Do humans feel more pain than animals?

Many researchers assume that the pain animals feel scales with size, or at least that animals are either partially conscious or completely unconscious of the pain they are suffering. But is the idea that pain scales with size actually true?

Jeremy Bentham once asked:

“The question is not, Can they [animals] reason? nor, Can they talk? but, Can they suffer?” (1)

I will now try to answer his problematic question.
I work in a laboratory that studies cellular growth and proliferation in Drosophila melanogaster, more commonly known as the fruit fly. During the vast amount of time I spent observing flies, I understood that their behavior, in many aspects, does not differ from ours.
For example, flies are capable of short and long-term memory. They can be trained to learn that if they move towards a certain smell, they will receive an electro-shock. After a few rounds of trial, flies efficiently learn to avoid the smell they now associate with pain.
This already brings me to the first conclusion: even small animals, very evolutionarily distant from us, sense pain and learn to avoid it, just as children learn to stay away from fire.

Humans conduct their experimental research making use of animals to better understand the basic functioning of subcellular machineries, cells and organs. The way biological life works is very similar across species, especially among those that are closer on the evolutionary scale.
Many researchers assume that the pain animals feel scales with size, or at least that animals are either partially conscious or completely unconscious of the pain they are suffering*. In other words, the feeling of pain a mouse experiences should be minor when compared to the one felt by humans, and following the same line of thought, the pain of a fruit fly should be considered nearly irrelevant.
Working policies tend to reflect these assumptions: whereas researchers must obtain licenses when working with mice, they are not required to follow any legislation when working with flies. Researchers, in order to obtain these licenses, must describe the experimental procedures and justify both the progressive purpose for which they will inflict pain on animals, and the amount of pain inflicted.

But is the idea that pain scales with size actually true?
Perhaps, before answering this question we should reflect on the position of humans in the scale of evolution. Humans, as they possess the intellectual ability to reflect on themselves, also have the tendency to think they are occupying a privileged position within the realm of species on the planet.
Giacomo Leopardi, in his 1827 “Dialogue between a goblin and a gnome(2), depicts a hypothetical discussion about which species is the most important one, in light of recent human extinction. They elaborate about how humans went extinct due to their belief that the world was their possession, and the consequent use they made of it.
If we follow Leopardi, the idea that human physical pain is more important than the one felt by animals comes from the assumption that humans occupy the top position at the evolutionary table. The idea that our pain is stronger than that of other animals is derived from the idea that our pain is also more relevant.

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Scientifically speaking, pain in humans is generally assessed on the basis of what the patient communicates.
For large animals, several behavioral observations are used to estimate physical suffering. For small animals like fruit flies, indirect behavioral observations are taken into account (as with the aforementioned pain-avoidance example).
To assess the intensity of animal pain, we are basing our analysis on human-like traits.
If animals resemble humans in their responses to pain, the answer to the original question is certainly that pain does not scale with size. Although there is no proof of this, there is also no evidence of the opposite*.
In other words, we must consider the principle of equity between humans and animals. Along these lines, Joel Feinberg proposed that:

“A skeptic might deny that a toothache hurts a lion as much as it does a human being, but once one concedes that lion pain and human pain are equally pain – in the same sense and the same degree – then one cannot deny that they are equally evils in themselves.” (3)

Considering the discussion above, what should researchers and the public do?
We should first of all admit that we cannot assume that animal pain is less severe than human pain.
We should then question whether the suffering of animals in certain experimental settings is really worth it: the more pain inflicted in an experiment, the more value the experiment should have. Often animals are suffering for the purpose of a publication rather than for scientific progress or to soothe human pain (to fully understand this point read the article on Journals and Scientific Romanticism), and this is why it is enormously important to question the value of each individual research project.
Researchers should be discussing the implications of their research more often. Ethically, scientists are lacking knowledge about the consequences of their actions, and, most of all, are often lacking the willingness to question themselves.
The general public, together with researchers, should generate enough pressure to ensure that a scientific body exists to guarantee that the business of science does not prevail over the ethical and progressive aspects of science. This body, local or – even better – international, should scrutinise the different research proposals, and verify that animal pain is at the very least inflicted for a progressive purpose.
Indeed, it is intrinsically paradoxical that, in order to identify pharmacological treatments to cure illnesses and alleviate pain in humans and in other animals, animals used in experiments must suffer. It is therefore an essential part of the job of a scientist to disentangle this intricate matter, by asking questions such as: “Am I doing something good, for real?”

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Independently of the conclusion the reader has drawn at this point, nobody should forget the essential evidence: animals do not care about all of this. Pain is an actual sensation that does not depend on any ethical discussion humans may have, including this very article. Pain is something real in a given time and existence, and the value of an experiment does not make it any better for that individual mouse or fruit fly in pain. Coming back to Leopardi (2), we may discuss how just it is to inflict pain, but by doing that we would again postulate our supremacy over the planet.

*Some may argue (4) that pain is valuable only when an organism is able to reflect on it, making use of its consciousness. I left this debate out for two reasons: 1) The idea that humans are the only animals possessing consciousness is not proven. 2) This is a primitive idea. Pain is a physical sensation and consciousness does not add to or remove anything from it.


Federico Germani



1) Bentham, J., “Introduction to the Principles of Morals and Legislation”, 1789.

2) Leopardi, G., “Dialogue between a goblin and a gnome”, Small moral works (original: “Dialogo di un folletto e di uno gnomo”, Operette Morali), 1827.

3) Feinberg, J., Rights, “Justice, and the Bounds of Liberty: Essays in Social Philosophy”, 1980.

For a comprehensive and awesome discussion on ethics and animal pain, please read:

4) Tannenbaum, J., “Ethics and pain research in animals”, ILAR, 1999.

Received: 28.10.18, Ready: 09.11.18, Editors: SR, AFB.

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