At 6-9 months, human infants know the meanings of many common nouns

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 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.

Fig. 1.

Experimental setup. The child sat on her parent's lap and was presented with images and sounds from a computer equipped with an eye tracker and speakers. The experimenter sat behind a screen and was not visible to the infant. The experimenter controlled presentation of stimuli and monitored the child on a live-feed camera. A backup video recording of the session was made to allow for confirmation of the validity of gross characteristics of the eye-tracking data stream. The figure shows an example of images presented on a paired-picture trial testing “banana” or “hair.”

Fig. 2.

Subject and item-pair means for 6- to 7- and 8- to 9-mo-olds. All data (A–H) were calculated over a window from 367 to 3,500 ms post target word onset. Subject mean difference scores are shown for paired-picture trials for 6- to 7-mo-olds (A) and for 8- to 9-mo-olds (B). Subject mean increases in target looking, corrected for baseline looking, are displayed for scene trials for 6- to 7-mo-olds (C) and for 8- to 9-mo-olds (D). Item-pair mean difference scores are shown for paired-picture trials for 6- to 7-mo-olds (E) and for 8- to 9-mo-olds (F). Item mean increases in target looking, corrected for baseline looking, are given for scene trials for 6- to 7-mo-olds (G) and for 8- to 9-mo-olds (H). (E–H) Error bars represent bootstrapped nonparametric 95% confidence intervals. On the right of each subplot is a histogram of the responses in the main plot; all histograms show more positive than negative responses for each subset of subjects and of item pairs.

Fig. 3.

Time course of infants’ picture fixation on paired-picture trials, averaged over infants in four age groups. The ordinate shows the mean proportion of infants who were looking at the named (target) picture at each moment in time. Error bars indicate SEMs, with means computed over subjects in each age range. At all four ages, target fixation rose from about 0.50 (chance) shortly after the onset of the spoken word. Overall, accuracy in fixating the named picture increased with age across the age groups. See SI Text for further details.