What is recursion?
Hauser, Chomsky, and Fitch (2002) use what they call ‘‘the comparative method’’,1 comparing data on a range of biologically diverse species and linguistic vs. non-linguistic cognitive domains to distinguish between a faculty of language in the broad sense (FLB) and a faculty of language in the narrow sense (FLN). According to these authors, most, if not all, of FLB is based on mechanisms shared with non-human animals, ‘‘with differences of quantity rather than kind’’, while FLN is specific to humans. The following example illustrates their method: They observe that much of phonology is likely to be part of FLB, rather than FLN, either because phonological mechanisms are shared with other cognitive domains (not ably music and dance), or because the relevant phenomena appear in other species, particularly bird and whale ‘‘song’’ (Fitch, Hauser, and Chomsky in press: 16). An important hypothesis emanating from this research is that FLN ‘‘comprises only the computational mechanisms of recursion as they appear in narrow syntax and the mappings to the interfaces’’, and that this is the only uniquely human component of the faculty of language, being specific both to language and to humans.2 On this account, almost everything essential to human language can also be found in other animals, while recursion is believed to distinguish language from the capacities of non-human animals.3 Recursion is responsible for discrete infinity, that is, for an open-ended and limitless system of communication: It means that sentences are built up of discrete units: There are 6-word sentences and 7-word sentences, but no 6.5-word sentences, and there is no non-arbitrary upper bound to sentence length (Hauser, Chomsky, and Fitch 2002: 1571).
How did recursion evolve in human language? Is it the result of a sudden or a gradual process? One part of the work on language evolution has focused on the question of whether human language evolved by gradual, adaptive extension of pre-existing communication or other systems, or whether it (or important components of it) was exapted away from other capacities, such as spatial, numerical, or other functions. So the question is: Where does recursion come from?
A number of answers have been proposed to this question. One line of answers invokes symbolic reference as a prerequisite for the rise of recursion: Rather than being a necessary ingredient for the emergence of language, recursion (or ‘‘non-degrading recursivity’’) is suggested to be a consequence of symbolic reference and/or symbolic verbal language (Deacon 2003: 126). Hauser, Chomsky, and Fitch (2002: 1569, 1573–4) rightly observe that proponents of the idea that recursion (more precisely, their ‘‘FLN’’) is an adaptation would need to supply additional data or arguments to support this viewpoint, and that existing hypotheses are hard-pressed to explain how ‘‘the capacity of language for infinite generativity’’ would have resulted from a series of gradual modiWcations.4 But much the same applies to the idea that recursion is the result of exaptation of any kind, or that the rise of discrete infinity was non-gradual. There is simply no appropriate evidence available to decide in favor of one or the other.
We will not be concerned with how recursion as a cognitive mechanism arose in humans or in human languages; this is an issue that is clearly beyond our methodology. Rather, we wish to show that there are some data applicable to a study of the genesis of recursive structures in human languages, and that these data are in support of the gradualist hypothesis. Accordingly, we will be restricted to recursion as it manifests itself in languages, more precisely in morphosyntax, ignoring the question of whether there may not be additional manifestations of it.
1 With reference to linguistics, this use of the term ‘‘comparative method’’ is somewhat unfortunate since the term is commonly used for quite a different method, namely one based on the regularity of sound change and sound correspondences.
2 These authors argue that FLN includes the core grammatical computations as they appear in narrow syntax and the mapping to the interfaces, and that these computations are limited to recursion (Hauser, Chomsky, and Fitch 2002: 1570; Fitch, Hauser, and Chomsky inpress: 3). Furthermore, they suspect (Hauser, Chomsky, and Fitch 2002: 1574) that many details of language, such as subjacency, Wh-movement, or the existence of garden-path sentences ‘‘may represent by-products’’ of FLN. These authors speculate further that recursion may derive from some computational mechanism that was used by some earlier species for navigation, social cognition, or some other purpose.
3 The authors are careful to observe that they do not define FLN as recursion by ‘‘theoretical Wat’’; rather, they wish to offer this as a plausible, falsifiable hypothesis worthy of empirical exploration: ‘‘We hypothesize that ‘at a minimum, then, FLN includes the capacity of recursion’, because this is what virtually all modern approaches to language... have agreed upon, at a minimum. Whatever else might be necessary for human language, the mechanisms underlying discrete infinity are a critical capability of FLB, and quite plausibly of FLN’’ (Hauser, Chomsky, and Fitch 2002: 1573).
4 At the same time, these authors note: ‘‘However, the available data suggest a much stronger continuity between animals and humans with respect to speech than previously believed. We argue that the continuity hypothesis thus deserves the status of a null hypothesis.’’ (Hauser, Chomsky, and Fitch 2002: 1574).
