This paper investigates the question of whether the Wave Model of linguistic change can furnish a valid idealization for the patterning of variation in interlanguage systems and, thereby, a scale for the determination of their degree of naturalness. Using cross-sectional data from 162 French-speaking adult learners, the paper analyzes the variability in the evolution of the English possessive determiners HIS and HER. The main findings are as follows:
1. The Wave Model furnishes a valid idealization for acquisitional change.
2. The developmental continuum reveals that L2, acquisition appears to involve a greater scope of variability. Change may be initiated in a more marked environment at a point when the rule change process is not yet fully completed in the less marked environment. Nonetheless, the proportion of learners showing variable application of the mature rule in only one environment at a time as opposed to those with variable application in both is more than 2:1.
3. Less natural systems evidence rule blockage and/or compartmentalization of the marked environment; rule conflict between noncontiguous parts of the rule, and violation of markedness constraints.
4. Premature adoption of gender marking on the possessive determiners or, more generally, failure to approach the target language rule with an unmarked, initial hypothesis is one important source of unnaturalness in rule variability.
The data analysis leads up to a number of other issues in interlanguage research such as inter-learner variation, the ‘acquisition-learning’ distinction, and the relationship between the Simple Codes Hypothesis and unsuccessful L2, acquisition.