In section 8.1 of the target article, Christiansen & Chater (C&C) suggest that phylogeny (how language evolved in the species over time) begets ontogeny (developmental properties of day-to-day learning by the child), inferring that certain underlying mechanisms for these very different processes are the same. Focusing on L1 acquisition, I argue that the authors' elegant conceptualization and simulations of the logical problem of language evolution do not necessarily translate to the untidy developmental facts of language acquisition in the current linguistic environment. Specifically, it may be premature on C&C's part to advocate general-purpose cognitive mechanisms alone to explain child language learning and the repertoire of rule types that governs it.

As a first step, terms such as “language-specific constraints” and “general-cognitive constraints” must be regarded with caution. While domain specificity is often readily identified, domain generality is frequently a “moving target,” precisely because it is definable by degree. That is, we say that a mechanism is domain-specific so long as it only does what it evolved to do, or as long as it is only used for learning within its domain. But we can only say that a mechanism is more or less domain-general: It is more domain-general or neutral the more it generalizes to other tasks outside its domain. In principle, this effect may not be problematic – after all, lots of phylogenic and ontogenic distinctions involve matters of degree. In synchronic terms, any general mechanisms of cognition do not dispense with the specific ones; instead, the existence of general machinery presupposes the existence of functionally related specific mechanisms. When dealing with the countless complexities of linguistic structure, Skinner's box cannot afford to be empty: it must at least contain a random assortment of various entities (paraphrasing Satterfield & Saleemi Reference Satterfield and Saleemi2003). In other words, the basis from which L1 development commences had better be minimally equipped to supply learners with knowledge of grammatical functions, and Case systems, and so on, just in case children encounter these properties in their primary linguistic data.

Second, C&C state that

This view has several consequences in the developmental context that are not easy to explain. Consider the following cases: (1) Children in American English-speaking environments are widely attested to omit subject pronouns in tensed clauses (e.g., Toys in there; Hyams Reference Hyams1986). (2) They also insert an extra wh-expression in long-distance questions: for example, “What do you think what pigs eat?” (Crain & Thorton Reference Crain and Thorton1998). The structures in question seem to be at odds with “everybody else” – for example, with adult English, which contains no such features (although these are by no means “wild guesses,” since in the former case [1], languages like Standard Spanish and Italian pattern somewhat in this way; and in the latter case [2], certain German dialects would be appropriate). However, it would be computationally “easier” to simply have a zero or deleted Complementizer in this site, rather than the wh-word. These tokens beg the question of why children should exhibit such tendencies at all. Perhaps the task requires children to initially exert multiple types of “biases” in order to obtain the maximal advantages of the grammar (Gawlitzek-Maiwald & Tracy Reference Gawlitzek-Maiwald and Tracy2005; Roeper Reference Roeper1999; Saleemi Reference Saleemi and Singh2002; Satterfield Reference Satterfield1999a; Reference Satterfield1999b; Yang Reference Yang, Dale and Church1999; Reference Yang2002).

On common ground with C&C, it is doubtful that a highly language (domain)-specific Universal Grammar (UG) functions as the sole machinery in the child's task of language acquisition. However, this position need not exclude the possibility that children pick which apparatuses they use, more-generalized or less-generalized tools, and to what degree, in a flexible and adaptive manner. Keeping this postulation in mind, consider the critical period in L1 acquisition. It has been successfully argued that not all aspects of language display critical period effects. Specifically, the acquisition of lexical items, the concatenation of words, and the nuts and bolts of semantically based principles of word order seem immune to age-related “atrophy” of language acquisition (Hudson & Newport Reference Hudson, Newport, Greenhill, Littlefield and Tano1999; Jackendoff Reference Jackendoff2002; Sorace Reference Sorace, Doughty and Long2003). However, the capacity to acquire other parts of language in late language learning, such as the inflectional system or real intricacies in phrase structure, appears to be largely diminished (Lardiere Reference Lardiere1998; Reference Lardiere and Archibald2000). These common L1 versus L2 conditions become at once isolatable by viewing language acquisition as a multi-layered construction with distinct learning mechanisms.

Ultimately, the solution to the logical problem of language acquisition may reside in the possibility of possessing several initial knowledge states in which domain-general mechanisms interact with domain-specific components designed to acquire the most arbitrary and least systematic knowledge of the target grammar. States that stabilize over several progressive stages could then emerge, with the help of additional domain-general learning mechanisms, which handle increasingly more systematic principles of the grammar. In sum, each transitional state may be shaped by the myriad effects of learning, or, more appropriately, of acquisition, which can be viewed as the combination of experience and internally determined processes such as maturation, learning mechanisms, and language-specific computations.