Based on several studies done in SLA, Nunan (cited in Nunan, 1987) has proved that there is a predetermined order of acquisition for certain grammatical morphemes, but the input that the learners received from the naturalistic settings and/or the classrooms may not appear to have any great influence on the development of these morphemes. Johnston (1987), and Towell and Hawkins (1994) have also confirmed that the underlying process of acquisition is independent of the order in which rules are taught in classes.
Håkansson (2002) has then pointed out that the teaching syllabus sometimes mismatches the learners’ capacity for learning. Some teaching materials are unable to
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relate explicit instructions to the build-up knowledge of the language system, and then relate the instructions to the creation of linguistic competence (Towell and Hawkins, 1994; Hawkins, 2003). Therefore, Pienemann (cited in Hyltenstam and Pienemann, 1985) has introduced the principles of syllabus construction that (1) new structures should build on previous ones; (2) simple structures should be taught before complex ones; and 3) interaction between grammatical structures should allow the expected structures to be introduced naturally. However, there are various measures to define what simple is and what should be introduced straight after the old structures (Pienemann, 1998b). Therefore, these basic principles remain ambiguous in teaching.
From the beginning of 1980s, Pienemann and his colleagues started to propose an interesting explanation, termed as the Teachability Hypothesis, for the disparity between instruction and acquisition, in terms of a discussion of the language processing constraints (Nicholas, 1985; Pienemann, 1985). Corder (1981: 77) has already addressed that ‘effective language teaching must work with, rather than against, natural processes, facilitate and expedite rather than impede learning’. Accordingly, Pienemann (1984, 1985 and 1989) has claimed that learners should be taught what can be processed and is therefore learnable on the basis of the universal processing route.
Theoretically, the Teachability Hypothesis was tested in a classroom study (Pienemann, 1984) in which ten Italian-speaking children learnt German L2 at the developmental levels ranging from X (a certain developmental stage) to X+2. All these participants were instructed to learn structures of X+3. In the end, the research results showed that only the participants whose current language was at stage X+2 could possibly acquire the structures of X+3, which indicated that the processing prerequisite for the structures of X+3 is the acquisition of stage X+2. Similarly, X+2 cannot be introduced without processing the structures at stage X+1 (Kawaguchi, 2005c).
This study has proved that the constraint in language development could not be overridden by any type of input or instructions (Clahsen, 1984; Pienemann, 1984). In
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relation to PT, the Teachability Hypothesis has further predicted that stages of SLA cannot be skipped nor altered through teaching intervention, because of the cumulative nature of the processing strategies. In other words, the intervention of language teaching should be constrained by ‘teachers’ judgments as to the potential target forms’ learnability (hence teachability)’ (Long, 2007: 123).
Learners cannot run before they can walk. It has accentuated the fact that a given construction will effectively allow the learners to benefit from teaching only when the learners are ‘well-prepared’ (Smith, 1994). As Pienemann (1998c) has concluded, only if the learner has reached the stage where he/she is ready to work in his or her linguistic system will the instruction have an effect on acquisition. Therefore, according to the processing hierarchy and the learner’s psychological constraints, the teaching sequence is required to ally with the order of acquisition.
In fact, the Teachability Hypothesis has provided the order of the presentation of the structural contents, i.e. what grammatical items to teach and when. Following this hypothesis, grammatical ‘items can only be learned when they are one stage ahead of a learner’s present processing capacity’ (Nunan, 1987: 89). Consequently, the teaching contents should be better structured and should fit in to reflect the developmental stages, since the instruction normally attempts to promote the L2 development (Pienemann cited in Hyltenstam and Pienemann, 1985; Smith, 1994).
Berti and Di Biase (2002) have conducted a study to verify the efficacy of form-focused instruction in L2 teaching when the grammatical items to be focused upon are decided on the basis of the learners’ developmental stages. Interestingly, learners in Berti and Di Biase’s study were able to move from stage 1 to stage 3 in 18 weeks following the form-focused instruction under the umbrella of PT. Notwithstanding that the Teachability Hypothesis constrains the possible influence of instruction on the acquisition process; it does not negotiate its influence on the speed of acquisition, the frequency of rule application and the different contexts in which the rule has to be applied, wherever the interlanguage development fulfills the requirements for such an influence (Pienemann cited in Pfaff, 1986).
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It is seen that formal instruction can advance language acquisition when the learner is developmentally ready for a particular structure (which is evident from a number of experiments and longitudinal and cross-sectional studies (Pienemann, 1998c; Kawaguchi, 2005c)). However, research has shown that learners need to ‘build up all processing strategies in a lockstep fashion, but the presence of all processing procedures does not guarantee that the structure will emerge at that point’ (Pienemann, 1998c: 251). In other words, the Teachability Hypothesis defines constraints on teachability which do not predict sufficient conditions for teaching to be successful. Therefore, from this perspective, one can easily agree with the view that the same input may have an effect on one learner but not on another.
In addition, the delayed effect of the instruction could also occur with a diversity of explanations, ‘among [which] are frequency related features of classroom discourse (Ellis, 1984), the difference between input and intake (Krashen, 1981 and 1989), the psycholinguistic constraints on speech processing and teachability (Pienemann, 1984, 1989, and 1998c)’ (Zhang, 2002b: 36).
As a consequence, the explanatory power of the Teachability Hypothesis is based on the hierarchical nature of the processability hierarchy. Given that every processing procedure in that hierarchy forms a key prerequisite for the next stage higher, none of the stages/prerequisites can be skipped through instruction. In other words, the effect of teaching is constrained by processability. Besides, the limits on the effects of instruction on SLA should be observed and analyzed to be incorporated into PT.