Our objective is to advance the life course analytical framework by demonstrating a model for testing two of its tenets. The first is whether the individual's developmental history conditions the response to a turning point event. The second is whether the influence of a major life event upon an individual's developmental course depends upon the timing of the event. We test both propositions in an analysis of the effect of grade retention on a child's trajectory of physical aggression. Our analysis is based on data from a longitudinal study of 1,037 boys from schools in the lowest socioeconomic areas in Montreal, Canada. We find clear evidence that a developmental history of physical aggression conditions the child's response to grade retention. The evidence on whether the timing of retention affects this response is less clear.