The mutual exclusivity assumption is a very useful constraint for learning basic-level category terms: children can map a novel word on to an object whose label has not yet been learned. It can also help children exclude a newly labeled object from the extension of an originally overextended old category and establish a new category. However, to learn words other than basic-level category terms, for example subordinate and superordinate category names, this assumption must be overridden. In this paper, to explore under what circumstances children adhere to mutual exclusivity and under what circumstances children override this principle, we discuss two series of studies. One series demonstrated 31/2-year-old children's failure to consider salient contextual/pragmatic information due to their adherence to mutual exclusivity. In contrast, the second series demonstrated that 36-month-old children, by using shape similarity, could override mutual exclusivity and establish a subordinate category. By considering the discrepancy between the two studies, we explore the meta-mechanism by which young children control the application of mutual exclusivity.
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