These studies also indicated that young infants link count nouns specifically to commonalities that underlie object categories and that infants' expectations for count nouns may be more precise than for other word categories, such as adjectives, perhaps because the mappings for other word categories tend to be more variable, both across languages and across development. These findings concur with most current theories of language acquisition, which suggest that the noun category may be established earlier and through different mechanisms than other grammatical categories, and that the noun category underlies the establishment both of reference and of other grammatical categories. Based on studies of infants acquiring English, French, or Spanish, researchers found that performance on novel nouns was the same across all three languages. Objects may vary across cultures, and words will vary across languages, but the fundamental process of mapping words to meaning is similar across both culture and language. Using data from more than 1,000 children under the age of three years and longitudinal data on 146 monozygotic and 86 dizygotic twins, researchers are examining the heritability of certain aspects of language development.
Current results suggest that vocabulary growth is partly heritable.
Specifically, monozygotic twins achieve vocabulary milestones in closer synchrony, have more similar vocabularies in terms of relative proportion of nouns, acquire new words at a more similar rate, and combine words productively in closer synchrony than do dizygotic twins. However, heritability is much higher for productive word combinations than for vocabulary, presumably because words can be learned only from the environment, while combining words into new combinations requires active mental processing. This variation may be due to genetic factors. MIT researchers report similar findings in magnetoencephalographic measurements of brain activity during normal subjects' production of regular and irregular forms, which supports the theory that the acquisition of words and rules are separate processes that may be differentially disrupted in language disorders, and that these differences relate to differences in brain organization. Finally, their evidence suggests that timing in language development may be part of a genetically-influenced maturational clock, which may help explain why different children acquire language on different schedules.