This paper has two major objectives: (1) to summarise Processability Theory, a processing-oriented approach to explaining language development and (2) to utilise this theory in the comparison of development in LI and L2 acquisition. Proponents of the Fundamental Difference Hypothesis (between L1 and L2) assume that L1 development can be explained with reference to Universal Grammar (UG) which, in their view, is inaccessible to L2 learners. Instead, they claim that a second language develops on the basis of language processing strategies.

I will show that the fundamentally different developmental paths inherent in first and second language acquisition can both be explained on the basis of the same language processing mechanics (as specified in Processability Theory). I will demonstrate that the developmental differences between L1 and L2 are caused by the qualitatively different early structural hypotheses which propagate through the acquisition process. The concept of “propagation of structural features” will be viewed as “generative entrenchment,” a logical-mathematical concept, which has proved to be highly productive in examining other kinds of developmental processes.

The tension between considering language acquisition to be a process of learning or a process of development is the visible surface of a theoretical iceberg. The difference between learning and development is neither simple nor self-evident, yet the implications that follow from a commitment to one or the other are considerable. Development is usually construed as an unfolding, the impassive evolution of “being.” Children develop physically, cognitively, emotionally almost in spite of themselves; they may not develop to their full potential, and they may require massive environmental support, but even in cases of benign neglect, children will normally become taller, smarter, and more stable. Learning implies a measure of control and responsibility. It depends on experience and on the availability of resources: conceptual, social, intellectual, and so on.

Responding to an invitation to comment on Pienemann's paper leaves me on the horns of a dilemma: I think the approach is novel and shows how future second language acquisition (SLA) research can be done. However, it is difficult to judge from the paper what the exact contribution of the theory is to be.

A problem when reacting to a contribution as rich and as highly interesting as Pienemann's paper, is that it is much easier to react to what's not in it than to what is. An example is the relation between the theory proposed and the way linguistic knowledge is acquired: “[Processability theory] … is not designed to contribute anything to the question of the innate or learnt origin of linguistic knowledge or the inferential processes by which linguistic input is converted into linguistic knowledge.” It is not easy to interpret the full consequences of the model if we are not to take into account the inferential processes by which the learner adds new information to his/her system. Maybe here the dividing lines are drawn too strictly: if the hypothesis space of the learner is restricted by the procedures that can be applied, one wonders at what level in the cognitive system these restrictions are located. Does the conceptualizer already know what the system is capable of doing, or is it uninformed (as follows from a strict interpretation of Levelt's proposal of an incremental system without feedforward)? In the latter case, this would mean that the learners/speakers would still try to express their communicative intentions and only later on in the process would things go wrong, for example, because the S-procedure cannot be carried out.

Let me begin my comments on Pienemann's keynote paper by expressing my admiration for the scholar who has developed Processability Theory (PT) over a period of some fifteen years with great determination and perseverance. What in earlier publications (e.g. Pienemann, 1985, 1987) appeared to me to be a rather disparate set of principles aiming to account for a limited set of empirical data (the well known sequence of five word orders of the ZISA study), has now evolved into a coherent theory which meets the demands of falsifiability, as PT's claims are formulated in sufficient detail to allow SLA researchers to put them to empirical test. PT comprises a number of principles of great generality, accounting, in principle, for the acquisition of any structure in any language, thereby exceeding the limits of the ZISA data of natural German L2 acquisition. As such, it is to be hoped that PT will have a healthy influence on the field of SLA research, as this field, in the last few years, has perhaps been dominated too much by the issue of whether L2 learners have access to Universal Grammar.

When you compare the behavior of two different age groups which are trying to master the same sensori-motor or cognitive skill, you are likely to discover varying learning routes: different stages, different intervals between stages, or even different orderings of stages. Such heterogeneous learning trajectories may be caused by at least six different types of factors:

(1) Initial state: the kinds and levels of skills the learners have available at the onset of the learning episode.

(2) Learning mechanisms: rule-based, inductive, connectionist, parameter setting, and so on.

(3) Input and feedback characteristics: learning stimuli, information about success and failure.

(4) Information processing mechanisms: capacity limitations, attentional biases, response preferences.

(5) Energetic variables: motivation, emotional reactions.

(6) Final state: the fine-structure of kinds and levels of subskills at the end of the learning episode.

This applies to language acquisition as well. First and second language learners probably differ on all six factors. Nevertheless, the debate between advocates and opponents of the Fundamental Difference Hypothesis concerning L1 and L2 acquisition have looked almost exclusively at the first two factors. Those who believe that L1 learners have access to Universal Grammar whereas L2 learners rely on language processing strategies, postulate different learning mechanisms (UG parameter setting in L1, more general inductive strategies in L2 learning). Pienemann opposes this view and, based on his Processability Theory, argues that L1 and L2 learners start out from different initial states: they come to the grammar learning task with different structural hypotheses (SOV versus SVO as basic word order of German).

Manfred Pienemann offers an interesting, fair-minded, and systematic account of the role of processability constraints in explaining language development. Thus he opens up a wide field of research in the area of the interaction of grammatical knowledge and processing constraints, and Pienemann should be applauded for this kick-off. In my view, this field should be explored in a non-partisan manner. Rather than pushing either grammar or processing at the expense of the other, we should be looking at the division of labour between different mental capacities, and at potential sites of overlap, conflict, and convergence. L2 development is crucial in this research, because it may show how much can be accomplished with non-grammatical processing. I will limit myself here to a number of disjointed remarks on some directions for research.

Pienemann's Processability Theory is an interesting and well thought out attempt to account for the developmental problem in L1 and L2. If correct, it would relieve the genetically specified knowledge base of what he views as its burdensome task of having to specify a great deal of genetically encoded linguistic knowledge and transfer at least part of that task to the processing mechanisms available for the acquisition of procedural skills needed for language processing. He posits a set of processing resources forming an implicational hierarchy such that the resource of a certain level is a prerequisite for the functioning of the next higher level. He posits further that the processing devices will be acquired in their predetermined sequence of activation in the production process. A consequence of this is that acquirers of both the first and the second language will follow the same sequence of acquisition of the processing hierarchy. The differences in the courses of development can be attributed, he says, to the knowledge base with which the learner comes to the acquisition task. In the case of L1, he says, many would see UG as the knowledge base; in the case of L2, only the surface syntactic features of the target language are available (and, presumably, knowledge of the L1).

The main thesis Pienemann puts forward is that while L1 acquisition and L2 acquisition of a particular language (e.g. German) exhibit non-parallel paths of development, such a difference does not entail that the processing resources claimed to be responsible for transitioning from one stage to the next are any different in the two situations. The cause of the difference in routes, he suggests, is the difference in departure points. In the German case at hand, L1 acquirers begin with an SOV grammar, while L2 acquisition, according to Pienemann, starts off SVO (“canonical order”). “Generative entrenchment” of SVO is what makes the ensuing development distinct from that of L1 German: “once a decision has been made and a new structure has been added, it is very costly, if not impossible, for the developmental process to move to a different developmental path.” I think there's something very right about this general picture – but also that part of it is “wrong-headed.”

Given that my paper touched upon many aspects of L1 an L2 acquisition, it is not surprising that the commentaries have raised a diverse range of issues. Nevertheless, one should not lose sight of the fact that the paper has a tightly defined focus, namely to delineate the influence of language processing factors and developmental dynamics on developmental trajectories in L1 and L2 acquisition, and this is done in a very explicit manner. As Carroll rightly puts it in her commentary, this “[e]xplicitness allows us to formulate questions which could otherwise not be asked.”

We construct a production model of code-switched discourse, in order to explain empirical observations about switch point distribution, well-formedness of monolingual fragments, conservation of constituent structure and lack of constraint between successive switch points. The two monolingual grammars are modeled by congruent context-free phrase structure grammars. Each code-switched sentence makes alternate reference to two virtual monolingual sentences, one in each language, and, in the immediate vicinity of switch points, to their constituent structures. Imposing conservative conditions on when a constituent should be labeled as to language, and invoking a constraint against real-time “look-ahead” from one code-switch to the next, we prove that the existence of a consistent tree labeling, easily monitored by the speaker, implies a constraint on local equivalence of constituent order in the two languages around a switch point. This constitutes an explanation, without invoking a “code-switching grammar”, of the observed tendency for equivalence-point code-switching.

A series of three lexical decision experiments showed that interlingual homographs may be recognized faster than, slower than, or as fast as monolingual control words depending on task requirements and language intermixing. In Experiment 1, Dutch bilingual participants performed an English lexical decision task including English/Dutch homographs, cognates, and purely English control words. Reaction times to interlingual homographs were unaffected by the frequency of the Dutch reading and did not differ from monolingual controls. In contrast, cognates were recognized faster than controls. In Experiment 2, Dutch participants again performed an English lexical decision task on homographs, but, apart from nonwords, Dutch words were included which required a “no” reaction. Strong inhibition effects were obtained which depended on the relative frequency difference of the two readings of the homograph. These turned into frequency-dependent facilitation effects in Experiment 3, where participants performed a general lexical decision task, responding “yes” if a word of either language was presented. It is argued that bilingual word recognition models can only account for the series of experiments if they explain how lexical processing is affected by task demands and stimulus list composition.