The Faculty of Language: What’s Special about it?
Cognition, in press
Supported by NIH grant HD-18381. We thank Stephen Anderson, Paul Bloom, Susan Carey, Andrew Carstairs-McCarthy, Matt Cartmill, Noam Chomsky, Barbara Citko, Peter Culicover, Dan Dennett, Tecumseh Fitch, Randy Gallistel, David Geary, Tim German, Henry Gleitman, Lila Gleitman, Adele Goldberg, Marc Hauser, Greg Hickok, David Kemmerer, Patricia Kuhl, Shalom Lappin, Philip Lieberman, Alec Marantz, Martin Nowak, Paul Postal, Robert Provine, Robert Remez, Ben Shenoy, Elizabeth Spelke, Lynn Stein, J. D. Trout, Athena Vouloumanos, and Cognition referees for helpful comments
and discussion. Supported by NIH grants HD 18381 (Pinker) and DC 03660 (Jackendoff). Authors‘ addresses: Steven Pinker, Dept. of Psychology, Harvard University, William James Hall 970, Cambridge MA 02138, firstname.lastname@example.org; Ray Jackendoff, Dept. of Psychology, Brandeis University, Waltham MA, 02454, email@example.com.
We examine the question of which aspects of language are uniquely human and uniquely linguistic in light of recent suggestions by Hauser, Chomsky, and Fitch that the only such aspect is syntactic recursion, the rest of language being either specific to humans but not to language (e.g., words and concepts) or not specific to humans (e.g., speech perception). We find the hypothesis problematic. It ignores the many aspects of grammar that are not recursive, such as phonology, morphology, case, agreement, and many properties of words. It is inconsistent with the anatomy and neural control of the human vocal tract. And it is weakened by experiments suggesting that speech perception cannot be reduced to primate audition, that word learning cannot be reduced to fact learning, and that at least one gene involved in speech and language was evolutionarily selected in the human lineage but is not specific to recursion. The recursion-only claim, we suggest, is motivated by Chomsky‘s recent approach to syntax, the Minimalist Program, which de-
emphasizes the same aspects of language. The approach, however, is sufficiently problematic that it cannot be used to support claims about evolution. We contest related arguments that language is not an adaptation, namely that it is ―perfect,‖ nonredundant, unusable in any partial form, and badly designed for communication. The hypothesis that language is a complex adaptation for communication which evolved piecemeal avoids all these problems.
The Faculty of Language: What’s Special about it?
1. The Issue of What is Special to Language
The most fundamental question in the study of the human language faculty is its place in the natural world: what kind of biological system it is, and how it relates to other systems in our own species and others. This question embraces a number of more specific ones (Osherson & Wasow, 1976). The first is which aspects of the faculty are learned from environmental input and which aspects arise from the innate design of the brain (including the ability to learn the learned parts). To take a clear example, the fact that a canine pet is called dog in English but chien in French is learned, but the fact that words
can be learned at all hinges on the predisposition of children to interpret the noises made by others as meaningful signals.
A second question is what parts of a person‘s language ability (learned or built-in)
are specific to language and what parts belong to more general abilities. Words, for example, are specifically a part of language, but the use of the lungs and the vocal cords, although necessary for spoken language, are not limited to language. The answers to this question will often not be dichotomous. The vocal tract, for example, is clearly not exclusively used for language, yet in the course of human evolution it may have been tuned to subserve language at the expense of other functions such as breathing and swallowing.
A third question is which aspects of the language capacity are uniquely human, and which are shared with other groups of animals, either homologously, by inheritance from a common ancestor, or analogously, by adaptation to a common function. This dimension cuts across the others. The system of sound distinctions found in human languages is both specific to language and uniquely human (partly because of the unique anatomy of the human vocal tract). The sensitive period for learning language may be specific to certain aspects of language, but it has analogues in developmental phenomena throughout the animal kingdom, most notably bird song. The capacity for forming concepts is necessary for language, as it provides the system of meaning that language expresses, but it is not specific to language: it is also used in reasoning about the world. And since other primates engage in such reasoning, it is not uniquely human (though parts of it may be). As with the first two questions, answers will seldom be dichotomous. They will often specify mixtures of shared and unique attributes, reflecting the evolutionary process in which an ancestral primate design was retained, modified, augmented, or lost in the human lineage. Answers to this question have clear implications for the evolution of language. If the language faculty has many features that are specific to language itself, it would suggest that the faculty was a target of natural selection. But if represents a minor extension of capacities that existed in the ancestral primate lineage, it could be the result of a chance mutation that became fixed in the species through drift or other non-adaptive evolutionary mechanisms (Pinker & Bloom, 1990).
In a recent article in Science, Marc Hauser, Noam Chomsky, and Tecumseh Fitch
(2002) offer a hypothesis about what is special about language, with reflections on its evolutionary genesis. The article (henceforth HCF) has attracted much attention both in the popular press (Kenneally, 2003; Wade, 2003) and among other language scientists. HCF differentiate (as we do) between aspects of language that are special to language (the ―Narrow Language Faculty‖ or FLN) and the faculty of language in its entirety,
including parts that are shared with other psychological abilities (the ―Broad Language Faculty‖ or FLB). The abstract of HCF makes the extraordinary proposal that the narrow language faculty ―only includes recursion and is the only uniquely human component of
the faculty of language.‖ (Recursion refers to a procedure that calls itself, or to a 1constituent that contains a constituent of the same kind.) In the article itself, the
starkness of this hypothesis is mitigated only slightly. The authors suggest that ―most, if
not all, of FLB is based on mechanisms shared with nonhuman animals…In contrast, we suggest that FLN – the computational mechanism of recursion – is recently evolved and
unique to our species‖ (p. 1573). Similarly (p. 1573), ―We propose in this hypothesis that
FLN comprises only the core computational mechanisms of recursion as they appear in narrow syntax and the mappings to the interfaces‖ (i.e., the interfaces with mechanisms 2of speech perception, speech production, conceptual knowledge, and intentions).
In other words, HCF are suggesting that recursion is the mechanism responsible for everything that distinguishes language both from other human capacities and from the capacities of animals. (These assertions are largely independent: there may be parts of the narrow language faculty other than recursion even if the narrow faculty is the only part that is uniquely human; and the narrow faculty might consist only of recursion even if parts of the broad faculty are uniquely human as well.) The authors go on to speculate that the recursion mechanism defining what is special about language may not even have evolved for language itself but for other cognitive abilities such as navigation, number, or social relationships.
1 Theoretical computer scientists often distinguish between tail recursion and true recursion. Roughly, in
tail recursion, a procedure invokes another instance of itself as a final step (or, in the context of language, a constituent contains an identical kind of constituent at its periphery). In true recursion, a procedure invokes an instance of itself in mid-computation and then must resume the original procedure from where it left off (or a constituent contains has an identical kind of constituent embedded inside it). True recursion requires a computational device with a stack of pointers (or an equivalent mechanism) to keep track of where to return after an embedded procedure has been executed. Tail recursion can be mimicked (at least in input-output behavior or ―weak generative capacity‖) by a computational device that implements simple iteration, where one instance of a procedure can be completed and forgotten by the time the next instance has begun. Tail recursion, however, cannot be mimicked by iteration when it comes to computations that require more than duplicating input-output behavior (―strong generative capacity‖), such as inferences that depend on the grouping and labeling of constituents. 2 It is possible to parse this sentence as saying that FLN consists of recursion and, in addition, the
mappings to the interfaces, rather than recursion as it appears in the mappings to the interfaces. But this
interpretation is more strained, and is inconsistent with the preceding two quotations, which simply identify the narrow language faculty with recursion.
HCF‘s hypothesis appears to be a radical departure from Chomsky‘s earlier position that language is a complex ability for which the human brain, and only the human brain, is specialized:
A human language is a system of remarkable complexity. To come to
know a human language would be an extraordinary intellectual
achievement for a creature not specifically designed to accomplish this
task. A normal child acquires this knowledge on relatively slight exposure
and without specific training. He can then quite effortlessly make use of
an intricate structure of specific rules and guiding principles to convey his
thoughts and feelings to others, arousing in them novel ideas and subtle
perceptions and judgments (Chomsky, 1975, p. 4).
3Similarly, Chomsky‘s frequent use of the terms ―language faculty‖ and ―mental organ‖
underscore his belief that language is distinct from other cognitive abilities, and therefore distinct from the abilities of species that share those abilities but lack the ability to acquire languages. For example:
It would be surprising indeed if we were to find that the principles
governing [linguistic] phenomena are operative in other cognitive systems,
although there might be certain loose analogies, perhaps in terms of figure
and ground, or properties of memory, as we see when the relevant
principles are made explicit. Such examples illustrate … that there is good
reason to suppose that the functioning of the language faculty is guided by
special principles specific to this domain … (Chomsky, 1980, p. 44).
Indeed, the position that very little is special to language, and that the special bits are minor modifications of other cognitive processes, is one that Chomsky‘s strongest critics have counterposed to his for years. Not surprisingly, many have viewed the Science paper
as a major recantation (e.g., Goldberg, 2003).
The HCF paper presents us with an opportunity to reexamine the question of what is special about language. As HCF note (p. 1572), the two of us have both advanced a position rather different from theirs, namely that the language faculty, like other biological systems showing signs of complex adaptive design (Dawkins, 1986; Williams, 1966), is a system of co-adapted traits that evolved by natural selection (Jackendoff, 1992, 1994, 2002; Pinker, 1994b, 2003; Pinker & Bloom, 1990). Specifically, the language faculty evolved in the human lineage for the communication of complex propositions. HCF contrast this idea with their recursion-only hypothesis, which ―has the interesting
effect of nullifying the argument from design, and thus rendering the status of FLN as an adaptation open to question‖ (p. 1573).
In this paper we analyze HCF‘s recursion-only hypothesis, and conclude that it is
hard to sustain. We will show that there is considerably more of language that is special,
3 ―We may usefully think of the language faculty, the number faculty, and others, as `mental organs,‘ analogous to the heart or the visual system or the system of motor coordination and planning‖ (Chomsky,
1980, p. 39).
though still, we think, a plausible product of the processes of evolution. We will assess the key bodies of evidence, coming to a different reading from HCF‘s, and then consider how they arrived at their position.
Despite our disagreements over the recursion-only hypothesis, there is much in the paper with which we are sympathetic. We agree that it is conceptually useful to distinguish between the language faculty in its broad and narrow sense, to dissect the broad language faculty into sensorimotor, conceptual, and grammatical components, and to differentiate among the issues of shared versus unique abilities, gradual versus saltational evolution, and continuity versus change of evolutionary function. The rigorous laboratory study of possible homologues and analogues of aspects of language in other species is a hallmark of the research programs of Hauser and Fitch, and we agree that they promise major advances in our understanding of the evolution of language. Our disagreement specifically centers on the hypothesis that recursion is the only aspect of language that is special to it, that it evolved for functions other than language, and that this nullifies ―the argument from design‖ that sees language as an adaptation.
The claims of HCF are carefully hedged, and the authors could argue that they are not actually advocating the recursion-only hypothesis but merely suggesting that it be
entertained or speculating that it may turn out to be correct in the long run. We are not so much interested in pinning down who believes what as in accepting HCF‘s invitation to take the hypothesis itself seriously.
2. What’s Special: A Brief Examination of the Evidence
We organize our discussion in line with HCF, distinguishing the conceptual, sensorimotor, and specifically linguistic aspects of the language faculty in turn.
2.1. Conceptual structure. Let us begin with the messages that language
expresses: mental representations in the form of conceptual structure (or, as HCF put it, outputs of the ―conceptual-intentional system‖). The primate literature, incisively
analyzed in HCF, gives us good reason to believe that some of the foundations of the human conceptual system are present in other primates, such as the major subsystems dealing with spatial, causal, and social reasoning. If chimpanzees could talk, they would have things to talk about that we would recognize.
HCF also argue that some aspects of the human conceptual system, such as Theory of Mind (intuitive psychology) and parts of intuitive physics, are absent in monkeys, and questionable or at best rudimentary in chimpanzees. They are special to humans, though not special to language. We add that many other conceptual systems, though not yet systematically studied in nonhuman primates, are conspicuous in human verbal interactions while being hard to discern in any aspect of primates‘ naturalistic behavior. They include essences (a major component of intuitive biology and chemistry), ownership, multi-part tools, fatherhood, romantic love, and most moral and deontic concepts. It is possible that these abilities, like Theory of Mind, are absent or discernable
only in rudimentary form in other primates. These too would be uniquely human aspects of the language faculty in its broad sense, but would be part of a system for nonlinguistic reasoning about the world rather than for language itself.
In addition, there are domains of human concepts which are probably unlearnable without language (Jackendoff, 1996). For example, the notion of a ―week‖ depends on
counting time periods that cannot all be perceived at once; we doubt that such a concept could be developed or learned without the mediation of language. More striking is the possibility that numbers themselves (beyond those that can be subitized) are parasitic on language – that they depend on learning the sequence of number words, the syntax of number phrases, or both (Bloom, 1994a; Wiese, 2004) (though see Grinstead et al., 1997; 2004, for a contrary view). Vast domains of human understanding, including the
supernatural and sacred, the specifics of folk and formal science, human-specific kinship systems (such as the distinction between cross- and parallel cousins), and formal social roles (such as ―justice of the peace‖ and ―treasurer‖), can be acquired only with the help 4of language. The overall picture is that there is a substrate of conceptual structure in chimps, overlain by some uniquely human but not necessarily language-based subsystems, in turn overlain by subsystems that depend on the pre-existence of linguistic expression. So here we more or less concur with HCF, while recognizing a more ramified situation.
2.2. Speech perception. HCF implicitly reject Alvin Liberman‘s hypothesis that
―Speech is Special‖ (SiS). According to SiS, speech recognition is a mode of perception
that is distinct from our inherited primate auditory analyzers in being adapted to recover the articulatory intentions of a human speaker (Liberman, 1985, 1991; Liberman et al., 1967; Liberman & Mattingly, 1989). One of the first kinds of evidence adduced for SiS, dating to the 1950s, was the existence of categorical phoneme perception (Liberman et al.,
) are 1967), in which pairs of phonemes differing in say, voicing (e.g., p and b
discriminated more accurately than pairs of stimuli separated by the same physical difference (in this case, in voice-onset time) but falling into the same phonemic category (both voiced, or both unvoiced). This particular bit of evidence for human uniqueness was deflated in the 1970s by findings that chinchillas make similar discriminations (Kuhl & Miller, 1975). HCF cite this as evidence against SiS, together with three other findings: that certain animals can make auditory distinctions based on formant frequency, that tamarin monkeys can learn to discriminate the gross rhythms of different languages, and that monkeys can perceive formants in their own species‘ vocalizations.
These phenomena suggest that at least some aspects of the ability to perceive speech were present long before the advent of language. Of course, some version of this conclusion is unavoidable: human ancestors began with a primate auditory system, adapted to perform complex analyses of the auditory world, and it is inconceivable that a system for speech perception in humans could have begun de novo. HCF go further and
4 We leave open whether such concepts are simply impossible without language or whether they are within the expressive power of the conceptual system but require language as a crutch to attain them. They certainly cannot be shared via ostension, so language is in either case necessary for their cultural transmission.
suggest that there have been no evolutionary changes to the mammalian auditory system
for the function of speech perception in humans. They suggest that this null hypothesis has withstood all attempts to reject it. We are not so sure.
Most experiments testing the perception of human speech by nonhuman animals have them discriminate pairs of speech sounds, often after extensive operant conditioning (supervised learning). It is not surprising that some animals can do so, or even that their perceptual boundaries resemble those of humans, since auditory analyzers suited for nonspeech distinctions might suffice to discriminate among speech sounds, even if the analyzers humans use are different (Trout, 2001, 2003b). For example, a mammalian circuit that uses onset asynchrony to distinguish two overlapping auditory events from one event with a complex timbre might be sufficient to discriminate voiced from unvoiced consonants (Bregman & Pinker, 1978). But humans do not just make one-bit discriminations between pairs of phonemes. Rather, they can process a continuous, information-rich stream of speech. In doing so, they rapidly distinguish individual words from tens of thousands of distracters despite the absence of acoustic cues for phoneme and word boundaries, while compensating in real time for the distortions introduced by coarticulation and by variations in the age, sex, accent, identity, and emotional state of the speaker. And all of this is accomplished by children as a product of unsupervised learning. A monkey‘s ability to be trained to discriminate pairs of phonemes provides little evidence that its auditory system would be up to the task accomplished by humans. It would be extraordinarily difficult at present to conduct experiments that fairly compared a primate‘s ability to a human‘s, fully testing the null hypothesis.
Moreover, there is considerable evidence that has cast doubt on the null
hypothesis (Anderson, 2004; Liberman, 1985, 1991; Remez, 1989, 1994; Trout, 2001, 2003b). First, speech and sound are phenomenologically different: under certain conditions, a given sound can be perceived simultaneously as part of a syllable and as a nonspeechlike chirp (Liberman & Mattingly, 1989), or a stretch of sound can be heard to flip qualitatively between speech and nonspeech (Remez et al., 2001).
Second, in humans the perception of speech dissociates in a number of ways from the perception of auditory events (the latter presumably using the analyzers we share with other primates). Neuroimaging and brain-damage studies suggest that partly distinct sets of brain areas subserve speech and nonspeech sounds (Hickok & Poeppel, 2000; Poeppel, 2001; Trout, 2001; Vouloumanos et al., 2001). A clear example is pure word deafness, in which a neurological patient has lost the ability to analyze speech while recognizing other environmental sounds (Hickok & Poeppel, 2000; Poeppel, 2001). Cases of amusia and auditory agnosia, in which patients can understand speech yet fail to appreciate music or recognize environmental sounds (Peretz, Gagnon, & Bouchard, 1998; Poeppel, 2001), show that speech and non-speech perception in fact doubly dissociate.
Third, many of the complex hallmarks of speech perception appear early in infancy (Eimas & MIller, 1992; Miller & Eimas, 1983). Recent studies suggest that young infants, including neonates, prefer speech sounds to nonspeech sounds with similar spectral and temporal properties. These include sounds that would have been
indistinguishable in the womb, so the preference cannot be explained by learning in utero (Vouloumanos & Werker, 2004a, 2004b).
Fourth, comparisons among primates turn up significant differences between their abilities to perceive speech and our abilities. For example, monkeys fail to categorize consonants according to place of articulation using formant transitions alone (Sinnott & Williamson, 1999). They discriminate /ra/ from /la/ at a different boundary from the one salient to humans (Sinnott & Brown, 1997). They fail to segregate the initial consonant from the vowel when compensating for syllable length in discriminating phonemes (Sinnott, Brown, & Borneman, 1998). They fail to trade off the duration of the silent gap with the formant transition in perceiving stop consonants within consonant clusters (Sinnott & Saporita, 2000). They fail to show the asymmetrical ―magnet effect‖ that
characterizes infants‘ discrimination of speech sounds varying in acoustic similarity to prototype vowels (Kuhl, 1991). And their subjective similarity spaces among vowels (measured by discrimination reaction times analyzed by multidimensional scaling) is very different from that of humans (Sinnott et al., 1997). Chimpanzees, too, have a subjective similarity space for vowels that differs from humans‘, and, like macaques, have difficulty
discriminating vowel pairs differing in advancement or frontness (Kojima & Kiritani, 51989). Quail (Trout, 2003a) and budgerigars (Dooling & Brown, 1990) that have been
trained to discriminate human speech sounds also show patterns of discrimination and generalization that differ from those of humans. A recent review of research on speech perception in humans, chinchillas, budgerigars, and quail showed that the phoneme boundaries for humans and animals differed in more than a third of the studies (Sinnott, 1998). These findings must be qualified by the fact that human speech perception necessarily reflects the effects of experience listening to a specific language, and it is difficult to equate such experience between humans and other animals. Nonetheless, if findings of similarities between humans and animals trained on human speech contrasts are taken as evidence that primate audition is a sufficient basis for human speech perception, findings of differences following such training must be taken as weakening such a conclusion.
2.3. Speech production. Turning to the articulatory side of speech, HCF cite two
arguments against evolutionary adaptation for language in the human lineage. One is that some birds and primates produce formants (time-varying acoustic energy bands) in their vocalizations by manipulating the supralaryngeal vocal tract, a talent formerly thought to be uniquely human. Nonetheless, by all accounts such manipulations represent a minuscule fraction of the intricate gestures of lips, velum, larynx, and tip, body, and root of the tongue executed by speakers of all human languages (Browman & Goldstein, 1992; Hauser, 1996). Nonhuman primates are also notoriously resistant to training of their vocalizations (Hauser, 1996), and as HCF themselves note, they show no ability to learn vocalizations through imitation. HCF try to downplay the difference between humans and primates by pointing out that vocal imitation is not uniquely human. But this is irrelevant
5 R. Remez, commenting in this reference on the work of (Kluender, 1994), notes that Kluender‘s trained
quail failed to distinguish labial and palatal phonemes. He also suggests that the quail‘s ability to distinguish other place-of-articulation distinctions may hinge on their detecting the salient apical bursts that initiate stop consonants rather than the formant transitions that suffice for such discriminations in humans.
to the question of whether vocal imitation evolved for language in the human lineage. The other species that evolved comparable talents, namely certain birds and porpoises, are not ancestral to humans, and must have evolved their talents independently of what took place in human evolution.
Other evidence, not mentioned by HCF, also suggests that vocal production has been adapted for speech in humans. In comparison with extant apes and pre-sapiens
hominids, modern humans have an enlarged region of the spinal cord responsible for the voluntary control over breathing required by speech production (MacLarnon & Hewitt, 61999). Humans also display greater cortical control over articulation and breathing, compared with the largely subcortical control found in other primates (Deacon, 1997). And as Darwin noted, the innate vocal babbling of human infants is one of the clearest signs that ―man has an instinctive tendency to speak.‖
To reconcile the recursion-only hypothesis with the fact that vocal learning and imitation are distinctively human (among primates), HCF refer to a ―capacity for vocal imitation‖ and assign it to the ―broad language faculty‖ which subsumes non-language-
specific abilities. But this is questionable. Humans are not notably talented at vocal imitation in general, only at imitating speech sounds (and perhaps melodies). For example, most humans lack the ability (found in some birds) to convincingly reproduce environmental sounds. Even the ability to convincingly imitate a foreign or regional accent is the exception rather than the rule among human adults, and adults are notoriously poor at imitating the phonetics of a second language. Thus ―capacity for vocal imitation‖ in humans thus might better be described as a capacity to learn to produce speech, contradicting the idea that grammatical recursion is the only human-specific and language-specific component of the language faculty.
HCF‘s second argument against human adaptations for speech production is the discovery that the descended human larynx (which allows a large space of discriminable vowels, while compromising other functions) can be found in certain other mammalian species, where it may have evolved to exaggerate perceived size. HCF note that while a descended larynx ―undoubtedly plays an important role in speech production in modern
humans, it need not have first evolved for this function‖ but may be an example of ―preadaptation‖ (in which a trait originally was selected for a function other than the one it currently serves). But this suggestion, even if correct, does not speak to the issue of whether the human vocal tract was evolutionarily shaped to subserve human language. Modifications of function are ubiquitous in natural selection (for example, primate hands, bear paws, and bat wings are adaptations that evolved by natural selection from the fins of fish), so the fact that a trait was initially shaped by selection for one function does not imply that it was not subsequently shaped by selection for another function. Thus even if the larynx originally descended to exaggerate size, that says nothing about whether its current anatomical position was subsequently maintained, extended, or altered by selection pressures to enhance speech.
6 The fact that Homo erectus had a spinal cord like that of other primates rules out an alternative hypothesis in which the change was an adaptation to bipedal locomotion.