The order in which stimuli are presented in an experiment has long been recognized to influence behavior. Previous accounts have often attributed the effect of stimulus order to the mechanisms with which people process information. We propose that stimulus order influences cognition because it is an important cue for learning the underlying structure of a task environment. In particular, stimulus order can be used to infer a "stimulus bundle"-a sequence of consecutive stimuli that share the same underlying latent cluster. We describe a clustering model that successfully explains the perception of streak shooting in basketball games, along with two other cognitive phenomena, as the outcome of finding the statistically optimal bundle representation. We argue that the perspective of viewing stimulus order as a cue may hold the key to explaining behaviors that seemingly deviate from normative theories of cognition and that in task domains where the assumption of stimulus bundles is intuitively appropriate, it can improve the explanatory power of existing models.H uman cognition typically takes place in an environment where information becomes available over time. The sequential nature of information availability implies that stimulus order-the order in which a sequence of stimuli is presentedmay strongly influence the interpretation of a task environment. Consider the weather patterns in Fig. 1 as an example. If we were to record the dominant weather phenomenon in each month of a year, order 1, as shown in Fig. 1, would likely represent a typical city in upstate New York, such as Rochester. Given this sequence of stimuli, it is relatively easy for an observer to infer the existence of "seasons" underlying the weather pattern-that is, winter seems to be in effect from November to April, followed by spring/summer, and then by fall. However, if we reorder this set of stimuli from order 1 to order 2 as shown in Fig. 1, the inference of seasons becomes much less apparent. Even when the exact same set of stimuli is presented in both sequences, the difference in stimulus order has led to a dramatically different interpretation about its underlying structure. Why does stimulus order affect human cognition in this interesting way?The effect of stimulus order on cognition has often been viewed as a consequence of the way that the human mind processes information (1-8). For example, Elliott and Anderson (5) accounted for the effect of stimulus order on category learning by incorporating the mechanisms of memory decay into a categorization model. Memory decay intuitively captures the assumption that the human mind tends to be influenced by nearby stimuli more than by distant ones. It has also been hypothesized that when a sequence of stimuli is ordered in such a way that the similarity between successive stimuli is maximized, it offers a processing benefit because the stimuli being kept in working memory will be similar to each other and thus allow for easier abstraction of concepts (4). In addition, the effects of stimulus ord...