the source of contextual interference is not well understood. at present, it is primarily an empirical phenomenon. most accounts assume that it emerges because blocked practice is not sufficiently demanding to produce optimal effort or attention. consistent with this view, contextual interference is reduced or eliminated with more complex tasks (see wulf & shea, 2002). across the various accounts of this complexity effect, the dominant theme is that as complexity increases, learners benefit more from the opportunity to repeat and refine their responses on successive trials. there is also debate regarding whether children show the effects of contextual interference as adult learners. some studies suggest children show normal contextual interference (e.g., edwards et al., 1986) whereas others show no effect in children (e.g., del rey et al., 1983). moreover, as with adults, more difficult or complex tasks show less contextual interference (magill & hall, 1990). thus, the absence of contextual interference in some studies with children may simply be another manifestation of the task complexity effect.