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.Clark, E. V. (2009). Lexical meaning. In E. L. Bavin (Ed.), The Cambridge Handbook of Child Language (pp. 283-300). New York: Cambridge University Press. 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.Hirsh-Pasek, K., Golinkoff, R. M., & Hollich, G. (2000). An emergentist coalition model for word learning: Mapping words to objects is a product of the interaction of multiple cues. In R. M. Golinkoff, K. Hirsh-Pasek, L. Bloom, L. B. Smith, A. L. Woordward, N. Akhtar,…G. Hollich (Eds.), Becoming a word learner: A debate on lexical acquisition (pp. 136-164). New York: Oxford University Press. 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."Golinkoff, R.M., Hirsh-Pasek, K., Bailey, L.M., & Wenger, N.R. (1992). Young children and adults use lexical principles to learn new nouns. Developmental Psychology, 28, 99-108.
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.Markman, E.M., & Wachtel, G.F. (1988). Children’s use of mutual exclusivity to constrain the meanings of words. Cognitive Psychology, 20, 121-157. 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.Merriman, W. E., Bowman, L. L., & MacWhinney, B. (1989). The mutual exclusivity bias in children’s word learning. Monographs of the Society for Research in Child Development, 54(3-4, Serial No. 220). 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.Callanan, M.A., & Sabbagh, M.A. (2004). Multiple labels for objects in conversations with young children: parents' language and children's developing expectations about word meanings. Developmental Psychology, 40, 746-763. Markman, Wasow, and Hansen (2003) found that “lexical constraints enable babies to learn words even under non-optimal conditions.”Markman, E.M., Wasow, J.L., & Hansen, M.B. (2003). Use of the mutual exclusivity assumption by young word learners. Cognitive Psychology, 47, 241-275.
One study (Stevens & Karmiloff-Smith, 1997) examined the use of lexical constraints in word learning in children with Williams syndrome, a rare neurodevelopmental disorder.Stevens, T., & Karmiloff-Smith, A. (1997). Word learning in a special population: do individuals with Williams syndrome obey lexical constraints? Journal of Child Language, 24, 737-765. Although they did not abide by other constraints such as the whole object or taxonomic constraints, they did exhibit fast mapping and mutual exclusivity.
Age and mutual exclusivity Halberda (2003) was one of the first studies to examine the limiting role of age on children's exhibition of mutual exclusivity.Halberda, J. (2003). The development of a word-learning strategy. Cognition, 87, B23-B34. He found a linear relationship between age and performance; that is, the older the child, the more likely they were to use mutual exclusivity in word learning. Grouped by age, 14-, 16-, and 17-month-olds were tested in a preferential looking paradigm in which they were presented with a familiar object and novel object, asked to “look at the dax,” and were recorded based on which object they looked at longest. The 17-month-olds looked at the novel object more than their original baseline preferences. The 16-month-olds looked at rates no higher than chance. The 14-month-olds actually looked at the familiar object (a car) after being asked to find the “dax.”
Other studies have offered different timelines for different areas of development of this constraint in children. Bion, Borovsky, and Fernald (2013) claimed that their findings suggested that “the ability to find the referent of a novel word in ambiguous contexts is a skill that improves from 18 to 30 months of age.”Bion, R.H., Borovsky, A., & Fernald, A. (2013). Fast mapping, slow learning: Disambiguation of novel word-object mappings in relation to vocabulary learning at 18, 24, and 30 months. Cognition, 126, 39-53. The 18-month-olds in their sample did not reliably look at the novel object after labeling, whereas 24-month-olds did. Liittschwager and Markman (1994) found evidence of 16-month-olds exhibiting mutual exclusivity, suggesting that differences in methods could account for this discrepancy in results across studies.Liittschwager, J.C., & Markman, E.M. (1994). 16-month-old and 24-month-old – Use of mutual exclusivity as a default assumption in 2nd-label learning. Developmental Psychology, 30, 955-968. They also found age differences in which children can override mutual exclusivity and give second labels to objects. They presented 16- and 24-month-olds with an object, familiar or unfamiliar, which was given a novel label. It was hypothesized that the children presented with a familiar object would have more trouble accepting the new label than those presented with the unfamiliar object. They found that their hypothesis was correct for the 16-month-olds but not the 24-month-olds. As the researchers put it, the 24-month-olds were able to “override this default assumption” of mutual exclusivity and give a second label to an object. However, they also learned that the 24-month-olds were only able to do this when the information load was low; that is, when they were given multiple objects to learn the labels of, they had similar rates of difficulty in learning the second label as the 16-month-olds. It is well known that adults have less difficulty in overriding the assumption of mutual exclusivity, but this research suggests that this ability to override it may begin as early as age 2.
A number of studies have shown that mutual exclusivity is not a bias exhibited in children alone. Halberda (2006) is one such study which showed that adults also exhibit mutually exclusivity.Halberda, J. (2006). Is this a dax which I see before me? Use of the logical argument disjunctive syllogism supports word-learning in children and adults. Cognitive Psychology, 53, 310-344. Specifically, they found that adults systematically avoided assigning the novel label to a known distractor and instead showed a significant looking preference to assigning said label to novel objects. Golinkoff, Hirsh-Pasek, Bailey, and Wenger (1992) also sought to determine whether adults would exhibit mutual exclusivity at similar rates to children. They found that every adult chose the novel object as the referent for the novel term in every trial. The adults’ performance was better than the 2.5-year-olds of their study, who performed slightly worse but still well above chance.
Bialystok and colleagues (2010) found that the younger children in their sample of 3- and 4.5-year-olds showed a higher tendency of exhibiting mutual exclusivity than the older children.Bialystok, E., Barac, R., Blaye, A., & Poulin-Dubois, D. (2010). Word mapping and executive functioning in young monolingual and bilingual children. Journal of Cognition and Development, 11, 485-508.
Bion, Borovsky, and Fernald (2013) claimed that their findings suggested that “the ability to find the referent of a novel word in ambiguous contexts is a skill that improves from 18 to 30 months of age.” The 18-month-olds in their sample did not reliably look at the novel object after labeling, whereas 24-month-olds did. The 24-month-olds did not show evidence of having remembered, or retained, the novel label after a waiting period, whereas 30-month-olds showed some signs of retention.