Linguistics 001 23: First Language Acquisition
Stages of language acquisition in children
In nearly all cases, children's language development follows a predictable sequence. However, there is a great deal of variation in the age at which children reach a given milestone. Furthermore, each child's development is usually characterized by gradual acquisition of particular abilities: thus "correct" use of English verbal inflection will emerge over a period of a year or more, starting from a stage where vebal inflections are always left out, and ending in a stage where they are nearly always used correctly.
There are also many different ways to characterize the developmental sequence. On the production side, one way to name the stages is as follows, focusing primarily on the unfolding of lexical and syntactic knowledge:
Vocalizations in the first year of life
At birth, the infant vocal tract is in some ways more like that of an ape than that of an adult human. Compare the diagram of the infant vocal tract shown on the left to diagrams of adult human and ape.
In particular, the tip of the velum reaches or overlaps with the tip of the epiglottis. As the infant grows, the tract gradually reshapes itself in the adult pattern.
During the first two months of life, infant vocalizations are mainly expressions of discomfort (crying and fussing), along with sounds produced as a by-product of reflexive or vegetative actions such as coughing, sucking, swallowing and burping. There are some nonreflexive, nondistress sounds produced with a lowered velum and a closed or nearly closed mouth, giving the impression of a syllabic nasal or a nasalized vowel.
During the period from about 2-4 months, infants begin making "comfort sounds", typically in response to pleasurable interaction with a caregiver. The earliest comfort sounds may be grunts or sighs, with later versions being more vowel-like "coos". The vocal tract is held in a fixed position. Initially comfort sounds are brief and produced in isolation, but later appear in series separated by glottal stops. Laughter appears around 4 months.
During the period from 4-7 months, infants typically engage in "vocal play", manipulating pitch (to produce "squeals" and "growls"), loudness (producing "yells"), and also manipulating tract closures to produce friction noises, nasal murmurs, "raspberries" and "snorts".
At about seven months, "canonical babbling" appears: infants start to make extended sounds that are chopped up rhythmically by oral articulations into syllable-like sequences, opening and closing their jaws, lips and tongue. The range of sounds produced are heard as stop-like and glide-like. Fricatives, affricates and liquids are more rarely heard, and clusters are even rarer. Vowels tend to be low and open, at least in the beginning.
Repeated sequences are often produced, such as [bababa] or [nanana], as well as "variegated" sequences in which the characteristics of the consonant-like articulations are varied. The variegated sequences are initially rare and become more common later on.
Both vocal play and babbling are produced more often in interactions with caregivers, but infants will also produce them when they are alone.
No other animal does anything like babbling. It has often been hypothesized that vocal play and babbling have the function of "practicing" speech-like gestures, helping the infant to gain control of the motor systems involved, and to learn the acoustical consequences of different gestures.
One word (holophrastic) stage
At about ten months, infants start to utter recognizable words. Some word-like vocalizations that do not correlate well with words in the local language may consistently be used by particular infants to express particular emotional states: one infant is reported to have used to express pleasure, and another is said to have used to express "distress or discomfort". For the most part, recognizable words are used in a context that seems to involve naming: "duck" while the child hits a toy duck off the edge of the bath; "sweep" while the child sweeps with a broom; "car" while the child looks out of the living room window at cars moving on the street below; "papa" when the child hears the doorbell.
Young children often use words in ways that are too narrow or too broad: "bottle" used only for plastic bottles; "teddy" used only for a particular bear; "dog" used for lambs, cats, and cows as well as dogs; "kick" used for pushing and for wing-flapping as well as for kicking. These underextensions and overextensions develop and change over time in an individual child's usage.
Perception vs. production
Clever experiments have shown that most infants can give evidence (for instance, by gaze direction) of understanding some words at the age of 4-9 months, often even before babbling begins. In fact, the development of phonological abilities begins even earlier. Newborns can distinguish speech from non-speech, and can also distinguish among speech sounds (e.g. [t] vs. [d] or [t] vs. [k]); within a couple of months of birth, infants can distinguish speech in their native language from speech in other languages.
Early linguistic interaction with mothers, fathers and other caregivers is almost certainly important in establishing and consolidating these early abilities, long before the child is giving any indication of language abilities.
Rate of vocabulary development
In the beginning, infants add active vocabulary somewhat gradually. Here are measures of active vocabulary development in two studies. The Nelson study was based on diaries kept by mothers of all of their children's utterances, while the Fenson study is based on asking mothers to check words on a list to indicate which they think their child produces.
There is often a spurt of vocabulary acquisition during the second year. Early words are acquired at a rate of 1-3 per week (as measured by production diaries); in many cases the rate may suddenly increase to 8-10 new words per week, after 40 or so words have been learned. However, some children show a more steady rate of acquisition during these early stages. The rate of vocabulary acquisition definitely does accelerate in the third year and beyond: a plausible estimate would be an average of 10 words a day during pre-school and elementary school years.
Sex differences in vocabulary acquisition
Against a background of enormous individual variation, girl babies tend to learn more words faster than boy babies do; but the difference disappears over time.
Svetlana Lutchmaya, Simon Baron-Cohen and Peter Raggat ("Foetal testosterone and vocabulary size in 18- and 24-month infants", Infant Behavior and Development 24:418-424, 2002) found that in a sample of 18-month-olds, boys' average vocabulary size was 41.8 words (range from 0 to 222, standard deviation 50.1), while girls' average was 86.8 (range from 2 to 318, standard deviation 83.2). By 24 months, the difference had narrowed to a boys' mean of 196.8 (range 0 to 414, standard deviation 126.8) vs. a girls' mean of 275.1 (range 15 to 415, SD=121.6). In other words, the girls' advantage in average values had shrunk from 86.8/41.8 = 2.1 to 275.1/196.8 = 1.5.
As time passes, the difference disappears entirely, and then emerges again in the opposite direction, with males showing larger average vocabularies during college years (though again against the background of within-group variation that's much larger than the across-group differences). Here's table 6 from Janet Shibley Hyde and Marcia C. Linn, "Gender Differences in Verbal Ability: A Meta-Analysis", Psychological Bulletin, 104:1 53-69 (1988).
Perception vs. production again
Benedict (1979) asked mothers to keep a diary indicating not only what words children produced, but what words they gave evidence of understanding. Her results indicate that at the time when children were producing 10 words, they were estimated to understand 60 words; and there was an average gap of five months between the time when a child understood 50 words and the time when (s)he produced 50 words.
All of these methods (maternal diaries and checklists) probably tend to underestimate the number of words about young children actually know something, although they also may overestimate the number of words to which they attribute adult-like meanings.
Combining words: the emergence of syntax
During the second year, word combinations begin to appear. Novel combinations (where we can be sure that the result is not being treated as a single word) appear sporadically as early as 14 months. At 18 months, 11% of parents say that their child is often combining words, and 46% say that (s)he is sometimes combining words. By 25 months, almost all children are sometimes combining words, but about 20% are still not doing so "often."
Early multi-unit utterances
In some cases, early multiple-unit utterances can be seen as concatenations of individual naming actions that might just as well have occured alone: "mommy" and "hat" might be combined as "mommy hat"; "shirt" and "wet" might be combined as "shirt wet". However, these combinations tend to occur in an order that is appropriate for the language being learned:
Some combinations with certain closed-class morphemes begin to occur as well: "my turn", "in there", etc. However, these are the closed-class words such as pronouns and prepositions that have semantic content in their own right that is not too different from that of open-class words. The more purely grammatical morphemes -- verbal inflections and verbal auxiliaries, nominal determiners, complementizers etc. -- are typically absent.
Since the earliest multi-unit utterances are almost always two morphemes long -- two being the first number after one! -- this period is sometimes called the "two-word stage". Quite soon, however, children begin sometimes producing utterances with more than two elements, and it is not clear that the period in which most utterances have either one or two lexical elements should really be treated as a separate stage.
In the early multi-word stage, children who are asked to repeat sentences may simply leave out the determiners, modals and verbal auxiliaries, verbal inflections, etc., and often pronouns as well. The same pattern can be seen in their own spontaneous utterances:
The pattern of leaving out most grammatical/functional morphemes is called "telegraphic", and so people also sometimes refer to the early multi-word stage as the "telegraphic stage".
Acquisition of grammatical elements and the corresponding structures
At about the age of two, children first begin to use grammatical elements. In English, this includes finite auxiliaries ("is", "was"), verbal tense and agreement affixes ("-ed" and '-s'), nominative pronouns ("I", "she"), complementizers ("that", "where"), and determiners ("the", "a"). The process is usually a somewhat gradual one, in which the more telegraphic patterns alternate with adult or adult-like forms, sometimes in adjacent utterances:
Over a year to a year and a half, sentences get longer, grammatical elements are less often omitted and less often inserted incorrectly, and multiple-clause sentences become commoner.
Perception vs. production again
Several studies have shown that children who regularly omit grammatical elements in their speech, nevertheless expect these elements in what they hear from adults, in the sense that their sentence comprehension suffers if the grammatical elements are missing or absent.
Often morphological inflections include a regular case ("walk/walked", "open/opened") and some irregular or exceptional cases ("go/went", "throw/threw", "hold/held"). In the beginning, such words will be used in their root form. As inflections first start being added, both regular and irregular patterns are found. At a certain point, it is common for children to over-generalize the regular case, producing forms like "bringed", "goed"; "foots", "mouses", etc. At this stage, the child's speech may actually become less correct by adult standards than it was earlier, because of over-regularization.
This over-regularization, like most other aspects of children's developing grammar, is typically resistant to correction:
CHILD: My teacher holded the baby rabbits and we patted them. ADULT: Did you say your teacher held the baby rabbits. CHILD: Yes. ADULT: What did you say she did? CHILD: She holded the baby rabbits and we patted them. ADULT: Did you say she held them tightly? CHILD: No, she holded them loosely.
A good starting point for more information about child language acquisition is the CHILDES web site at CMU, where you can find out about downloading the raw materials of child language research, and also search a specialized child language bibliography.
A recent article in the NYT Magazine (Paul Tough, "What it takes to make a student", 11/26/2006) discusses at length some well-known studies about social-class differences in language acquisition (Betty Hart and Todd Risley, "Meaningful Differences in the Everyday Experience of Young American Children" (1995); Betty Hart, "A Natural History of Early Language Experience", Topics in Early Childhood Special Education, 20(1), 2000; "The early Catastrophe: the 30 Million Word Gap", American Educator, 27(1) pp. 4-9, 2003). The abstract from the 2003 paper:
42 is not a very large sample, and there are many other questions to ask, but this work suggests that we should be concerned about possible lasting effects of cultural differences in children's linguistic environment.
Another, more recent, study suggesting the same conclusion is Martha J. Farah, et al., ("Childhood poverty: Specific associations with neurocognitive development", Brain Research 1110(1) 166-174, September 2006). Prof. Farah and her co-workers "administered a battery of tasks designed to tax specific neurocognitive systems to healthy low and middle SES [socio-economic status] children screened for medical history and matched for age, gender and ethnicity".
All the participants in this study were African-American girls between the ages of 10 and 13. As the graph above indicates, the difference in performance on the "Language" part of the test battery between middle SES and low SES girls represented an effect size of about 0.95.
There were two language-related tasks:
This finding is consistent with a lasting effect of differences like those in the Hart & Risly study, ,though other explanations are also possible.