In recent weeks, I have been touring the United States and the United Kingdom, promoting "Animal Liberation Now," my new book on the ways in which we are inflicting suffering on hundreds of billions of nonhuman animals, especially in factory farms. The persistence of this vast, entirely unnecessary suffering is one of the great moral issues of our time.

Some people doubt this claim because they think that humans matter incomparably more than animals. But it is increasingly accepted, among the general public and ethicists, that preventing suffering is morally important, irrespective of the species of the being who is suffering.

But this concern for other species raises a much more difficult question: how do we compare the suffering of animals of different species? Given our limited time and resources, how many chickens should we help instead of helping one human? An equally perplexing question can be asked about the decision to focus on preventing the suffering of some animals -- say, pigs -- rather than others, when we are harming many more of the latter. For example, each year we raise, in close confinement, and then kill, without prior stunning, more than 15 times as many fish as the entire human population of the planet. What moral standing does their suffering have?

Here, we need to consider some essential philosophic and neuroscientific issues. How do our own capacities to suffer differ from those of animals, and how can we compare the capacities for suffering of different kinds of animals? We also need to ask: Is preventing suffering all that matters, or do other things matter, too, like thwarting individuals’ future-oriented preferences? If it is wrong to thwart such preferences, then the differences between humans and other animals, and between different species of nonhuman animals, may become more significant.

Rethink Priorities, an American think tank, has been studying these questions. Researchers surveyed hundreds of studies of animal sentience to provide a basis for estimating how much we should weigh the suffering of a range of animals, relative to the weight we give to the suffering of humans.

To simplify the task, they assumed the utilitarian view that promoting happiness and preventing suffering is all that matters. They also assumed that we should not care any less about suffering experienced by an animal only because it is not human. Provided that the intensity and duration of suffering are equal, we should care about it equally, no matter who experiences it.

To investigate how much animals suffer, as compared to humans, the researchers began by accepting, in accordance with current scientific understanding, that the basic neurochemistry that underlies emotional feelings is essentially similar, at least in all mammals. The researchers then looked at both behavioral and physiological evidence of the suffering of various species of animals.

In Animal Liberation Now, I discuss one clear example of behavioral evidence. Commercially raised chickens have been bred to grow so fast that their immature leg bones have difficulty in supporting their weight. As a result, they often become lame, trying not to put weight on one leg.

In one study, chickens taken from commercial flocks were offered two differently colored foods, one of which contained an anti-inflammatory drug. Lame birds soon learned to choose more of the food with the drug, and their limping decreased in proportion to the dose they consumed. This indicates that lame chickens are likely to be in pain when they walk and suggests a close parallel with the effect that relieving pain has in humans.

Other relevant behavioral evidence indicating pain can include changes in heart rate, blood pressure, pupil dilation, perspiration, hormonal levels, and metabolic activity. Here, too, chickens bred for rapid growth serve as an example: the skeletal, heart, and lung problems that they experience are evidence of suffering.

Drawing on this evidence, the researchers at Rethink Priorities developed a set of estimates for how we might weigh the suffering of some representative animals. If we assign the value 1.00 to humans (and round the estimates to two decimal places, to mitigate any misplaced sense of exactitude), their estimates suggest that we might value pigs at 0.52, chickens at 0.33, and octopuses at 0.21. They ranked all fish at less than 0.1, with carp at 0.09 and salmon at 0.06. Interestingly, they ranked bees between these two fish species, at 0.07, and crayfish lower, at 0.04.

Obviously, this effort to draw on the best available empirical research to compare the capacities of animals of different species, and even of different phyla, is open to challenge. But the importance of the effort is indisputable. The possibility of our own species’ moral progress may depend on it.

Peter Singer

Peter Singer, a professor of bioethics at Princeton University, is founder of the charity The Life You Can Save and the author, most recently, of “Animal Liberation Now” (Bodley Head, 2023). -- Ed.

(Project Syndicate)