It is difficult to know exactly how much a dog can learn, and recent data keep pushing the limits beyond what we felt was possible before. Perhaps one breakthrough, in terms of our ability to assess the intelligence of dogs came about in the early 1990s. At that time it dawned upon me that one way to learn about the limits of canine abilities was to use tests that were already developed for assessing human infants, and to modify them so that they could be used for dogs. The idea was that if a dog could pass a particular test, then not only would he have clearly demonstrated that he has the fact particular mental ability, but it might be possible to assign a human mental age to his performance, which might give us a better understanding of the dog's mental capacity. A number of canine behavioral researchers ultimately adopted the same strategy.
My own first research using this technique began when I was looking at canine language learning ability, as described in my book The Intelligence of Dogs. I began by modifying the MacArthur Communicative Development Inventory, which contained several forms to assess language and communication ability in very young children, not only in terms of the use of words, but also in terms of gestures. Using only family pets, which were not specifically trained to understand language and gestures, I came to the conclusion that dogs had the mental ability roughly equivalent to a human two-year-old. Further work led me to believe that the most intelligent dogs might have the mental abilities similar to a human two-and-a-half-year-old child. I cautioned, that we really didn't know how far we could push the dog's abilities until we specifically tried to train a dog for maximum comprehension of human language.
If my estimates were correct, then it should be possible to train a bright dog to understand 200 words or more. Several years later scientific reports appeared confirming that a border collie named Ricco, who had been specifically trained to improve his vocabulary, had a language ability in that range. Since that time a number of researchers have tried to see just how much language a dog can learn. At the time of this writing, perhaps the most linguistically advanced dog is a border collie named Chaser, owned by a retired psychologist named John Pilley. Chaser's vocabulary is around 1000 words, which would be the equivalent of what we might expect from a human three-year-old. Not only does Chaser understand single words, but also concepts and categories like "ball", which may include a number of items of different sizes and textures. Chaser's skills did not come easily, however, but required a lot of training, with Dr. Pilley often spending four or more hours a day working with the dog. It therefore seems likely that his research is pushing the limits of what a dog can be trained to understand.
The idea of a dog's learning ability hovering around that of a two to three year old child has to be understood within a certain set of limits. Dogs are more athletic and physically accomplished than a human child of that age, and therefore can learn jumping and swimming feats which the child cannot be expected to complete, even though the concept "to jump" or "to swim" would be understood by the child. On the other hand the child has better manipulative abilities than the dog thanks to our fingers and opposing thumb. Also some tasks depend upon sensory ability, so asking the dog to learn things depending upon fine color discrimination, or the child to learn tasks based upon scent discrimination would obviously be inappropriate.
In addition it is important to limit our conclusions to mental and intellectual abilities, since in terms of social consciousness, with their interest in sex, dominance, and ranking in a social group, dogs are more like human teenagers in their intellectual functioning.
If we are to draw a conclusion from this kind of research, it is that dogs have the mental ability approximating that of humans between two and three years of age, at least as far as language, object recognition, and concept formation. That means that if we are posing a problem, or teaching a task, which would be too difficult for a human two or three-year-old to solve or learn, then it is likely to be beyond the dog's capacity as well.
Stanley Coren is the author of many books including: Born to Bark, The Modern Dog, Why Do Dogs Have Wet Noses? The Pawprints of History, How Dogs Think, How To Speak Dog, Why We Love the Dogs We Do, What Do Dogs Know? The Intelligence of Dogs, Why Does My Dog Act That Way? Understanding Dogs for Dummies, Sleep Thieves, The Left-hander Syndrome
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