compromise. In it, the four members of Rethink Priorities’ invertebrates team—Daniela R. Waldhorn, Marcus A. Davis, Peter Hurford, and Jason Schukraft—outline their views on the value, feasibility, and danger of quantitative estimates of invertebrate sentience. Marcus and Peter provide numerical estimates of sentience for each of the taxa we investigated for our invertebrate sentience project, Daniela offers a qualitative ranking of the same taxa, and Jason argues that we are not yet in a position to deliver estimates that are actionable and robust enough to outweigh the (slight but non-negligible) harm that publishing such estimates prematurely might engender.

What follows are the personal opinions of individual researchers. Officially, Rethink Priorities does not have a position on the explicit probability that various invertebrates are sentient.

Daniela R. Waldhorn

1. Vertebrates

There is an ample and detailed body of empirical data which justifies believing that non-human vertebrates are sentient. In particular, there is solid neuro-anatomical, physiological and behavioral evidence that vertebrates like cows and chickens are conscious. There is also a growing trend to recognize that these animals do not only experience physical suffering (and pleasure) but also have emotional lives (see e.g. Marino, 2017; Proctor et al., 2013).

Based on existing evidence and the generalized acceptance of the Cambridge Declaration on Consciousness (2012)[3], my overall conclusions regarding the probabilities of consciousness for these animals are presented as follows:

2. Invertebrates

When we consider invertebrates, the debate about whether they are conscious becomes much more complex. First, it must be conceded that the numerous invertebrate species and their diversity impose severe constraints to justifiable generalizations about the presence of consciousness in this group of animals. Second, the scientific literature about sentience in invertebrates is not only scarce but fragmentary–that is to say, the extent to which invertebrates have been investigated varies. Thus, there are some particular species about which there is a comparatively great deal of knowledge (e.g., fruit flies), whereas much less research has focused on individuals of other taxa (see our Summary of findings Part 1 and Part 2).

The existing gaps in this field of research entail that we face significant constraints when assessing the probability that an invertebrate taxon is conscious. In my opinion, the current state of knowledge is not mature enough for any informative numerical estimation of consciousness among invertebrates. Furthermore, there is a risk that such estimates lead to an oversimplification of the problem and an underestimation of the need for further investigation. As it was stated in our summary of findings, the available evidence is not enough for determining with sufficient certainty whether most invertebrate taxa are conscious or not. Yet it does allow us to identify certain aspects as potentially relevant for consciousness and flag certain areas that should be further investigated if we want to arrive at more robust conclusions.

Nevertheless, our literature review allows us to conclude, for instance, that there is relatively strong evidence that octopuses are conscious. We can even assert that octopuses are more likely to be conscious than other invertebrates, like earthworms. Hence, although, as stated, this field is not mature enough to make robust estimations of sentience, we can still make some relevant comparisons. Therefore, I propose developing a framework for examining existing evidence for or against consciousness in particular invertebrate taxa.

To contribute to this discussion, here I present a preliminary and generic attempt to such a framework. It consists of five broad categories of answers to the question of whether invertebrate organisms of a particular taxon are sentient or not. These categories are “very probably yes”, “probably yes”, “possibly yes”, “possibly no”, and “probably no”. A sixth category, “very probably no” is later introduced to assess consciousness in other non-invertebrate taxa included for comparison purposes. These six categories range from highest to lowest assigned probabilities of consciousness.

Each category is established by appeal to specific criteria. Hence, existing evidence of consciousness of an invertebrate taxon is assessed in light of those criteria. The different taxa have been classified depending on the degree to which they meet them. These allows for comparisons between these taxa. Additionally, this framework can be useful for assessing new evidence that may update our views, and for studying other invertebrate taxa as well.

It must be stressed that differences between adjacent categories do not necessarily represent equal intervals in the underlying “scale” which gives rise to these groups. However, since the categories are ordered from higher to lesser probability, this framework allows us to claim, for example, that honey bees are more likely to be conscious individuals than jellyfish. This and similar judgements reflect my interpretation of the available evidence in light of the selected criteria. New evidence may change how current knowledge is understood and how organisms of a given taxon are classified. Therefore, the fact that an organism falls into a category should not be interpreted as a definitive answer to that organism’s probabilities of being conscious.

In what follows, I try —to a limited degree— to describe the criteria that define each category. I devote space to elaborate on such criteria because a practical framework for categorizing our findings requires clear and defined indicators. Otherwise, if a criterion is not correctly specified, what counts or not as evidence and what the evidence points to will be subject to the researcher’s interpretation. However, please keep in mind that this is a preliminary work, and I am not an expert. Presumably, specialized knowledge, further empirical research and the development of a fundamental theory of consciousness may contribute to improve our conceptual clarity and to better weigh the evidence we possess about consciousness in non-human individuals.

Finally, the fact that several organisms fall into the same category does not necessarily entail that the existing evidence for them is similar. There are cases in which our evidence for some taxa is stronger than for others, even in the same category. To better understand why a specific taxon falls into a given category, see our ‘Invertebrate Sentience Table’ and our Summary of findings Part 2, where it is further described which potentially consciousness-indicating features are found in each taxon.

Very probably yes

I include in this category taxa that meet the following criteria:

  • Given the current state of our knowledge, there is direct evidence that individuals of these taxa exhibit features which, according to expert agreement, seem to be necessary –although not sufficient– for consciousness (Bateson, 1991; Broom, 2013; EFSA, 2005; Elwood, 2011; Fiorito, 1986; Sneddon et al., 2014; Sneddon, 2017). These features are:

    • Neuroanatomical structures and physiological functions, such as nociceptors or equivalent structures, centralized information processing, vertebrate midbrain-like function, and physiological responses to nociception or handling. Additionally, it is expected that conscious individuals have opioid-like receptors and analgesics reduce their nociceptive reflexes and avoidant behaviors;

    • Behavioral responses that are potential indicators of pain experience, such as defensive behavior or fighting back, and moving away from noxious stimuli. These reactions seem to take into account a noxious stimulus’ intensity and direction. Other observed behaviors include pain relief learning, and long-term behavior alteration to avoid a noxious stimulus.

  • In several cases, direct evidence of those features is incomplete. Still, we find an important body of information about other potentially consciousness-indicating features that, broadly, account for the organism’s ability to display complex and flexible behaviors. Additionally, other behavioral indicators suggest that these organisms may experience emotional states and have cognitive skills. This criterion includes:

    • The individual shows different reactions to noxious stimuli, depending on exogenous or endogenous changes (motivational trade-offs);

    • Proxy indicators of memory are observed, such as ‘spatial memory’ and ‘long-term behavior alteration to avoid noxious stimulus (24+ hours)’ (about the importance of memory for assessing consciousness, see Baars (2003) or Stein et al., (2016));

    • There is evidence of additional forms of learning different from habituation, sensitization and mere associative learning. That is to say, the organism can learn in forms that are more likely to require consciousness, such as operant conditioning with an unfamiliar action, observational learning and contextual learning;

    • Individuals may display certain benchmarks of cognitive sophistication, such as tool use[4];

    • As conscious pain is plausibly correlated with certain kinds of behaviors, so, too, we expect that various emotional states are correlated to an array of behavioral indicators. Hence, behaviors that suggest different positive and/​or negative mood states are also observed in these organisms;

    • Various forms of navigational skills are identified.

  • In addition, these organisms also respond to several drugs in a manner similar to humans;

  • There may be other behavioral evidence of consciousness, such as goal-directed behavior, communicational behavior and forms of interaction that probably require consciousness;

  • Consistent with the above, there is some agreement among prominent scientists and philosophers that these animals may be conscious.

Given the current evidence I assign a high credence to individuals of the following taxa being sentient:

Recent research has shown that octopuses are highly intelligent and potentially capable of experiencing pain. This has led to the inclusion of octopuses in animal protection legislation in some jurisdictions (e.g. in the European Union, see Directive 2010/​63/​EU), and their recognition as conscious individuals in the Cambridge Declaration on Consciousness (2012).

Probably yes

In these cases, the defined criteria are:

  • Although incomplete, there is direct evidence that individuals of these taxa exhibit features which, according to expert agreement, seem to be necessary –although not sufficient– for consciousness (Bateson, 1991; Broom, 2013; EFSA, 2005; Elwood, 2011; Fiorito, 1986; Sneddon et al., 2014; Sneddon, 2017) (see the first criterion of the ‘very probably yes’ category);

  • There is an important body of information about other potentially consciousness-indicating features that, broadly, account for the organism’s ability to display complex and flexible behaviors. Additionally, other behavioral indicators suggest that these organisms may experience emotional states and have cognitive skills (see the first criterion of the ‘very probably yes’ category). However, this sort of evidence is not as abundant as in the previous case (‘very probably yes’). Or, for some features, it is discussed whether a specific behavior is a conscious reaction or an automatic response (e.g., autotomy or grooming, as forms of protective behavior). Alternatively, it may happen that available evidence is indicative of consciousness but it is still relatively recent;

  • In addition, these organisms also respond to some drugs in a manner similar to humans;

  • There may be other behavioral evidence of consciousness, such as goal-directed behavior, communicational behavior and forms of social interaction and organization that probably requires consciousness;

  • Hence, some prominent scientists and philosophers claim that these animals may be conscious. However, there does not seem to be as widespread an agreement on this as that observed for the previous category (‘very probably yes’).

Given the current evidence I considered that it is probable that individuals of the following taxa are conscious:

Honey bees display considerable learning abilities, complex social behaviours and even communicational behaviors. They are probably conscious but we have little evidence of the presence of a pain system, if any. Regarding decapod crustaceans (e.g., crabs, crayfish), recent research has shown that they are potentially capable of experiencing pain. According to the European Food Safety Authority (2005), these animals exhibit complex behaviors and should be legally protected. A similar position is held by the British Veterinary Association (2017). Fruit flies, for their part, have been widely used as biological models. New research provides compelling evidence suggesting that they not only feel pain, but also experience chronic pain that lasts long after an initial injury has healed (Khuong et al., 2019). Behaviorally, however, fruit flies do not seem to exhibit such complex reactions as those observed in honey bees or decapods.

Possibly yes

In these cases:

  • Several potentially consciousness-indicating features are found through these taxa, suggesting that these individuals may be sentient. Most of this evidence refers to behavioral observations. These behavioral indicators include:

    • Responses such as moving away, escaping and avoidance, that seem to account for noxious stimuli intensity and direction;

    • Proxy indicators of memory, such as ‘spatial memory’ and ‘long-term behavior alteration to avoid noxious stimulus (24+ hours)’ (about the importance of memory for assessing consciousness, see Baars (2003) or Stein et al., (2016));

    • There is evidence of additional forms of learning different from habituation, sensitization and mere associative learning. That is to say, the organism can learn in forms that are more likely to require consciousness, such as operant conditioning with an unfamiliar action, observational learning and contextual learning:

    • The individual shows different reactions to noxious stimuli, depending on exogenous or endogenous changes (‘motivational trade-offs’);

    • There may be other indirect behavioral evidence of consciousness, such as navigational skills or forms of social interaction and organization that probably require consciousness.

  • However, in these cases, the neuroanatomical structures and neuroanatomical functioning related to valenced experience remain unclear. In general, the physiology or structures that seem to correlate with the presence of sentience (e.g., nociceptors), if they exist, have not been identified yet;

  • Nevertheless, limited evidence of the neurophysiological bases of consciousness should not be confused with their absence. Further studies should assess some open questions about the neuroanatomical and physiological bases of existing behavioral findings.

Hence, the case for the taxa in this category being conscious is not as strongly supported as in previous taxa. I therefore assign a lower credence to their being conscious:

Social ants, and to a lesser extent, spiders (especially, jumping spiders) and cockroaches, show considerable learning abilities and complex social behaviour. The small size of the brain does not necessarily mean poor functioning, since their nerve cells are very small. However, we have little evidence of a pain system (if any). Of these three taxa, the available evidence is weaker for cockroaches.

Possibly no

  • There is limited evidence about features which, according to expert agreement, seem to be necessary –although not sufficient– for consciousness. In particular, some neuroanatomical and physiological features may be observed (e.g., nociceptors). However, there is too little evidence for us to positive conclude that these organisms possess a pain system;

  • Behaviorally, different nociceptive responses may be observed, along with expressions of associative learning. Nevertheless, the existing evidence about motivational tradeoffs, mood state behaviors or cognitive sophistication is highly limited;

  • In general, there is very limited scientific evidence related to the presence of valenced experiences in these individuals. Still, there is more positive than negative evidence about the possibility that these individuals are sentient.

Sea hares (Aplysia) are an example of a taxon falling into this category, as per the above criteria.

Aplysia are the most active marine gastropod molluscs. Much research has been carried out on the nervous system of sea hares and their relatives. Evidence of learning and flexibility of behaviour is considerable but there are also studies showing very rigid responses.

Probably no

  • There is highly restricted evidence of features that –according to expert agreement– seem to be necessary for consciousness. This reduces in a significant manner the likelihood that these animals are sentient;

  • Similarly, information about other anatomical, physiological and behavioral features is scarce. Other potentially indirect evidence of consciousness has not been not observed either;

  • We may not encounter direct negative evidence against any potentially consciousness-indicating features in these organisms, but indirect research suggests that some features may not be present or that there is a low chance of them to be found in these animals;

  • Broadly, these animals’ pain system (if they have any) has not been sufficiently studied. And in spite of certain behavioral evidence, it is hypothesized that observed reactions may be reflexes.

According to these criteria, the following taxa fall into this category:

Regarding earthworms, there is limited research about them, since they are not usually considered sentient organisms. Most of the existing evidence points out that their noxious-stimuli-related responses are highly rigid.** **However, of the three taxa listed in this category, current evidence is slightly stronger for earthworms, especially because of the relative complexity of their nervous system.

In the case of C. elegans, despite being a widely studied animal, evidence of their being conscious is weak and their behavioral reactions are simple and highly stereotyped. Finally, about moon jellyfish, although they display some noxious stimuli reactions and have a nerve net that allows them to detect various stimuli, they do not possess a central nervous system or other equivalent structure that centralizes information processing.

3. Other organisms

As explained in a previous post, we investigated plants (kingdom Plantae), prokaryotes and protists to give a sense of how often potentially consciousness-indicating features are found in organisms that are widely believed to lack consciousness. Regarding these three broad taxa**:**

  • There is no solid evidence that individuals of these taxa exhibit features which, according to expert agreement, seem to be necessary –although not sufficient– for consciousness. In particular:

    • The physiology and structures that seem to correlate invariably with the presence of sentience (e.g., nociceptors) are not observed. No equivalent structures or functioning are found either;

    • Behaviorally, the organism may display some basic noxious stimuli reactions (e.g., moving away), but these responses do not seem to account for noxious stimulus intensity and direction.

  • Similarly, information about other anatomical, physiological and behavioral features is not found. Other potentially indirect evidence of consciousness is not observed either;

  • Consistent with the above, there is a general agreement among the scientific community that these organisms are not conscious.

Following these criteria, plants, prokaryotes and protists are very probably not conscious:

4. Results

The following graph summarizes my previous estimations:

Marcus A. Davis

Estimating the Probability of Sentience

Humanity currently doesn’t understand what fundamentally generates the ability for humans to experience the world. Even were we to gain such an understanding, unless we also gather the ability to understand everything that could plausibly generate such experiences, as opposed to merely what does in humans, it will still be difficult to ascertain if other creatures who have different neuroarchitecture than humans are also experiencing the world. For these reasons and others, I’m very uncertain about the value of providing quantitative estimates for the taxa we’ve studied but still think it’s worth doing so on balance. To me, the case for doing so is based primarily on reasoning transparency as opposed to others using my estimates as placeholders in their calculations. Given the uncertainty around perceptions of invertebrate sentience, for others considering our research and actions it’s useful to see how this project affected, or did not affect, my beliefs and what those beliefs are with respect to invertebrates.

However, there are several other additional qualifications worth attaching to my estimates including the following:

  • I’m by no means an expert on consciousness nor am I an expert on any of the classes of invertebrates we are considering.

  • In general, I don’t believe my estimates are very likely to be well calibrated, and my best guess isn’t necessarily very stable in all cases. Even relative to the difficulties of generally being well-calibrated, I find my subjective estimates likely aren’t very meaningful and I’ve accordingly provided relatively wide estimates for how probable these creatures are to be sentient. Even with this caveat, the range is still more of a guideline for my subjective impression than a declaration of what all agents would estimate given their engagement with the literature.

  • For many groups of invertebrates, the amount that is currently unknown suggests that any attempt to place a particular number is largely a forecast about what future studies may reveal as opposed to reflective of what current studies have revealed.

  • Given the uncertainty, the absolute point estimate I provide of each taxa’s sentience is probably less valuable than the relative rankings of a given taxon compare to each other.

  • Extrapolating from the taxa we examined to other invertebrate taxa, while tempting, is not advisable. I set my estimates for the specific creatures under consideration, and without considering how others might try to extrapolate them to other creatures, even other organisms within the same genus or family we considered.

  • These estimates do not represent the moral weight I believe these creatures would possess provided they are in fact sentient. I’ve not attempted to estimate moral weight for these groups but were I to do so the answer could be affected by a range of features including intelligence, internal clock speed, social complexity, and affective complexity.

Finally, given these considerations, I would add, sticking my specific point estimates (or all of RP’s) directly into a cost-effectiveness analysis is probably foolhardy. Ranges would serve you better, but even then you reduce these estimates to an easy calculation used for expected value at your own hazard.

How I’ve updated since the beginning of this project

Overall, I found some evidence of invertebrate behavior relatively surprising, particularly some responses to drugs, certain tool use, and long-term changes due to learning. Though I didn’t create point estimates for each of these taxa before beginning this project, I suspect the combination of work this area caused me to, at least, update positively for ants, cockroaches, honeybees, fruit flies, and octopuses.

However, perhaps my largest surprise wasn’t an update toward or against a particular type of animal, rather it was based on the extent of conditioned learning behavior that is more or less exhibited by all taxa we considered, including single-celled organisms and animal bodies detached from brain communication, including the lower body of a mouse with a severed spine. While one could take this as weak evidence of widespread sentience, this updated me toward thinking many of these behaviors aren’t very impressive and they were thus largely disregarded in contemplating the positive case for sentience.

Further notes on specific taxa

For chickens and cows, my priors about their probability of sentience remained approximately where it was when we began this project. This isn’t a surprise given they weren’t our focus and I didn’t learn much new information about chicken or cow capacities during our work.

For the remaining non-invertebrate taxa—plants, protists, prokaryotes—they possess particularly weak evidence of sentience, and lack central nervous systems and brains. My estimates reflect this doubt in their sentience. However, one significant issue I encountered was while they vary in abilities and thus there’s a good case a precise and well-calibrated estimate of sentience would vary, generally attempting to estimate the probability that any of these taxa are sentient amounts to an attempt to estimate the probability that more or less everything I understand about sentience is incorrect. Are the odds of such an occurrence one in ten thousand? One in one million? Or, are they one in one hundred billion? I don’t have a relevant reference class to draw from to answer such a question and thus it would likely be more accurate to surmise I take the probability that any of these groups is sentient is vanishingly unlikely and, for all intents and purposes, approximately zero.

Peter Hurford

Some of my colleagues have declined to produce estimates for the likelihood of sentience for various invertebrates, instead suggesting that the interested reader go through our evidence and create their own estimates. I’m somewhat sympathetic to this view—there is a large amount of uncertainty involved and judgement calls must be made on both personal values and weighing the evidence we have. Furthermore, keep in mind that yet to be resolved questions around moral weight, Pascal’s mugging, and generalizability also complicate how these probabilities will be used in practice. Lastly, I don’t want you to overweight my opinions or think of me as some sort of expert where I am not. While I may have my personal probabilities, I respect that there may still be a wide variety of disagreement on these probabilities, implied values, and implied views on how to make decisions based on these. I invite people to come to their own conclusions.

That being said, I still wish to offer my own probabilities. For one, I think we owe the interested reader the clearest possible takeaways to do as much work for them and save them time—as long as we provide the appropriate caveats and warnings (you have been warned). Additionally, I think stating clear probabilities can be a clear way to explain and express my personal worldview and help make disagreements more precise, similar to what was done by Muehlhauser (2017), Section 4.2. I hope that the way we have structured this exercise—by presenting individual opinions that showcase disagreement among our own team and that go above and beyond to state the many caveats and uncertainties—will be enough to dissuade you from overweighting our opinions. I believe you deserve to know where I am personally coming from—more precisely and quantitatively.

Overall, I believe that many kinds of invertebrates are a lot more likely to be capable of phenomenal consciousness than I thought, given that these taxa are a lot more behaviorally sophisticated than I thought, and it becomes difficult to point to certain sophistications that cows and chickens display that honeybees do not. My general “case against” primarily rests upon what I think is a substantial chance for systematic bias in the papers we cite toward finding startling publishable conclusions, plus thinking that maybe a fundamental theory of consciousness will eventually emerge that leads me to put much more reduced epistemic rate on behavioral evidence, plus some remaining credence to my uninformed prior intuition for invertebrates are not all that sophisticated (e.g., intuitions from phylogenetic distance, intuitions that complex behavior may not actually be that complex, difficulties figuring out the complexity of behaviors).

I should also endeavor to repeat that I don’t have any particular reason to think these probabilities are well-calibrated or even all that useful, except as a more detailed expression of my current views and how they changed (in some cases, significantly). I’m definitely not an expert in thinking probabilistically and I’m unsure of the extent to which these probabilities do capture my views—though I do think they capture my views more clearly than I would express qualitatively in words, hence my reasoning for giving them.

While I would like to share my probabilities both before and after starting this research, I unfortunately did not actually record probabilities prior to starting the research. I tried my best to reconstruct them now as I would’ve thought about them then, to at least provide a rough guide as to how my views have changed.

I think my explanation for changing my views comes from two sources. First is the large amount of research that went into our table—I found myself particularly weighing centralized information processing, contextual learning abilities, social learning, long-term behavior alteration, flexible tool use, and play behavior when attributing phenomenal consciousness to some organisms and not others. Also, while I weigh neuron count now much less as a factor than I previously did, I still think it is plausible that the capacity for phenomenal consciousness may scale to some degree with higher neuron count.

Second was just generally thinking about the role and function of consciousness for another year lead me to become more confident in certain areas. For example, while I am now more skeptical of a clear line being established between chickens and honeybees, I am more certain we can establish a clear line between honeybees and plants that excludes the plausibility of plant sentience. While looking into research on plant sentience did make me more impressed at the capabilities of plants, it also helped me understand how much different they are than the capabilities of other invertebrates and how seemingly complex behavior can likely result without consciousness.

*I added chimpanzees here because they were in Muehlhauser (2017), Section 4.2 and seem useful as a point of comparison, but they weren’t actually studied in our work.

Jason Schukraft

The Issue

What are the odds an octopus has the capacity to experience pleasure and pain? How much likelier is it that flies are sentient compared to earthworms? Given the available evidence, what is the rational credence in the proposition crayfish are phenomenally conscious?

Answers to questions like these could be extremely useful. Invertebrates outnumber vertebrates by a wide margin, but population counts alone aren’t the whole story. If invertebrates lack the capacity for valenced experience—that is, their experiential states never take on a positive or negative affect (either because they don’t have experiential states or those states are always neutral)—then invertebrates aren’t moral patients and don’t possess intrinsic moral worth.[5] If we knew the rational probability, conditional on the available evidence, that different groups of invertebrates had the capacity for valenced experience, we could multiply that probability by the (estimated) number of animals in each group. That would yield the expected number of animals for each taxon, and this figure could form the basis for evaluating how to divide scarce resources among different groups of animals.[6]

Nonetheless, I think we shouldn’t (yet) try to answer these types of questions.

More precisely, I contend that publishing specific estimates of invertebrate sentience (e.g., assigning each taxon a ‘sentience score’) would be, at this stage of investigation, at best unhelpful and probably actively counterproductive. The benefits are slim, and the risks are non-negligible. To be clear: I don’t believe it’s a bad idea to think about probabilities of sentience. In fact, anyone directly working on invertebrate sentience ought to be periodically recording their own estimates for various groups of animals so that they can see how their credences change over time. I also believe that we ought to aspire, as a field, to be able to provide robust, actionable estimates of invertebrate sentience. I just don’t think we’re there yet.

The Problem

Undoubtedly, the work of Rethink Priorities would be more digestible if we assigned a sentience score (either as a straightforward probability or as a position on an arbitrary scale so that such scores could be compared across taxa) for each of the groups of animals we investigated. Unfortunately, assigning objectively good sentience scores would be extraordinarily difficult. The 53 features we investigated are not equally important, and the context in which they are displayed often makes a substantial difference to their evidential weight. One would have to have an expert grasp on biology, philosophy, and neuroscience (as well as lots of time on their hands) to even justifiably begin such a scoring project. But without a clear methodology, it’s unclear how effectively the scores could be criticized or improved in the future. And because subjective experience is, well, subjective, strict calibration in this domain is necessarily impossible.

Of course, having studied the topic for some time now, I expect that my estimates would be better than the estimates of the average member of the EA community. If that’s true, then it’s tempting to conclude that making my estimates public would improve the community’s overall position on this topic. However, I think there are at least three reasons to be skeptical of this view.

(1) Sentience scores for specific taxa aren’t that useful

Rethink Priorities investigated the evidence of sentience for 18 groups of organisms.[7] After deciding which types of organisms we wanted to investigate, we next needed to decide the taxonomic level at which we would investigate those organisms. We tried to drill down to a fairly narrow taxon (species, genus, or family) because, in general, the higher up the taxonomic hierarchy one goes, the more diverse a taxon becomes. If a taxon becomes too large, then saying that the taxon possesses some feature ceases to be informative. If 50 different arthropods each possess one (and only one) of the features we investigated, and each species possesses a different feature, a database with the category “arthropod” would give the misleading impression that arthropods definitely have the capacity for valenced experience.

But when it comes to actual interventions, some degree of generalization is going to be inevitable. It’s extremely unlikely that a cost-effective intervention is going to specifically target Drosophila melanogaster or Caenorhabditis elegans. It’s much more likely that the intervention would target some much larger group, like insects or nematodes. So a sentience score for Drosophila melanogaster or _Caenorhabditis elegans _wouldn’t actually provide much actionable information without some idea whether and to what extent that sentience score generalizes. But developing a justified sentience score for Drosophila melanogaster or Caenorhabditis elegans is hard enough; doing so for groups as large and diverse as insects or nematodes would require far more data than we gathered.[8]

Moreover, even well-justified generalized sentience scores aren’t super useful unless we have at least a rough handle on how different groups of animals compare in moral weight. Knowing that there is a 25% chance eusocial insects are sentient and a 95% chance birds are sentient doesn’t help one judge which interventions are most effective unless one also know approximately how many insect-life-days a bird-life-day is worth.

(2) Sentience scores would inevitably be over-emphasized

Another concern is that interested parties might skip straight to our (uncalibrated, somewhat unjustified, extremely speculative) numerical sentience estimates without taking the time to understand the nuance and intricacy of the issue. Numbers are seductive because they are so easy to manipulate (e.g., just stick them into a cost-effectiveness analysis!). But caveats don’t fit into spreadsheets. Numbers, even ranges of numbers, can’t convey subtlety or complexity or nuance. Withholding explicit sentience estimates forces the reader to take a more holistic approach to our work. For an enormously complicated subject like sentience, the details really matter. I would much prefer a reader to come away from one of our posts with a renewed appreciation for the complexity and importance of the issue rather than sentience credences that exactly match mine.

It’s difficult to present explicit estimates of invertebrate sentience in a way in which those estimates don’t steal the show. It’s hard to imagine a third party summarizing our work (either to herself or to others) without mentioning lines like ‘Rethink Priorities think there is an X% chance ants have the capacity for valenced experience.’ There are very few serious estimates of invertebrate sentience available, so members of the community might really fasten onto ours. But if we did publish our estimates, I would want them to be viewed as hypotheses to be further refined (or perhaps completely abandoned) as more evidence comes in rather than hard conclusions that our work definitively supports. Personally, I worry that assigning sentience scores sacrifices too much in the name of digestibility.

(3) Sentience scores might reduce our credibility with potential collaborators

The future of the invertebrate welfare cause area will depend in large part on our ability to collaborate with biologists, ethologists, and neuroscientists. There is a great deal of empirical information, from better population appraisals to studies on the self-administration of anxiolytic drugs, that needs to be gathered to generate an informed view of how to best help various invertebrates. But science, especially peer-reviewed science, is an inherently conservative enterprise. Scientists simply don’t publish things like probabilities of sentience. For a long time, even the topic of nonhuman sentience was taboo because it was seen as unverifiable. Without a clear, empirically-validated methodology behind them, such estimates would probably not make it into a reputable journal. Intuitions, even intuitions conditioned by careful reflection, are rarely admitted in the court of scientific opinion.

Rethink Priorities is a new, non-academic organization, and it is part of a movement that is—frankly—sort of weird. To collaborate with scientists, we first need to convince them that we are a legitimate research outfit. I don’t want to make that task more challenging by publishing estimates that introduce the perception that our research isn’t rigorous. And I don’t think that perception would be entirely unwarranted. Whenever I read a post and encounter an overly precise prediction for a complex event (e.g., ‘there is a 16% chance Latin America will dominate the plant-based seafood market by 2025’), I come away with the impression that the author doesn’t sufficiently appreciate the complexity of the forces at play. There may be no single subject more complicated than consciousness. I don’t want to reduce that complexity to a number.

The Solution

The unfortunate reality is that producing useful estimates of invertebrate sentience is going to be incredibly hard. Nevertheless, the cause is too important to give up on. There are just too many invertebrates to justifiably ignore. The best and only solution is just to keep working as diligently as we can on the issue until we are in a position to deliver informed, actionable estimates. And let’s not kid ourselves: we’re not there yet.

But it’s not as if our work makes no progress on the question. We’ve argued that coleoid cephalopods (i.e., squid, cuttlefish, and octopuses) are clearly the best candidates for phenomenal consciousness in the invertebrate world. There is also very intriguing evidence of sentience for decapod crustaceans (i.e., prawns, shrimp, crayfish, crabs, and lobsters) and eusocial insects (i.e., ants, termites, bees, and wasps). The evidence of sentience is much greater for these groups than it is for jellyfish, earthworms, or roundworms. There is basically no reason to think plants, protists, or prokaryotes are sentient. It’s still possible that they are, but if so, our best philosophical and neuroscientific theories of consciousness are radically mistaken. These initial claims are well-justified and supported by the available data. They give us a foothold from which to explore other questions, and they offer at least rough guidance on where to focus further research. For now, that’s enough.


This essay is a project of Rethink Priorities. It was written by Jason Schukraft, Peter Hurford, Marcus A. Davis, and Daniela R. Waldhorn. Thanks to David Moss and Adrià Voltes for helpful feedback. If you like our work, please consider subscribing to our newsletter. You can see all our work to date here.


  1. Before November 2018 there were only about 10-15 person-hours devoted to this project per week. Beginning in November 2018, there were closer to 40-50 person-hours per week devoted to this project. ↩︎

  2. For examples of opinion pieces published in a high-quality science journal, see, inter alia, “What does AI’s success playing complex board games tell brain scientists?”, “Governing the recreational dimension of global fisheries,” and “Why science needs philosophy,” all published in PNAS. ↩︎

  3. The Cambridge Declaration on Consciousness claims that “convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates.” ↩︎

  4. It should be noted that this criterion may, in itself, be the subject of further discussion. Which behaviors account for “cognitive sophistication” or assigning a “level of sophistication” to a given action is a complex matter that is beyond the scope of this article. Generally, it is assumed that the use of tools by octopuses is a display of cognitive sophistication. However, some scientists resist such an implication. In particular, Schnell & Clayton (2019) argue that, although octopuses do use tools (e.g., carrying coconut shells around as mobile dens), in the absence of controlled experiments, simpler explanations for this behavior cannot be ruled out. Further research is needed based on other than anecdotal observations. Hence, the conclusions here presented should be taken with caution. I thank Adrià Voltes for these points. ↩︎

  5. Strictly speaking, this argumentative move is too fast, but the details need not detain us. ↩︎

  6. Of course, final decisions would factor in lots more information, such as our best guesses about relative moral weight, the tractability of various interventions, flow-through effects of those actions, and so on. ↩︎

  7. I can’t just say ‘animals’ because we also investigated plants, protists, and prokaryotes. ↩︎

  8. Moreover, even well-justified generalized sentience scores aren’t super useful unless we have at least a rough handle on how different groups of animals compare in moral weight. Knowing that there is a 25% chance eusocial insects are sentient and a 95% chance birds are sentient doesn’t help me judge which interventions are most effective unless I also know approximately how many insect-lives a bird-life is worth. ↩︎