You must turn off your ad blocker to use Psych Web; however, we are taking pains to keep advertising minimal and unobtrusive (one ad at the top of each page) so interference to your reading should be minimal.

If you need instructions for turning off common ad-blocking programs, click here.

If you already know how to turn off your ad blocker, just hit the refresh icon or F5 after you do it, to see the page.

Psi man mascot

Early Comparative Psychology

Comparative psychology takes its name from the goal of comparing the behavior of different animal species. The field began shortly after the publication of Charles Darwin's books The Origin of Species (1859) and The Descent of Man (1874).

Darwin suggested that animal species changed over time. Late generations displayed the characteristics carried forward from earlier generations, so later generations resembled individuals who survived and reproduced.

The same process shaped intelligence, Darwin suggested. Therefore it was logical to trace the development of human intelligence from simpler forms that might still be found in non-human species.

What did Darwin write about mental faculties and their comparison between species?

Here is how Darwin put it in his book The Descent of Man (1874).

It is...highly probable that with mankind the intellectual faculties have been mainly and gradually perfected through natural selection...

Undoubtedly it would be interesting to trace the development of each separate faculty [intellectual skill] from the state in which it exists in lower animals to that which exists in man. (Darwin, 1874, pp. 128-129)


George John Romanes (1848-1894) was a friend and student of Darwin. He wrote a book called Animal Intelligence (1882) comparing mental abilities of animals ranging from snails to humans.

The book was rushed to press in 1882 so it could be published before Darwin's death that year. It contained previously unpublished quotations from Darwin's notes about animal intelligence, as well as reports of unusual feats of animal intelligence, collected by Romanes.

What was Romanes's "ejective inference"?

Romanes used what he called ejective inference : the assumption of similarity between animal minds and human minds. He wrote:

Starting from what I know subjec­tively of the operations of my own individual mind, and the activities which in my own organism they prompt, I proceed by analogy to infer from the observable activities displayed by other organisms what are the mental operations that underlie them. (Romanes, 1883/1977, in Roitblat, 1987)

Romanes's method of gathering data was to collect anecdotes. Anecdotes are stories that would appear in a brief newspaper article or letter from a friend, or as an aside in a scholarly article.

Anecdotes are not a very scientific form of evidence, and Romanes knew this. He felt that if he sifted through the anecdotes for the best and most revealing, they would be helpful for generating hypo­theses that could be investigated more systematically later.

What does it mean to "anthropo­morphize"?

This belief that animals had similar mental processes justified Romanes's "ejective inferences." Romanes did not hesitate to anthropomorphize or project human qualities upon animals.

For example, Romanes would refer to a rat that had just been freed from a cage as happy and carefree. The mental life of a rat was assumed to be like that of a human in the same situation, minus only language.

Romanes' attitude is still common. Humans find it natural to project our own experiences into animal minds. Consider the following excerpt from a report in the Science section of the New York Times:

The similarities between us and Rattus extend far beyond gross anatomy. They're surprisingly self-aware. They laugh when tickled, especially when they're young, and they have ticklish spots; tickle the nape of a rat pup's neck and it will squeal ultrasonically in a soundgram pattern like that of a human giggle. Rats dream as we dream, in epic narratives of navigation and thwarted efforts at escape...

Rats can learn to crave the same drugs that we do–alcohol, cocaine, nicotine, amphetamine–and they, like us, will sometimes indulge themselves to death. They're sociable, curious and love to be touched–nicely, that is. If a rat has been trained to associate a certain sound with a mild shock to its tail, and the bell tolls but the shock doesn't come, the rat will inhale deeply with what can only be called a sigh of relief. (Angier, 2007):

Each of the reporter's anthropo­morphic claims is based on a research finding. Researchers are usually reluctant to go the last mile and infer such human-like experiences. To the reporter, and most humans, ejective inference comes naturally.

Romanes's reliance upon anecdotes and his tendency to project human qualities upon animals was popular with the public. But it was not endorsed by most American psychologists.

At the time, in the 1890s, they were trying to move away from subjective approach­es such as introspection to make psychology more scientific. They saw Romanes's approach as a step backward toward folk science and speculation.

Nevertheless, Romanes was probably correct about the usefulness of anec­dotes in stimulating later research. Consider the ticklishness of rats, alluded to in Angier's report above.

No doubt that started as an anecdote. But it led to follow-up research. For example, Ishiyama and Brecht (2016) reported that tickling a rat results in activity in a particular area of the rat somatosensory cortex, the same area affected by tickling in humans.

Rat laughter was a 50kHz vocalization, much too high for humans to hear. It only occurs when there was no threatening stimulus in the area, confirming Darwin's speculation in 1972 that "the mind must be in a pleasurable condition" for ticklish laughter. Tickling was accompanied by a release of dopamine in the rat brain, indicating a state of enjoyment.

The Phylogenetic Scale

Implicit in the work of comparative psychologists from Darwin's day until about the 1930s was the idea that different species could be put on a scale of intelligence. The continuum from stupidest to brightest animals (lower to higher) was called the phylogenetic scale.

This idea goes back nearly 2400 years to Aristotle's notion of a Great Scale of Nature or Scala Naturae. Aristotle suggested that animals could be ordered from least (for example, worms and snails) to intermediate levels (such as dogs and cats) to the highest and most advanced level (humans).

Romanes, like Darwin, accepted this idea as common sense. He wanted to trace the development of intelligence as it moved "up the phylogenetic scale" from simple or primitive animals to complex or advanced animals.

What was the "phylogenetic scale"? How did it resemble the Scala Naturae of Aristotle?

On the surface, the phylogenetic or phyletic scale seems reasonable. It is true some species are much more capable than others, in particular ways.

However, modern psychologists recognize many different forms of intelligence. There could be a different phylogenetic scale for each type of intelligence. On a test of odor recog­nition, a bloodhound would rate as far more advanced than a human.

Why does the Scala Naturae make no sense from an evolutionary perspective?

The phylogenetic scale makes no sense from an evolutionary perspective. Putting the animals in a linear order (rat, cat, monkey, human) suggests that a rat, if it got a little smarter, would think like a cat. A smart cat might think like a monkey, and a smart monkey might think like a dull human.

But that is not so; each of those species thinks in different ways. And why should they think alike? The animals are not ancestral to each other and do not represent a single evolving lineage. No cat ever descended from a rat, no monkey was ever descended from a cat, and no human ever descended from a chimpanzee.

All these species have been evolving independently for millions of years. Different currently-existing species do not grow into each other, and there is no reason to expect their mental capacities to fall into a smooth progression (Hodos & Campbell, 1969).

Romanes believed in the phylogenetic scale and the principle of continuity. According to that assumption, all different animals think the same way. They differ only in speed of learning. If that was true, then it made sense to rank animals according to how fast they learned, from least to best, earthworm to human.

Today's animal researchers no longer endorse the principle of continuity. They find each species to be intelligent in its own ways, as needed to cope with its distinctive challenges and to reproduce.

Morgan's Canon

In 1894, the same year Romanes died, C. Lloyd Morgan published a book with a more sober point of view. Morgan's book was titled Introduction to Comparative Psychology.

Morgan drew a distinction between (1) objectively testable inferences from animal behavior, which were scientific and (2) untestable speculations about animal minds. He put Romanes's ejective inferences into the second category: untestable speculations.

What "modern" sounding distinction did Morgan draw, in his 1894 book?

To use a modern example, you might have a cat who comes running when it hears an electric can opener. You might infer that the cat is capable of hearing this sound from the other room and has associated its occurrence with the possible delivery of food.

This is scientific speculation because it is based on well-known principles of conditioning, and if you wanted to, you could do experiments to test it, such as associating a new sound to food delivery, or comparing the cat's response to different sounds.

Or you could do ejective inference, and infer that the cat knows that cans contain food, and the cat knows a can opener is what opens up a can. The ejective inference is not a scientific speculation because it inserts human-level thoughts into the mind of a cat. It also goes way beyond what is necessary to explain the cat's behavior.

Morgan recommended economy or simplicity in interpreting animal behavior. The following declaration from his 1894 textbook came to be known as Morgan's Canon. (A "canon" is a principle or law.)

In no case may we interpret an action as the outcome of a higher psychical faculty, if it can be interpreted as the outcome of the exercise of one that stands lower in the psychological scale. (Morgan, 1894, p.53)

This idea can be paraphrased as, "Never assume more complexity than you must, to explain animal cognition." Morgan's Canon is a close relative of Occam's Razor, the famous principle that simple explanations are to be preferred over complicated explanations, other things being equal.

What was the main idea of Morgan's Canon? How does it resemble Occam's Razor?

Armed with a canon and a razor, students can avoid anthropomorphi­zation. We should not project human-like qualities onto animals unless evidence supports that speculation, as in the case of rats emitting something like laughter when they are tickled, above.

However, another case involving rats is not so clear. When baby rats are separated from their mothers and exposed to cold, they emit ultrasonic vocalizations.

Knowing how human babies act when distressed, and how this motivates parents, we easily assume baby rats are crying out in distress, and the mother is therefore motivated to retrieve the pups to the warmth and comfort of the nest.

Blumberg, Sokoloff, Kirby and Kent (2000) found that the cries were actually due to abdominal pressure pulses. These pressure pulses were set off automatically when blood pressure dropped below a certain level. This happened whether or not the baby rat was in distressing conditions.

The cries restored blood pressure; they did not necessarily show distress. The authors noted that their findings "underscore the potential pitfalls of anthropomorphic interpretations"(p.78).

Of course, abdominal pressure pulses could serve multiple purposes. They could restore blood pressure, heat the baby rat a bit, and also produce noises that stimulate the mother to retrieve a baby outside the nest. Different functions are not always neatly separated in nature; a biological response may serve many functions at once.

What was Morgan's view of learning and consciousness?

Animal researchers of the 1890s were considerably less cautious than modern researchers, when it came to speculating about the animal mind. Despite his famous canon, Morgan himself was prone to making assump­tions about animal consciousness.

Morgan declared that any form of learning was evidence of conscious­ness. A chick might be unconscious the first time it made a peck after hatching from an egg, he said, but when the accuracy of pecking increased, this showed consciousness, "for only by appealing to consciousness can [pecks] be guided" (Morgan, 1896, p.130).

Morgan was wrong. Hess (1956) showed that chicks improved their pecking accuracy even if they are fitted with little prism goggles that diverted their pecks off to the side, so they always missed their target.

The chicks' improved pecking was due to maturing muscles, not consciousness, not even sensory feedback. Morgan did not discover that, because (contrary to his own advice) he did not look for objective evidence. He simply assumed the operation of "a higher psychical faculty" (consciousness).

What reaction to speculations about animal minds occurred? What was Loeb's view?

By 1900 scientists were reacting against speculations about animal conscious­ness. (You may recall that John B. Watson found it absurd when he was required to speculate about conscious­ness of rats running a maze.) Many scientists were ready to stop speculating about animal minds altogether.

Jacques Loeb (1900) argued for a mechanistic view. He proposed that animals were like biological machines. Only humans had minds and conscious­ness, he proposed.

Loeb explained that animal behavior was due to tropisms, automatic movements toward (or away from) a stimulus. The movement of moths toward light (or slugs toward beer) would be interpreted by Loeb as a tropism, a simple stimulus-response connection in the animal's nervous system. A stimulus led to a response, he said, and no mind was involved.

Loeb was reacting too far in the other direction, but his attitude was taken up by the early behaviorists like Watson who rejecting speculations about consciousness. It was over half a century until scientists were comfortable again talking about animal cognition.

Clever Hans

Scientists were growing cautious of speculations about mental abilities of animals, around 1900. However, the public showed an undiminished appetite for stories of animal consciousness.

Newspapers and magazines published serious articles about dogs and cats with human-like intelligence. Ten years after Morgan's Canon, many people believed newspaper stories about a German horse who could read and write.

The year was 1904 when retired Berlin schoolteacher Wilhelm von Osten presented Clever Hans to the world. The horse could apparently understand German, do arithmetic, interpret calen­dars, and perform other amazing intellectual feats.

Clever Hans with von Osten
Von Osten and his pupil, Clever Hans

Von Osten created a letter board on which each letter of the alphabet was assigned a number. The horse spelled out answers to questions by tapping its hoof the number of times corresponding to each letter, forming words and sen­tences.

How did von Osten train Hans?

At first von Osten instructed the horse by holding the horse's hoof, naming a letter, and tapping the appro­priate number of times. After many repetitions, the horse appeared to catch on. Von Osten withdrew his hand gradually. (This resembles the technique described in Chapter 5 as prompting and fading.)

Now the horse tapped the appropriate number of times whenever a letter was named. Soon Hans was spelling out words, then whole sentences, using hoof-taps for each letter of the alphabet. Von Osten went on to teach Hans arithmetic and other skills. Katz (1953) describes the apparent outcome:

It took about two years to complete the training which made the horse so famous. It mastered the four funda­mental rules of arithmetic, changed common fractions into decimals and back again, and could give the day of the month.

It knew how to tell the time by the clock and could, for example, answer the question, "Between which figures is the small hand when it is twenty-five to eight?"

The horse also had an astonishing appreciation of music. If one played the dominant seventh chord D, F, A, C, it shook its head and indicated that the C should be left out to turn it into a more harmonious minor chord. (p.13)

What remarkable abilities did Clever Hans appear to possess?

Who believed in Clever Hans? The psychiatrist Gustav Wolff did, among others.

He issued a statement declaring "that an animal can think in a human way and can express human ideas in human lan­guage." He was joined in this opinion by "prominent scholars, scientists, psychol­ogists, psychiatrists, medical doctors, and many others" (Hediger, 1981).

Why was it obvious no "cheating" was involved?

One reason so many prominent author­ities believed in Clever Hans was that the horse could perform in the absence of its trainer. This made it appear that no cheating was possible. A commission of zoologists evaluated the animal, and they concluded that no trickery was involved.

Oskar Pfungst from the Berlin Psycho­logical Institute finally solved the puzzle. He showed that Hans could answer a question only when somebody near the horse knew the answer.

If the question was presented to the horse by somebody who then departed from the scene, leaving only people who did not know the answer to the question, Hans never stopped tapping. Under those circumstances, Hans tapped slowly, hesitated and appeared to watch the observers very closely...and contin­ued to tap.

What did Pfungst conclude about Clever Hans's abilities?

Pfungst deduced that Hans was react­ing to tiny cues given off by observers. When Hans approached the correct number of hoof taps (completing a word, for example) the human ob­servers reacted in some way that the horse could detect.

Perhaps the observers raised and lowered their eyebrows, or took a deep breath. When Hans perceived these subtle cues, he would stop tapping to get his reward. Pfungst was able to get the horse to give any answers he chose by making similar tiny movements.

Using the terminology of animal trainers, Clever Hans was responding to a "no-go" or "stop what you are doing" signal. His reward for paying attention to these signals was the social reinforcement of an approving crowd.

How were "no-go" signals involved? Were the signals given consciously?

The crowd (and even von Osten) was not conscious of giving this type of signal. Pfungst concluded:

In the course of the long series of lessons in arithmetic, the horse must have learned to spot more and more accurately the tiny body movements with which the teacher unconsciously accompanied his own thinking. These movements the horse learned to utilize as cues....The horse's performance and the great accuracy it achieved in perceiving these tiniest of movements remain amazing. (In Katz, 1953, p.15)


In 2005 I heard from a gentleman in Dubai about a cat whose abilities rivaled those of Clever Hans.

Dear Dr. Dewey;

I want to inform you about a Persian cat with COGNITION. The uncanny abilities of this cat, CUTY BOY, attracts scientists and cat lovers around the world. Cuty Boy has many skills. He can count, can identify objects, can identify persons before we introduce them to him, can understand many languages and has many other skills. No scien­tists could define the amazing abilities of Cuty Boy.

As a teacher I am convinced of the skills of this cat. Psychologists, School Teachers, University Professors, Doctors, Journalists and many people from various fields have tested Cuty and are convinced of his skills, yet no one could define the phenomenon with Cuty Boy. Please understand, Cuty Boy does not possess the CLEVER HANS EFFECT.

What are some parallels between the stories of Clever Hans and Cuty Boy?

Cuty Boy's other alleged skills included a knowledge of algebra, plus the ability to do psychic readings. Cuty Boy answered questions by tilting his head toward a slip of paper that had the correct answer on it (when offered a choice of several slips of paper).

On other occasions Cuty Boy would touch noses with his owner, while held in his owner's lap, to answer "yes" to a question. When I heard this, I understood what was happening. I replied, in part:

Ah, but do you really know the story of Clever Hans? You should learn the details. The owner of Clever Hans was NOT cheating or deceiving people intentionally (and I assume Cuty Boy's owner is not, either).

The story is more interesting than that. It illustrates the wonderful ability of non-human animals to detect subtle cues. Cats are clearly capable of that, as I know (being a devoted owner of a talented cat).

Oscar Pfungst of the Berlin Psycho­logical Institute tested Clever Hans by making sure that nobody in the presence of the horse knew the question that Hans was asked. So, to find out if Cuty Boy is truly a cat Einstein who knows algebra, different languages, and things like that, you would have to ask him a question, then leave the room.

Have somebody who knows nothing about the question interpret Cuty Boy's answer. That would eliminate the possibility (the certainty) that Cuty Boy's behavior is a response to subtle cues, not an indication of true understanding.

What was Dr. Dewey's explanation?

If this was done, you would find that Cuty Boy's Einstein-like behavior would disappear, I'm afraid. In fact, Cuty Boy himself would probably disappear, due to all the commotion. Cats generally do not enjoy perform­ing for strangers.

Here's a discussion of the Clever Hans from my introductory psych­ology textbook... [and I included the page about Clever Hans]

My correspondent wrote back, insisting that Cuty Boy's talents were genuine. He also said his knowledge of English was not adequate to understand the testing procedure I recommended.

He remained friendly, granting me permission to use our correspondence, and he passed my e-mail address on to Cuty Boy's owner, Hema Mohan, then of Dubai. (The Mohans since relocated to their native India.)

Hema and I carried on a friendly corre­spondence, exchanging greetings and stories about our cats. Hema would write to ask if I was OK when a hurricane hit the U.S.

Cuty Boy, who I have to assume is no longer with us, was adorable and quite unusual. For one thing, he grew up with a parrot named Minu, with whom he posed in numerous photographs. He also allowed Hema to dress him in human-like outfits every day. He was a gorgeous specimen of Persian cat!

photos of Cuty-Boy from Hema
Cuty Boy with Minu (left) and in a typical outfit (right)


Angier, N. (2007, July 24) Smart, curious, ticklish. Rats? New York Times, p.F1.

Blumberg, M. S., Sokoloff, G., Kirby, R. F., & Kent, K. J. (2000) Distress vocalizations in infant rats: what's all the fuss about? Psychological Science, 11, 78-81.

Darwin, C. (1859) The Origin of Species. London: John Murray.

Darwin, C. (1872) The Expression of Emotion in Man and Animals. London: John Murray.

Darwin, C. (1874) Descent of Man and Selection in Relation to Sex. New York: D. Appleton and Co.

Hediger, H. K. P. (1981) The Clever Hans phenomenon from an animal psychologist's point of view. New York Academy of Sciences, 364, 1-17.

Hess, E. H. (1956, July) Space Perception in the Chick. Scientific American, 195,, 71-80.

Hodos, W. & Campbell, C. B. G. (1969). Scala Naturae: Why there is no theory in comparative psychology. Psychological Review, 76, 337-349.

Ishiyama, S. & Brecht, M. (2016) Neural correlates of ticklishness in the rat somatosensory cortex. Science, 354, 757-760.

Katz, D. (1953) Animals and Men. London: Penguin Books.

Loeb, J. (1900) Comparative Physiology of the Brain and Comparative Psychology. New York: G.P. Putman's Sons.

Morgan, C. L. (1894) An Introduction to Comparative Psychology. New York: Scribner.

Romanes, G. J. (1982) Animal Intelligence. London: Kegan Paul Trench & Co.

Write to Dr. Dewey at

Don't see what you need? Psych Web has over 1,000 pages, so it may be elsewhere on the site. Do a site-specific Google search using the box below.