If you just spun the wheel, you probably became a nematode. If you excluded nematodes, you probably became an arthropod—maybe a copepod or an ant. Your chance of being human was roughly 1 in 137 billion. To put that in perspective, if you were being reborn every second, it would take over 4000 years before you'd expect to land on a human life even once. This vast difference in numbers raises a question: if most sentient beings on Earth are non-human, and if they can suffer, should their experiences matter as much as ours? Many of us assume human interests should take priority, but is that because we've thought it through—or simply because we happen to be human?

The Philosophical Argument

This simulator is based on John Rawls's "veil of ignorance" - a thought experiment where you imagine designing society's principles without knowing what position you'll occupy. The idea is that this approach encourages fairness and impartiality in establishing societal principles, as it forces people to consider the needs of all members of society, regardless of race, gender, wealth, or other factors. It especially forces people to consider the experiences of the least advantaged.

Mark Rowlands extends this to non-human animals. He argues that if Rawls's contractarianism is applied consistently, representatives behind the veil should be blind to species membership. The probability argument suggests that with quintillions of wild animals versus billions of humans, the statistical likelihood of being non-human is overwhelming. Behind the veil, any rational agent would want to minimize the worst possible outcomes— and when non-human animals make up 99%+ of individuals, their potential suffering becomes impossible to ignore.

Implications

Manmade suffering: Over 90 billion land animals are slaughtered annually for food—most in intensive conditions involving confinement, mutilation without anesthesia, and slaughter methods that often fail. Our daily choices about what we eat directly fund these systems. Behind the veil of ignorance, knowing you had a far greater chance of being a factory-farmed chicken than a human being, would you want a world in which humans continue to do this to animals?

Natural suffering: We often romanticise nature and the natural order of things. We don't recognize the intense and widespread suffering that is endured by all types of animals every day. Humans have built civilizations and systems to escape the most brutal aspects of nature. Knowing that you have an over 99% chance of being born a wild animal, would you create a world in which wild animal suffering is ignored entirely?

Note On Sentience

You may be skeptical about the sentience of some of the animals included in this simulator, particularly nematodes, arthropods, and annelids. Recent scientific evidence suggests at least a "realistic possibility" of insect sentience: research using the Birch et al. (2021) framework found strong evidence of pain in flies, mosquitoes, cockroaches, and termites; bees display emotion-like states, play behavior, and respond to analgesic drugs; and a 2024 declaration by leading consciousness researchers states that "empirical evidence indicates at least a realistic possibility" of insect consciousness.

Rethink Priorities estimates a 6.80% probability of sentience in the nematode C. elegans. Given the staggering populations, even small probabilities of sentience are worth giving weight to.

Note on Bacteria This simulator does not include bacteria, despite their incomprehensibly vast numbers (approximately 10³⁰ individuals). While the probability of bacterial sentience is estimated at only 0.462%, if bacteria were sentient, they would dominate this wheel so overwhelmingly that virtually nothing else would matter. The sheer scale—a billion times more bacteria than all animals combined—means that even this tiny probability could have enormous moral implications.

Of course, we can never be certain—direct proof of subjective experience is impossible to obtain. If you strongly believe any particular animals are not conscious, you can exclude them using the toggles above.

Want To Learn More?

• Jack Hancock's talk: What 99% of People Don't Know About Wild Animals
• Animal Ethics' article: Justice for all: what the veil of ignorance shows us about a just society
• Animal Ethics' book: Introduction to Wild Animal Suffering
• Oscar Horta's paper: Debunking the Idyllic View of Natural Processes: Population Dynamics and Suffering in the Wild
• The Effective Altruism community's work on animal suffering

How This Simulator Works

This simulator estimates what you'd most likely experience if you were randomly reborn as any sentient being alive today. It uses a two-stage selection process: (1) Select a category based on total population (e.g., Nematodes: 10²¹, Marine Arthropods: 10²⁰), and (2) Select a specific animal within that category, weighted by individual population estimates.

Population Data Sources

Category population totals are primarily based on the following sources:

• Brian Tomasik's comprehensive synthesis of wild animal populations
• Vasco Grilo's compiled population estimates
• Bar-On, Phillips, and Milo (2018), "The biomass distribution on Earth," PNAS
• UN FAO data for farmed animals via Our World in Data
• Multiple sources for companion animals (World Animal Foundation, ASPCA)

Specific animal outcomes within each category are representative examples selected to illustrate the diversity and typical experiences within that group. Their relative proportions are based on general ecological abundance patterns estimated by Claude (Anthropic), not exact population counts from peer-reviewed sources.

Category Population Estimates

• Nematodes: 10²¹ (1 sextillion) — Soil, marine, freshwater, and parasitic roundworms
• Marine Arthropods: 10²⁰ (100 quintillion) — Primarily copepods, plus krill and other small crustaceans
• Terrestrial Arthropods: 10¹⁹ (10 quintillion) — Insects (ants, beetles, flies, bees), spiders, mites
• Annelids: 10¹⁸ (1 quintillion) — Earthworms, polychaete worms, leeches
• Molluscs: 10¹⁸ (1 quintillion) — Snails, slugs, clams, mussels, oysters
• Fish: 10¹⁵ (1 quadrillion) — All wild marine and freshwater fish
• Reptiles: ~3 trillion — Lizards, snakes, turtles
• Amphibians: ~3 trillion — Frogs, toads, salamanders
• Birds: ~100 billion — All wild bird species
• Mammals: ~100 billion — Wild land and marine mammals (mostly rodents)
• Farmed Animals: ~485 billion — Chickens, fish, shrimp, and other animals raised for food
• Companion Animals: ~1.6 billion — Dogs and cats, including pets and strays
• Humans: ~8 billion — Current global population

Sentience and Uncertainty

Research on animal sentience suggests varying levels of confidence across groups. Rethink Priorities' Welfare Range and Probability of Sentience Distributions provide evidence-based assessments of different animals' capacity for welfare, considering factors like neural architecture, behavioral indicators of sentience, and cognitive abilities. There is high confidence that vertebrates (fish, amphibians, reptiles, birds, mammals) can suffer, moderate confidence for many arthropods (especially those with larger nervous systems like bees and crabs), and higher uncertainty for simpler invertebrates like nematodes and mites.

This simulator includes all categories so you can explore the implications of different assumptions. Use the toggles to include or exclude categories based on your beliefs about which animals can suffer. The nematode toggle is particularly important—if nematodes are sentient, they vastly outnumber all other animals combined.

Limitations

This simulator represents only the 48 outcome types we've included across 13 categories. The true diversity of animal life is far greater—there are millions of species we haven't represented. Population estimates for wild animals are highly uncertain and can vary by orders of magnitude. The experiences described are generalizations; individual animals' lives vary considerably.

Despite these limitations, the core message remains: if you were randomly reborn as a sentient being, you would be overwhelmingly more likely to experience life as a small, short-lived creature facing predation, starvation, disease, or—if farmed—confinement and slaughter.