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INDIVIDUAL DIFFERENCES AND THEIR IMPLICATIONS

FOR THEORIES OF LANGUAGE DEVELOPMENT

Elizabeth BatesUniversity of California, San Diego

Philip S. Dale

University of Washington

Donna Thal

San Diego State University

Chapter 4 for Paul Fletcher & Brian MacWhinney (Eds.),Handbook of Child Language.

Oxford: Basil Blackwell, 1995.

Introduction

Like every other aspect of human development,language development is characterized by variation.Historically this variation has been largely ignored bystudents of child language, who have concentrated onthe remarkable similarities in sequence of developmentthat are usually observed across children acquiring agiven. language Individual differences in rate ofdevelopment and individual differences in learning

style have been left to applied practitioners such asspeech pathologists and special educators. We believeit is no accident that these professionals, concernedwith such important questions as the definition ofabnormality, the relationship of language to nonverbalcognition, and the role of environmental variables, havefound it essential to focus on variation.

It is our contention that quantitative and qualitativevariations within and across components of earlylanguage are also relevant, indeed essential, if we wantto understand the mechanisms that underlie normallanguage development. Far from simply reflectingnoise in our measuring instruments or variability inlow-level aspects of physiological maturation, the

variations that we will document here are substantial,stable, and have their own developmental course.

Because this variation is substantial, it is critical fordefining the boundary between normal and abnormaldevelopment; because it is stable, it provides a windowonto the correlates and (by inference) the causes ofdevelopmental change; and because it has its owndevelopmental course, it can be used to pinpoint criticaldevelopmental transitions that form the basis fortheories of learning and change.

Although we are well aware of the clinicalapplications that hinge on an adequate assessment ofnormal variation (i.e. one cannot define abnormalwithout an adequate definition of normal), ourprimary goal here will be an exploration of theimplications of individual differences for theories ofnormal language. We will concentrate on the earlystages of language learning, from the onset of wordcomprehension (around eight to ten months of age) tothe onset of grammar (from 2036 months). This is theperiod in which the most dramatic changes in languageability are observed, phenomena which have beenamply documented in small- and large-sample studies.It is also a period characterized by dramatic events inpostnatal brain development (e.g. synaptogenesis),which means that biological factors may play aparticularly important role in those aspects of languagethat change at the same time (Bates, Thal, andJanowsky, 1992). For these reasons, we stand a goodchance of discovering something interesting about theinterplay of biology and environment. The chapter isdivided into four parts, as follows:

1. Variations in rate within components of early

language. In this section we will review evidence for

variations in speed of development in wordcomprehension, word production, first wordcombinations, and the first stages of grammar. As weshall see, there are enormous individual differences inonset time and rate of growth in each of thesecomponents, variations large enough to challenge andconstrain the notion of a universal bioprogram(Bickerton, 1984) or a universal maturational timetable


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for early language development (Lenneberg, 1967).Such maturational accounts are insufficient, because thelinguistic variations that we observe in perfectly healthychildren are so much larger than the variations that areusually observed in other maturational milestones likecrawling or walking. At the same time, environmentalvariables (at least those that have been examined to

date) appear to account for only a modest proportion ofthe variance observed in early comprehension andproduction of language. At the risk of invitingaccusations of radical centrism, we conclude that thevariations observed in early language development areso large that they require substantial contributions fromboth genetic and environmental factors, with special

emphasis on their interaction.2. Dissociations between components of early

language. Having demonstrated large-scale variation in

rate of development within individual components, we

can go on to ask about the degree of association ordissociation in rate of development that is observed

between those components. In this section, we willlook for evidence of developmental asynchronybetween comprehension and production, and betweenlexical production and grammar. The purpose of thisinvestigation is to locate the seams and joints of thelanguage processor, i.e. components that can develop atdifferent rates because they depend on differentcognitive and/or neural mechanisms. Hence this sec-tion has implications for the hotly contested issues ofmodularity and the autonomy or interdependence oflinguistic and cognitive systems (Fodor, 1983).

3. Variations in learning style. Continuing our

search for the seams and joints of the languageprocessor, we will move on in this section to a brief

review of evidence for qualitative variations in learningstyle (aka. referential v. expressive style, nominal v.pronominal style, analytic v. holistic style). We endby concluding that stylistic variation is the emergentproperty of quantitative variations in the information-processing mechanisms that all children must have forsuccessful language learning.

4. Atypical populations: variation at the extremes

of the normal range. Finally, we will review evidence

on the same three themes (rate, dissociations and style)in the early stages of development for several quitedifferent atypical populations: early talkers (anonclinical but very unusual group), late talkers (manyof whom go on to qualify for a diagnosis of specific

language impairment), children with focal brain injury(to provide insights into the neural mechanisms thatunderlie individual differences in early language), andchildren with contrasting forms of mental retardation(i.e. Williams Syndrome, where language eventuallymoves ahead of many other cognitive domains; DownSyndrome, where language levels often fall behindmental as well as chronological age). This will be avery brief review of a large topic, but it will help toround out our understanding of the mechanisms

involved in early language development, across theperiod from first words to grammar (Bates, Brethertonand Snyder, 1988). We will conclude that most of thevariations observed in atypical populations representextensions of the variations that are also observed in thenormal range.

1. Variations in Rate

We will start with variations in rate of developmentwithin individual components, a form of variation thatis (at least in principle) easy to define and quantify. Infact, this apparently simple form of measurement posesa substantial methodological problem. Estimations ofvariability, even more than estimations of centraltendency, require a substantial sample size. Forobvious reasons, this is generally not possible forstudies of child language, which are exceptionally timeand labor intensive. The great majority of researchstudies have included fewer than 25 subjects; and many

of the most influential have been far smaller, i.e. single-case studies (e.g. Leopold, 1949) or studies of three orfour children (e.g. Bloom, 1970; Brown, 1973). Evenstudies nominally focussed on individual differenceshave continued the tradition of small samples or single-case studies (e.g. Peters, 1983). As illuminating asthese studies have been in defining those patterns ofindividual variation that are possible, the extent and

nature of such variation will remain controversial untillarge samples are available. For this reason, we willconcentrate here on a single study with a uniquely largesample of more than 1,800 children: the norming studyfor the MacArthur Communicative DevelopmentInventories (Fenson, Dale, Reznick, Thal, Bates,

Hartung, Pethick, and Reilly, 1993; Bates, Marchman,Thal, Fenson, Dale, Reznick, Reilly, and Hartung,1994; Marchman and Bates, 1994; Fenson, Bates, Dale,Thal and Reznick, 1994). These results are based ontwo parental report instruments (CDI:Infants, forchildren 816 months, and CDI:Toddlers, for children1630 months) that have been developed over a periodof more than 15 years. A variety of studies havedemonstrated the reliability and validity of thisinstrument and its immediate precedessors (Dale, Bates,Reznick, and Morisset, 1989; Dale, 1991; Camaioni,Caselli, Longobardi, and Volterra, 1991). For example,the vocabulary checklists correlate positively andsignificantly with laboratory assessments (both standard

tests and free speech) with coefficients ranging from+0.40 to +0.80; the grammatical complexity scalecorrelates with laboratory measures of Mean Length ofUtterance at +0.88 at 20 months and +0.76 at 24months. In hindsight, this high validity is hardlysurprising. Parents have a far larger dataset thanresearchers or clinicians can ever hope to assemble; it isalso far more representative of the child's ability, as it is

based on the child's behavior in a wide range of


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situations which call for an equally wide range oflanguage skills.

There are, of course, limitations to the kind ofinformation that can be obtained with parent report. AsBates et al. (1994) acknowledge,

We can say nothing here about phonological

development (e.g. segmental v. suprasegmentalapproaches to the analysis of speech), nor about thefrequency with which children use particularvocabulary types (i.e. type/token relations). Wecannot distinguish between imitations andspontaneous speech, nor can we specify the rangeof contexts in which individual lexical items areused (e.g. flexible and productive use v.memorized frames). However, we can provide anexceptionally clear view of developmental changesfrom 8 to 30 months of age, and we can establishthe boundaries of variation.......within and acrosslevels of development.

As long we keep these variations firmly in mind, andmake no attempts to generalize beyond the factors thatcan be studied reliably and accurately with parentalreport, a data base of this kind can be extraordinarilyuseful.

Methodology

Parents of 1,803 children between eight and 30 monthsparticipated in a norming study for these inventories,conducted in San Diego, Seattle, and New Haven(Fenson et al., 1993). Parents of 673 children between816 months completed the CDI:Infants; parents ofanother 1,130 children, between 1630 months,

completed the CDI:Toddlers. A minimum of 30 malesand 30 females are represented at each age level.Children with serious health problems or extensiveexposure to a language other than English wereexcluded from the study, and are not included in thenumbers just listed. The sample includes a widesocioeconomic range, although it is heavily weightedtoward families in the middle class (e.g. parents with atleast a high school education). For the presentdiscussion, we focus on two core subscales from theCDI:Infants (word comprehension, word production)and three subscales from the CDI:Toddlers (wordproduction, onset of word combinations, andgrammatical complexity).

Parents of 500 children in the original sample alsocompleted a second inventory approximately six weekslater. Parents of another 503 children completed asecond inventory approximately 6.5 months later. Ofthis latter group, 62 parents of children in the Infantsample completed the CDI:Infants a second time;parents of 217 children in the Infant sample completedthe CDI:Toddlers; and parents of 224 children in theToddler sample completed the CDI:Toddlers a second

time. This information was used to assess the cross-agestability of parental report.

The CDI:Infants includes a 396-item vocabularychecklist organized into 19 semantic categories. Ten ofthese categories comprise nouns (animal names,vehicles, toys, food and drink, clothing, body parts,furniture and rooms, small household items, outside

things and places to go, and people). Additionalcategories are included for sound effects and animalsounds, games and routines, verbs, adjectives,pronouns, question words, prepositions and locations,quantifiers, and words about time. Parents are asked toindicate which words the child u n d e r s t a n d s

(comprehension) and which words the child says and

understands (production). Note that we have excluded

a third theoretically plausible category, i.e. words thatthe child says but does not understand. This decisionreflects our discovery and acknowledgment of animportant limitation of parental report. In earlierversions of the CDI, we asked parents to distinguish

between words that the child imitates withoutcomprehension, and words that are producedspontaneously and productively. Our results made itclear that parents find it difficult to make a distinctionof this kind; indeed, most parents operate under theassumption that production reflects understanding. Wehave built that parental assumption into the finalversion of the CDI, but we realize that there is no wayto win on this matter. Degree of productivity is a subtledimension that must be studied with a differentmethodology, including in-depth observations oflanguage use and context and detailed interviews withparents that elicit information about the contexts inwhich words are used (see Snyder, Bates and

Bretherton, 1981 and Bates et al., 1988, for resultsusing the interview technique).The CDI:Toddlers includes a 680-word vocabulary

checklist, organized into 22 semantic categories. Thelarger number of categories on the toddler form is aresult of two sections (helping verbs, and connectingwords) and the division of outside things and placesinto separate sections. As vocabulary becomes larger, itis no longer possible for parents to monitorcomprehension vocabulary; they are asked only toindicate us e (production). The second part of the

toddler form is designed to assess morphological andsyntactic development. Only two measures from thispart will be discussed here. Parents are asked if their

child is combining words; they can respond not yet,sometimes, and often. If they respondsometimes or often, they proceed to a set of 37forced-choice recognition items in which they choosethe member of each pair that best reflects their child'scurrent level of language use (In each of the followingpairs, please mark the one that sounds MOST like theway your child talks right now). The 37 items includecontrasts in the use of bound morphemes (e.g. "Daddycar" v. Daddy's car), functors (e.g. Kitty sleeping v.


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Kitty is sleeping), and early-emerging complexsentence forms (e.g. Baby crying v. Baby crying cuzshe's sad).

Stability of individual differences

Before turning to evidence for the substantial variationin rate of development observed in early childlanguage, we should review evidence for thestability/reliability of this variation from the six-monthlongitudinal data collected in the norming study. Toevaluate continuity, we applied a very stringentmultiple regression analysis, controlling age (a roughindex of maturational status), gender (reflecting acombination of biological and cultural factors), and sixfamily and social class variables (birth order, SES,mother's education, father's education, mother'soccupation, and father's occupation), before the Time 1measure was entered as a predictor of the correspondingTime 2 measure. Five such analyses were performed:

InfantInfant Comprehension; Infant-Infant Production;Infant-Toddler Production; ToddlerToddlerProduction; and ToddlerToddler SentenceComplexity. In every case the earlier measure was ahighly significant (p < .001) predictor of the latermeasure, accounting for an additional 16.6 percent to31.1 percent of the variance on the final step. Thus, theindividual differences discussed below are unusuallyrobust, compared with other psychometric studies in thesame age range (McCall, Eichorn, and Hogarty, 1977).It also follows from these analyses that a substantialportion of the variation described below cannot beexplained by such traditional factors as age, gender, andsocial class.

Vocabulary comprehension

Figure 4.1 shows the mean developmental function forword comprehension between 8 and 16 months,together with functions that describe children who are1.28 standard deviations above or below the mean.This contrast (which we will use in most of the graphsthat follow) illustrates the developmental zone occupiedby approximately 80 percent of the sample (so that theprobability of falling outside this region is p < .10 ateither tail of the distribution).

For most children, robust evidence of word

comprehension first appears between eight and tenmonths of age. At the eight-month entry point, themean number of words that parents report incomprehension is 36, although the median (a moreconservative measure) is only 17. At ten months of agethe mean has surged to 67 words, with a median of 41.By the 16-month exit point for the CDI: Infants scale,children have a mean receptive vocabulary of 191, witha median of 169. These median scores are consistentwith other estimates of the onset of word

comprehension and its early growth (Benedict, 1979;Rescorla, 1981; Bates et al., 1988). However, the meansare relatively high, reflecting much more variability inthe onset and rate of receptive vocabulary growth thanmight have been expected from previous research. Forexample, the 1.28 standard deviation range at tenmonths of age goes from a low of zero words to a high

of 144. By 16 months the corresponding range is from78 to 303 words. An indication of the magnitude ofindividual differences is the fact that the overallcorrelation between word comprehension and age ispositive and significant (r = 0.60, p < 0.001), butaccounts for only 36 percent of the variance. Theremainder of the variance must be a combination (exactrecipe unknown) of true individual variation and thenoise and error of parental report.

Some evidence for the reality of this variationcomes from recent electrophysiological studies ofcomprehension. Mills, Coffey, and Neville (1993)tested a group of ten-month-old children, half of themearly comprehenders reported to understand at least

five to ten words from a short laboratory checklist, andhalf early noncomprehenders reported as notunderstanding these common words. Event-relatedbrain potentials (ERPs) were recorded while thechildren listed to familiar and unfamiliar words.Significant differences between familiar and unfamiliarwords were observed for children in the "earlycomprehenders" group, but not for children in thenoncomprehenders group. Thus the parental report ofcomprehension was correlated with anelectrophysiological measure of recognition. Of coursethis does not mean that high-performing ten-month-oldsunderstand the meaning of familiar words. The ERP inthis experiment is an index of recognition, nothingmore. However, it does suggest that parents who reporthigh comprehension are aware of their child's selectivesensitivity to speech.

The very high levels of word comprehensionreported for children at the upper end of the distributionat eight to ten months may also reflect a much lessinteresting factor. In particular, some parents mayadopt a different and more liberal definition ofunderstands than we had in mind, inferringcomprehension from nothing more than evidence forhigh attention and positive affect. There is someevidence that this overestimation may be morecharacteristic of parents with low education (see Fensonet al., 1994). Nevertheless, taken together, the Mills etal. (1993) electrophysiological data and the cross-agestability cited earlier demonstrate that a high proportionof the variance in early word comprehension isauthentic.

Vocabulary production

Figures 4.2 and 4.3 show the mean developmental func-tion and range of variation (1.28 standard deviations in


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either direction) for word production between eight and16 months (on the CDI: Infants), and between 1630months (on the CDI: Toddlers). The mean and medianfunctions are consistent with previous small-samplestudies, showing that expressive language does not getoff the ground for most children before 12 months ofage. For most children, there is slow growth in

vocabulary production from a mean of 1.8 words ateight months (with a median of 0), to a mean of tenwords at 12 months (with a median of 6), to a mean of64 words at 16 months (and a median of 40)

1. By 30

months, the mean score has increased to 534 words,with a median of 573 (notice that the median is nowhigher than the mean, indicating that the distribution isnow skewed by "late talkers" at the low end of thedistribution see section 5). The overall correlationbetween age and vocabulary is 0.47 (p < 0.001) for theInfant data, accounting for just 22 percent of thevariance. For the Toddler data, the age correlation is0.68 (p < 0.001), accounting for 46 percent of thevariance. Thus there is a substantial amount of age-independent variation.

Around these central tendencies, variation invocabulary production has a complex and interestingtime course. There is relatively little individualvariance at or before 12 months. The 1.28 standarddeviation window at one year of age extends from a lowof zero words to a high of 24. However, after 13months there is a dramatic increase in variability, dueprimarily to rapid growth in children at the high end ofthe distribution. At 16 months, for example, children inthe top tenth percentile have reported productivevocabularies of at least 154 words, while children in thelowest 10th percentile are still producing no words atall. This highly skewed distribution continues tocharacterize variation in expressive vocabulariesthroughout the 1630-month range, until ceiling effectsare operative. For example, at the two-year point (24months), the mean for reported expressive vocabularyon this measure is 312 words, but the 1.28 standarddeviation range goes from a low of 89 to a high of 534(see also Huttenlocher, Haight, Bryk, Seltzer, andLyons, 1991).

It seems likely that the nonlinearities in figures 4.2and 4.3 have something to do with a controversialphenomenon called the "vocabulary burst". Some timeduring the second year of life, many childrenexperience a marked acceleration in rate of word

learning (Nelson, 1973; Dromi, 1987; Gopnik andMeltzoff, 1987; Goldfield, and Reznick, 1990). Formost of the children that have been studied to date, thegrowth curve starts to rise somewhere between 50100words. The single best example in the literature comes

1Although the mean is higher at 16 months for theCDI:Toddlers list than for the same age on the CDI:Infantslist, the medians are quite similar: 44 and 40 words,respectively. The skewed nature of the distribution isresponsible for the differences in means.

from Dromi's detailed and exhaustive longitudinalstudy of vocabulary development in a single child (seealso van Geert, 1991). A number of contrastingproposals have been offered to account for thisnonlinear shift. For example, it has been argued thatchildren achieve a sudden insight into the idea thatthings have names (Dore, 1974), or alternatively, that

all objects ought to have a name (Baldwin andMarkman, 1989). Others have attributed this shift to amore general change in cognition, includingdevelopmental changes in the ability to categorizeobjects (Gopnik and Meltzoff, 1987) and/ordevelopmental shifts in representational capacity(Shore, 1986; Brownell, 1988). Yet another class ofexplanations revolve around reorganizations in thephonetic segmentation (Plunkett, 1993) and/orarticulatory ability (Menn, 1976). All of these accountsare interesting, but they suffer from two relatedproblems: lack of universality, and absence of aninflection point. Let us address each of these in turn,and then see what we can glean from the large cross-

sectional data base currently at our disposal.The first problem with the various explanations of

the vocabulary burst cited above revolves around thefact that not all children display a growth spurt of thiskind. Reznick and Goldfield, among others, haveargued from longitudinal data that the burst is notuniversal. In some children, vocabulary grows socontinuously that it is difficult to identify a single pointor narrow region of acceleration (see below). In othercases, vocabulary development is characterized by aseries of small bursts, a stairstep pattern that is difficultto reconcile with most single-factor theories of "theburst".

A second problem comes from the discontinuityimplied by the word "burst", and by most of thetheories that seek to explain accelerations in wordlearning. Van Geert (1991) and Bates and Carnevale(1993) have noted that individual growth curvesdescribed in longitudinal studies of vocabularydevelopment during the second year (see especiallyDromi, 1987) are best fit by a smoothly acceleratingexponential function, or by related nonlinear functionssuch as the quadratic or the logistic. By appropriatevariation in their parameters, such nonlinear modelscould lead to growth curves marked by apparentlyintense bursts, weaker bursts, or no significantacceleration at all. The key insight here is that there isno inflection point in the exponential portion of the so-called vocabulary burst, i.e. no single "take-off point"of the kind assumed by most of the theories citedabove. And yet, there does appear to be a region ofacceleration during the second year that cries out forexplanation.

Because the CDI data base is essentially cross-sectional (one cannot construct a growth curve fromtwo points), it cannot provide direct information aboutthe existence, incidence or prevalence of the legendary


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burst. If there are a large number of individual stepwisefunctions, they are masked by summation acrossindividual children. However, these data can providetwo indirect sources of information about the timingand nature of nonlinear changes in rate of development.

First, let us assume that accelerations in rate ofdevelopment become most evident between 50100

words. If this is the case, then some children in oursample are on their way to a vocabulary burst between1416 months of age (see figure 4.3) well before the1719 month transition predicted by many theories. Bythe same argument, the data in figure 4.3 suggest thatsome children in the third year of life are still wellbelow the 50100-word boundary where the putativeburst typically appears. Thus, if a vocabulary burstdoes occur, its timing is characterized by wideindividual variation. For some children, it may occur asearly as 14 months, for some as late as 2426 months.

A second set of facts derives from analyses ofstability over time in the MacArthur Norming Study.Reznick and Goldfield (1992) have reported

longitudinal data from the 500 parents who completed asecond inventory approximately six weeks after theinitial questionnaire. Correlations between initial andlater scores were computed separately for each agegroup, based on the child's age at first administration.Correlations for the CDI:Todders exceeded r = 0.90 atevery age. Correlations for the CDI:Infants were in the0.80.9 range for both comprehension and production,with a single exception: at 12 months, the test-retestcorrelation for infant vocabulary production dropped tor = 0.60. Analyses of our six-month longitudinal datawere consistent with these results, suggesting adiscontinuity in individual differences occurring atapproximately one year of age. The lower reliabilitycoefficient at 12 months may reflect a generalreorganization of infant cognition at the 12-monthboundary (see McCall, Eichorn and Hogarty, 1977 for asimilar 12-month discontinuity in longitudinal studiesusing the Bayley Scales of Infant Development).Alternatively, it may reflect a discontinuity in parentalperceptions of infant language. That is, the emergenceof meaningful speech at 12 months may cause someparents to reevaluate the criteria that they used beforethis point to define a "real word" (e.g. "I thought he said'mama' before, but what he is doing now is reallydifferent"). If this observation is correct, then the mostimportant discontinuity in early word production is theone that occurs between 1213 months of age andnot the one that is supposed to occur later in the secondyear. The putative vocabulary burst from 1620months may be the inevitable product of a growthfunction that is set in motion at the end of the first year,i.e. from 1012 months.

These statistical observations are consistent withsome informal observations of Dale and Thal (personalcommunication), both of whom have studied earlytalkers. It is difficult to locate children below the age

of 12 months with a demonstrable productivevocabulary of, say, 10 or more words. It is much easierto identify children at 1416 months with very largevocabularies of 100 words or more. We suspect thatthere may be a "bottleneck" or "gate" into referentialvocabulary which cannot be substantially accelerated.Once through that milestone, however, exceptional

ability may lead to a flowering of vocabulary. It is anintriguing, but speculative, hypothesis that thisphenomenon may be biologically specified.

Combining words

For English, with its relatively modest inflectionalmorphology and general absence of case markings, theinitial step in grammatical development for mostchildren is combining words (this generalization is notentirely accurate even for English, and is of coursehighly inaccurate for languages such as Japanese andHungarian.) In the MacArthur study, parents were

given three options for describing their child'scombinatorial language: not yet, sometimes, andoften. Parents appeared to interpret these terms quiteconsistently, even though they were given withoutprecise quantitative specification. Figure 4.4 shows theregular progression in parents' response to this question.There is a gap of approximately three to four monthsbetween the two criteria. It might have been expectedthat sometimes would be more easily quantified byparents (greater than zero) than often, hence morereliably judged. Contrary to this expectation, theoften criterion was more highly correlated with bothage (0.58) and total vocabulary size (0.73) than wassometimes (0.47 and 0.57 for age and vocabulary,

respectively). Fenson et al. have suggested that parentschoose often in response to their children's first use ofgenuinely productive combinatorial language, in whichindividual words from a particular syntactic or semanticcategory may be combined flexibly with a variety ofwords from one or more other categories to express aconsistent semantic relationship. In contrast, the choiceof sometimes may reflect the appearance ofnonproductive or rote combinations that do not reflect ageneralized and semantically consistent word-combining skill. Such rote combinations are likely tohave a stronger component of individual stylisticvariation than does productive combinatorial language(see below), and are therefore less strongly correlated

with other measures of language development.Whichever criterion is used, it is apparent that there

is wide variation in the onset of combinatoriallanguage. At 18 months, approximately 11 percent ofparents report that their child is often combining words,and another 46 percent report that their child issometimes combining words. Although this questionwas not included in the CDI:Infants, an extrapolation offigure 4.4 suggests that a subset of children arecombining words prior to 16 months. We assume that


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these early combinations are formulaic in nature,although there are sporadic reports in the literature ofrare but novel word combinations in children as youngas 14 months (e.g. the expression "Wadoo baba Water bottle" uttered by a 14-month-old child afterthrowing a playmate's bottle in a wading pool Bateset al., 1988). By 25 months, nearly all parents report

some combinations, but 19 percent are still notreporting combinations 'often.' Thus the much-usedclinical criterion of failure to combine words by agetwo years (e.g. Rescorla, 1989) corresponds roughly tothe lowest 10 percent of the CDI distribution.

Sentence complexity

Finally, we turn to grammatical development, as in-dexed by parental response to the 37 forced-choicerecognition items. Figure 4.5 illustrates the devel-opmental function and standard deviations for thisscale. The 37 items include bound morphemes, functor

words, and early-emerging complex sentence forms.The mean developmental function of figure 4.5 isconcordant with observational studies of early gram-matical development, but there is (once again) sub-stantial variation from the earliest period. Childrenfunctioning 1.28 standard deviations above the mean at16 months are at a level which will not be achieved bychildren one standard deviation below the mean until 28months. In other words, there is a full calendar yearseparating children at the high v. low end of the dis-tribution for sentence complexity.

To clarify the range of variation, and make it moremeaningful, we can convert this scale to a more familiarone, namely, Mean Length of Utterance (MLU). MLU

is widely used as a measure of early grammaticaldevelopment, despite a number of limitations (Crystal,1974; Klee and Fitzgerald, 1985). For purposes ofcharacterizing variation, the two most significantlimitations of MLU are the absence of good normativedata, and the potential for achieving similar MLUvalues with quite different syntactic abilities (andconversely, achieving different MLU values withsimilar syntactic abilities). Here we attempt tocharacterize variation in MLU with a much largersample than has been previously available, takingadvantage of the high correlation between sentencecomplexity and MLU. A correlation of 0.84 (p

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