  In Zacks, Tversky, and Iyer’s experiments, participants segmented an ongoing activity (like washing dishes) while watching it on film by pressing a key to mark “natural and meaningful” unit boundaries. The grain at which participants segmented the activity was manipulated between subjects: one group was asked to mark the largest meaningful units (coarse grain size), the other group the smallest units (fine grain size) which still could be considered to be complete events. The placement of perceived event boundaries provides information about the psychological status of events and their partonomic hierarchy. If the stream of action is perceived as consisting of discrete events, participants should place event boundaries consistently and this is what Zacks, Tversky, and Iyer (2001) found. Further, if participants make use of partonomic hierarchies, coarse event boundaries should be aligned with fine boundaries, since the end of each superordinate event is also the end of its last subevent. Again, this hypothesis was corroborated by a strong hierarchical bias effect. Events thus seem to be psychologically real and to be hierarchically structured. 