[Premack D. Human and animal cognition: continuity and discontinuity. Proc Natl Acad Sci U S A. 2007 Aug 28;104(35):13861-7. Epub 2007 Aug 23.]
Full Text: http://www.pnas.org/content/104/35/13861.full
Microscopic study of the human brain has revealed neural structures, enhanced wiring, and forms of connectivity among nerve cells not found in any animal, challenging the view that the human brain is simply an enlarged chimpanzee brain. On the other hand, cognitive studies have found animals to have abilities once thought unique to the human. This suggests a disparity between brain and mind. The suggestion is misleading. Cognitive research has not kept pace with neural research. Neural findings are based on microscopic study of the brain and are primarily cellular. Because cognition cannot be studied microscopically, we need to refine the study of cognition by using a different approach. In examining claims of similarity between animals and humans, one must ask: What are the dissimilarities? This approach prevents confusing similarity with equivalence. We follow this approach in examining eight cognitive cases—teaching, short-term memory, causal reasoning, planning, deception, transitive inference, theory of mind, and language—and find, in all cases, that similarities between animal and human abilities are small, dissimilarities large. There is no disparity between brain and mind.
Animal competencies are mainly adaptations restricted to a single goal. Human competencies are domain-general and serve numerous goals. For instance “planning” may be tied to episodic memory, suggesting a broad competence. However, if episodic memory is confined to one (or only a few) activities, planning itself will be a narrow competence. Differences in the evolutionary origins of animal and human abilities help explain why the one is tied to a single goal, and the other to indeterminately many goals.
In humans, teaching did not evolve in the context of food seeking (by stalking or coping with toxic food) as it apparently did in animals; but in a far broader context arguably involving TOM, language, and aesthetics. In causal reasoning, animal limitations are of a different kind: The sense of cause may originate in personal actions that result in a desirable or undesirable outcome. In the human, the sense graduates to impersonal actions: a rock that falls on a plant crushing it, a wind that blows out a flame, etc., to events that humans recognize as causal. Does the animal make the human transition, or does its sense of cause remain tied to personal action? This question awaits decisive evidence. Further, because animals have no concept of monotonic order, although “transitive inference” is widely found in animals, it cannot be based on logic or reasoning.
The broad range of cognitive cases, which includes teaching, causal reasoning, short-term memory, planning, TOM, etc., consistently shows fundamental limitations in the animal version of the human competence. There is no anomaly in the disparity—the disparity between human and animal cognition is compatible with the disparity between human and animal brain. The coming challenge is to understand the function of the cellular-level differences between human and animal brain. Work linking these neural changes to cognitive processes can now move forward.
The fact that adaptations have a single target distinguishes teaching by animals from teaching by humans. Human teaching is not an adaptation. It is a domain-general competence with indeterminately many targets. Further, the targets of teaching differ in every culture. Toilet training and table manners are widely taught in the western countries, whereas among the Kalahari San, walking and sitting are the key activities taught to the young (12).
Human teaching consists of three distinct actions: observation, judgement, and modification. A teacher observes the novice, judges his actions or products, and modifies them when they fall short of her standards. The human recognizes that the young are incompetent and therefore need to be taught; has the technology with which to teach; and is motivated to teach by deeply rooted aesthetic standards. Each of these actions has a distinct cognitive source.
The recognition that competence develops with age humans owe to their TOM: It enables them to both differentiate the mental conditions of other individuals, and to analyze the factors, such as age, intelligence, experience, etc., that cause the differences (13–18).
Humans can teach or modify the other one because they are both language-competent and expert in passive guidance (placing other’s body in desired positions).
The human motivation to teach is largely aesthetic (19, 20). A parent has a conception of a proper act or product and dislikes the appearance of an improper one. The evidence for such standards is twofold. First, humans “practice,” e.g., swing a golf club repeatedly, flip an omelet, sing a song, write a poem, etc., trying to improve their performance of a chosen activity. Second, humans seek to improve their appearance. The mirror is where they begin their day, combing their hair, applying makeup, etc. That humans have mental representations of preferred actions or appearances is suggested not only by the demands they make on themselves but by the corrections they make of children when teaching them. Teaching, the attempt to correct others, is the social side of the attempt to correct self.
It is no coincidence that humans both practice and teach, whereas other species do neither. A species that practices but does not teach—that corrects itself but does not correct others—will probably never be found. Nor will a species of the opposite kind, one that teaches but does not practice—corrects others but not itself. […]
Although short-term memory limits the number of units one can remember, it does not define the content of a unit. A language-trained chimpanzee exposed to the numbers 1–9 might remember, for example: 2, 6, 4, 3, 7, whereas humans might remember, for example, 21, 43, 96…; 214, 618, 109… ; 1012, 6680, 3456, etc. In language, content of the unit is even more open-ended. A chimpanzee may remember five words; a human may remember five phrases, five sentences, five stories, etc. Humans are able to make these expansions because they are capable of both recursive language and numbers. Thus, despite comparable limitations in short-term memory, animals and humans may differ dramatically in the amount of information they can remember. In addition, humans can, and often do, circumvent short-term memory with written language. Similar limitations in different species may have entirely different consequences depending on the other cognitive resources of the species (22). […]
An animal may recognize that a large rock is more likely to break a branch than a small one. But if the animal observes a large rock lying by a crushed plant, will it infer that the rock crushed the plant? There is no evidence that it will. The understanding of physical action is not the equivalent of
causal reasoning. […]
Although further research may reveal more development than is presently recognized, the cognitive elaboration leading to causal reasoning appears to be lacking in animals. […]
Complex planning differs from simple planning in these respects. It is social: two or more individuals form the plan, and the beneficiary of the plan is likely to be yet another individual, different from those who form the plan; the plan is not one-shot, but a series of plans; the plan extends not for hours but over years. Neither social nor sequential planning, nor planning that extends over long durations, is likely to be found in animals. […]
Whether the plover’s act is goal-directed could be determined by arranging two cases, one in which its display leads intruders away and another in which its displays do not succeed in leading intruders away. If, when the displays fail, the bird ceases to make them, the act is intentional. For intentional acts that fail to realize their goal extinguish. However, neither the potential intentionality of the plover’s display nor the fact that the plover can discriminate real intruders from fake ones changes the status of the display. It is an adaptation that serves only one goal. It is not comparable with human deception, a domain-general competence that can serve indeterminately many goals. […]
How is it possible for species that lack the concept of monotonicity to do transitive inference? They probably use a hard-wired mechanism and do not do it as humans do. Earlier, we suggested that animals are not capable of causal reasoning. How can they have a hard-wired mechanism for one kind of reasoning but not the other? Causal reasoning is more complex than transitive inference, involving not one simple inference, like transitive inference, but many inferences. Evolving a simple hard-wired mechanism is therefore less likely. […]
The advanced function most clearly associated with the reorganization of the human brain is complex social cognition. No less than language, it distinguishes humans from animals. […]
Chimpanzee mothers do not recognize that their infants lack knowledge and cannot therefore, for example, crack nuts with rocks. Therefore, they do not teach them. Chimpanzees do not have the concept of knowledge, do not distinguish a knowing individual from an ignorant one, and do not attribute the mental state of knowing, perception, and intention being the only mental states they attribute (22).
Humans attribute embedded mental states, such as, John thinks that Bill thinks that Henry believes that John should put his kids in Sunday school. Women think that men think that they think that men think that women’s orgasm is different. There is a behavioral counterpart to embedded mental states in human social behavior. One individual watches another individual watch yet another individual engage in some act. In the classroom, for instance, we may observe child A watch child B watch child C look at the teacher. In animals there is nothing comparable. In the wild, we sometimes see one chimpanzee infant watch its mother, another infant watch its mother, etc., but this is a string of independent acts, not a sequence in which each act is embedded in the preceding one. Animals neither attribute embedded mental states nor have embedded social behavior (22). […]
The grammar of a recursive language permits an endless compacting of information limited only by human memory.
The hierarchical organization of information is a related case. Humans divide biological objects into plants and animals, plants into fruit and vegetables, fruit into…, etc. Whereas chimpanzees sort, for example, apple, grape, etc., into one bin, bread, cupcake, etc., into another, thus recognizing categories (24), and category is a precursor of hierarchical information, there is no evidence that they recognize class-inclusion, which is another precursor of the hierarchical organization of information. Class-inclusion requires that the chimpanzee recognize, for instance, that although apple is included in fruit, fruit is not included in apple. Children apparently do not acquire class-inclusion until ≈5 years of age, suggesting that chimpanzees will not acquire it. A good rule of thumb is this: Concepts acquired by children after 3 years of age are never acquired by chimpanzees (49).
Is recursion an automatic part of human language, number, and organization of information, such that these systems have only a recursive form? Or do they occur in both recursive and nonrecursive forms, being recursive only under appropriate cultural pressure? Perhaps the answer is different for the three systems. Number is likely to differ from the other two cases because humans have an innate system (located in the left and right intraparietal sulci) for representing analogue quantities, a system they share with animals, as well as a second system for representing digital quantities, which they do not share with animals (50). […]
Because animals lack recursion (and human language is recursive), the animals’ lack of language is attributed to this factor. But recursion is not the only factor animals lack. If a species lacked language, even a nonrecursive language would be an enormous boon. Yet, chimpanzees have no language of any kind, recursive or nonrecursive.
A number of factors stand between animals and language. For instance, chimpanzees lack voluntary control of their voice. When a chimpanzee wants the attention of its trainer, it does not call; instead, it pounds on a resonant surface. Chimpanzees, therefore, could not have speech. But sign language is a possibility, for they do have voluntary control of their hands.
Chimpanzee sign language, however, could not be comparable with human sign language, because chimpanzees also lack voluntary control of their face, and in human sign language, facial expression plays grammatical roles, such as denoting the boundary of clauses (51).
A weaker form of sign that dropped facial expression and relied exclusively on hand signs would still pose a problem for the chimpanzee. The young animal could not imitate the hand signs of its mother. Most species can imitate the object or location that a model chooses, but there is a second level of imitation in which the novice must copy the motor act of the model (this would be the requirement in the case of sign language). Motor acts are more difficult to copy than objects or locations, because motor acts are ephemeral, and one must form a mental representation of the motor act and then copy the representation (52). Only humans imitate motor acts, although chimpanzees, when taught by humans, can do so. But the untrained chimpanzee cannot, so if a mutant chimpanzee with a simplified sign language were to appear, the other chimpanzees could not copy it.
Teaching is essential for language. Not for grammar, which arguably cannot be taught, but for words. Children are taught their initial words by their mother, and only later do they acquire words more or less on their own. Inasmuch as chimpanzees do not teach, even if they possessed all the other factors mentioned above, they could not have evolved language. In humans, the evolution of teaching evidently preceded that of language.