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.
Merriman and Bowman (1989) offered a list of ways in which mutual exclusivity may influence infants’ word learning.
[6] Markman, Wasow, and Hansen (2003) found that “lexical constraints enable babies to learn words even under non-optimal conditions.”[7] One study (Stevens & Karmiloff-Smith, 1997) examined the use of lexical constraints in word learning in children with Williams syndrome, a rare neurodevelopmental disorder.
[9] 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.
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.
[11] They also found age differences in which children can override mutual exclusivity and give second labels to objects.
Golinkoff, Hirsh-Pasek, Bailey, and Wenger (1992) also sought to determine whether adults would exhibit mutual exclusivity at similar rates to children.
Less research has been conducted specifically on children's and adults’ retention of word-object relations learned through fast mapping.
[15] They were tested three times on whether they successfully mapped the label to the object, once immediately after being presented with the word-object relation, once a week later, and once a month later.
Byers-Heinlein and Werker (2009) was one of the first studies to examine the differences between the uses of mutual exclusivity in 17- and 18-month-old monolingual, bilingual, and trilingual children.
Likewise, their bilingual subjects exhibited the constraint more often than their trilingual counterparts, who showed no reliance on mutual exclusivity.
Other studies have built upon this research by examining monolingual and bilingual children's use of mutual exclusivity at ages older than 17 months.
A 2017 study examined 2- to 4-year-olds and found that the bilingual children tended to rely less on mutual exclusivity than their monolingual counterparts.
These studies used a disambiguation task in which the children were presented with a familiar and unfamiliar object and asked to assign a novel label to one or the other.
Yow and colleagues (2017) examined 4.5-year-olds’ use of the constraint, but they did not simply present the children with novel objects and labels or teach them the word-object association.
[20] Instead, the experimenters looked at the novel object when saying the novel label, so the children needed to use the speaker's gaze to exhibit mutual exclusivity.
The researchers suspected that this “bilingual advantage” was a result of their increased sensitivity to communicative contexts compared to monolingual children.
Kaminski, Call, and Fischer (2004) tested a dog's ability to fast map new names to new objects.
Some theorists believe that children possess this bias from the start of word learning, whereas others argue that it is slowly acquired throughout early childhood.
[5] Diesendruck and Markson (2001) argued that children's avoidance of lexical overlap may actually be caused by the way researchers speak to them.
Others have argued that documented use of mutual exclusivity being mostly exhibited by older children lessens the scope of the assumption.
[11] When they tested this, they found that the amount of information given to the children impacted their ability to learn second words for objects.
McMurray, Horst, and Samuelson (2012) offered an alternative assumption in researching lexical constraints and biases.
Some researchers have criticized the indirect role of mutual exclusivity in word learning because an alternative explanation could account for children's motivation to assign novel labels to novel objects.