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).