Mutual exclusivity is a word learning constraint that involves the tendency to assign one label/name, and in turn avoid assigning a second label, to a single object. Mutual exclusivity is often discussed as one of three main lexical constraints, or word learning biases, that are believed to play major roles in word learning, the other two being the whole-object and taxonomic constraints. This assumption is typically first seen in the early stages of word learning by toddlers, but it is not limited to young childhood. Mutual exclusivity is often discussed by domain-specific accounts of language as limiting children's hypotheses about the possible meanings of words. It is generally accepted that mutual exclusivity alone cannot account for the complexity of word learning but is instead “more like heuristics in problem-solving."
History Research on mutual exclusivity as a lexical constraint first began in the 1980s. Markman and Wachtel (1988) designed what some consider to be the seminal study of mutual exclusivity. It involved six studies that analyzed children's willingness to assign an unfamiliar label to familiar or unfamiliar objects. In their first study, they placed two objects in front of 3- to 4-year-old children, one that had a label known to the children (familiar object) and one that did not have a known label (novel object). Once shown both objects, the children were then told "show me the [nonsense syllable]" to see which object the children would assign this new label. The children had a significant tendency to "reject a second label for [a familiar] object, treating it, instead, as a label for a novel object." This is often referred to as a simple disambiguation task. In their remaining five studies, they examined whether familiarity to an object would [mediate?] whether or not children would assign a new label to the object as a name for the object itself or a label of a part of it. For example, Study 2 involved a familiar condition and unfamiliar condition. In the familiar condition, children were presented with a familiar object (in this case "fish") and asked whether a novel label ("dorsal fin") applied to the whole object or "just this part" (pointing to the fin). In the unfamiliar condition, children were presented with an unfamiliar object ("lung") and asked whether a novel label ("trachea") applied to the whole object or a part of it. Methods of studies 3-6 were general variations of this design and ultimately found similar results. When the object was familiar to the children, they overwhelmingly assigned the novel part label to a part of the object, rather than the whole object itself. When the object was unfamiliar to the children, they overwhelmingly assigned the novel part label to the whole object itself, rather than one of its parts. These studies set a precedent for how many researchers have studied mutual exclusivity to date.
Function Merriman and Bowman (1989) offered a list of ways in which mutual exclusivity may influence infants’ word learning. First, mutual exclusivity might influence the infant's decision about the reference of a new word. For example, if a child is faced with a familiar object for which they already have a label, and an unfamiliar object, which they have not yet labeled, they are likely to map a novel word onto the unfamiliar object. This is called the disambiguation effect. Next, the bias might cause an infant to change the extension of a familiar word. In this case, a child might remove 'wolf' from their extension of 'dog' upon hearing an animal labeled a 'wolf.' This can be called the correction effect. As well, the bias might compel an infant to reject a new word, for instance, rejecting 'wolf' in favor of 'dog.' This is the rejection effect. Lastly, mutual exclusivity might influence word generalization. In this case, if a child knows an object to have a particular name, the child should not generalize other names to it. This is known as the restriction effect.
Associations with vocabulary learning Some studies have found positive correlations between children's use of mutual exclusivity and their learned vocabulary, while others have found the opposite. In Callanan's and Sabbagh's (2004) study on parents’ facilitation of mutual exclusivity with their 12- to 24-month-olds, they found that having parents who gave two labels for an object to their child was positively correlated with having a productive vocabulary. Markman, Wasow, and Hansen (2003) found that “lexical constraints enable babies to learn words even under non-optimal conditions.” One study (Stevens & Karmiloff-Smith, 1997) examined the use of lexical constraints in word learning in children with Williams syndrome, a rare neurodevelopmental disorder. Although they did not abide by other constraints such as the whole object or taxonomic constraints, they did exhibit fast mapping and mutual exclusivity.
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