Learning word meanings at 6 months?

Forthcoming in PNAS, an article by Elika Bergelson and Daniel Swingley arguing that "At 6–9 months, human infants know the meanings of many common nouns" with obvious implications for the study of cultural learning. The authors link their findings to the recent discovery of mindreading abilities in infants at the same early age  (see for instance for instance Kovács, Téglás , and Endress, [2010] 'The social sense: Susceptibility to others’ beliefs in human infants and adults. Science 330:1830–1834)  

Abstract: It is widely accepted that infants begin learning their native language not by learning words, but by discovering features of the speech signal: consonants, vowels, and combinations of these sounds. Learning to understand words, as opposed to just perceiving their sounds, is said to come later, between 9 and 15 mo of age, when infants develop a capacity for interpreting others’ goals and intentions. Here, we demonstrate that this consensus about the developmental sequence of human language learning is flawed: in fact, infants already know the meanings of several common words from the age of 6 mo onward. We presented 6- to 9-mo-old infants with sets of pictures to view while their parent named a picture in each set. Over this entire age range, infants directed their gaze to the named pictures, indicating their understanding of spoken words. Because the words were not trained in the laboratory, the results show that even young infants learn ordinary words through daily experience with language. This surprising accomplishment indicates that, contrary to prevailing beliefs, either infants can already grasp the referential intentions of adults at 6 mo [the explanation the authors prefer] or infants can learn words before this ability emerges. The precocious discovery of word meanings suggests a perspective in which learning vocabulary and learning the sound structure of spoken language go hand in hand as language acquisition begins.

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