Introduction

Children's understanding and production of their first words is undeniably one of the most heralded achievements in early development. The onset of language officially marks the transition from "infancy" (which derives from the Latin root infans, meaning "unable to speak") to early childhood, and radically alters the child's social world. Words enable children to share meanings with others and to participate in cultural learning in unprecedented ways. The fact that all normally developing children acquire language suggests that there exist universal properties that support children's language development, including the opportunity to communicate with others and direct and indirect access to analyzable language models (e.g., Meisel, 1995). Nonetheless, children vary enormously in the course of their language growth in terms of how quickly they achieve specific milestones, their receptive and productive vocabulary sizes, and in their eventual levels of competence in syntactic, semantic, and pragmatic aspects of language (Bloom, 1993; Hoff-Ginsberg, 1997; Tamis-LeMonda & Bornstein, 2002). For example, some children speak as early as 8 months while others express their first words several months later (e.g., Acredolo, Goodwyn, Horobin, & Emmons, 1999; Dapretto & Bjork, 2000). Bloom's (1993) research on the language achievements of12 children illustrates the substantial variation in the age of onset of children's first words. Although children expressed their first conventional words (i.e., adult word forms) at the start of the second year on average, some children began to speak as early as 10 months while others did not achieve the first word milestone until 18 months.

Early differences in children's language are not transitory. Language achievements in the first years of life portend children's later abilities in literacy, school readiness, and cognitive development (e.g., Chall, Jacobs, & Baldwin, 1990; Bornstein & Haynes, 1998). In particular, children's oral language skills, which include spoken vocabulary and discourse, have been positively associated with reading and phonemic awareness (e.g., Beals, DeTemple, & Dickinson, 1994; Scarborough, 1989; Whitehurst & Lonigan, 2003), as well as with more developed abilities to construct meanings from conversations, reading, and writing (Beals et al., 1994). Research indicates strong associations between early vocabulary development and later reading comprehension (e.g., Anderson & Freebody, 1983). Moreover, parents' assessments of children's language skills between 3 and 4 years of age relate to teachers' assessments of vocabulary skills in second and fourth grades (e.g., Dickinson & DeTemple, 1998). Children with poor language skills are at a higher risk of literacy failure on school entry than are children with more developed oral language abilities (e.g., Snow, Burns, & Griffin, 1998). These effects are far reaching, such that children who exhibit delays at the onset of schooling are more likely to experience grade retention, special education placement, and failure to complete high school (Adams, 1990; McGill-Franzen, 1987).

The magnitude of associations from early child language to later school performance has incited decades of inquiry into the sources of early individual differences. Research on the social context of language development has underscored the role of more sophisticated language partners, most notably parents, in guiding novice toddlers toward understanding the meanings expressed in shared conversations (Bruner, 1974, 1983; Vygotsky, 1962). A cornerstone of this research has been the careful description of children's language environments in terms of the words and phrases that children hear and the ways that those words are presented to children. This literature has produced irrefutable evidence for the centrality of children's exposure to language for their receptive and productive language growth, literacy, and cognitive development (e.g., Bloom, 1993; Bornstein, 1985; Carpenter, Nagell, & Tomasello, 1998; Hart & Risley, 1995; Huttenlocher, Haight, Bryk, Seltzer, & Lyons, 1991; Landry, Smith, Miller-Loncar, & Swank, 1997; Nelson, 1973, 1988; Snow, 1986; Woodward & Mark-man, 1998).

In addition to studying the contributions that children themselves make to their language progress and learning (e.g., Bornstein et al., 2004), we have investigated the influence of parents' verbal responsiveness on children's early language development. Parents' verbal replies that are prompt, contingent, and appropriate to children's activities (i.e., responsiveness) predict children's language achievements within and across time. Promptness refers to the timing of parents' responses relative to children's behavior. Prompt responses facilitate children's processing of incoming verbal information due to the temporal contiguity between auditory and other perceptual experiences. Contingency refers to the dependence of parents' reactions on child behaviors. Contingent responses are those that evolve out of moments of shared attention and bear direct relevance to what the child is doing (e.g., "Look at the doll!" as the child looks to a doll). Contingency can be contrasted with asynchronous replies (e.g., "Look at the doll!" as a child reaches for a ball), which even if prompt, may actually interfere with the process of mapping words to referents, particularly during early stages of language (Tamis-LeMonda & Bornstein, 2002; Tomasello & Farrar, 1986). Appropriateness refers to maternal replies that are positively connected to the child's behavior. A mother who states, "You're feeding your bear!" as a child directs a spoon to the mouth of a toy bear is providing constructive information, whereas a mother who admonishes "Put that down!" is not.

In this chapter, we describe variation in children's language achievements across the first 3 years, and document findings on the significance of parents' responsiveness for children's early language. The chapter begins with a brief overview of the methods and measures that we have used to study children's language and parents' responsiveness, followed by a presentation of research findings across successive periods of language growth. Research is ordered chronologically, moving from infants' understanding of words (~9-10 months), to production of first words (~13 months), to the "vocabulary spurt" (~18 months), to first "sentences" and decontextualized uses of language (end of the second year). We end by describing our research on the language and cognitive development of 2- and 3-year-old children from low-income families, as children living in poverty are typically found to lag behind their more advantaged peers in their language abilities already in the toddler years. This recent work emphasizes the roles of both fathers and mothers in children's language development, and extends conceptualizations of responsive parenting to include the broader learning environments of young children.

METHODS FOR STUDYING CHILDREN'S LANGUAGE AND PARENTING

Over the past 15 years, we have conducted a series of longitudinal investigations on children's early language and communicative development in the context of parenting in both middle- and low-income populations. Middle-income families were recruited from private pediatric groups in New York City, and families from low-income backgrounds were recruited from community agencies as part of the Early Head Start National Evaluation Study (Love, Kisker, Ross, Schochet, Brooks-Gunn, Paulsell, et al., 2002). Samples ranged from 40 to over 1,000 children across studies. Families were visited in their homes every few months where children were assessed for their language gains and parents were coded on their verbal responsiveness. In the first year, visits occurred when infants were 5-6 months and/or 9 months, depending on the study. In the second year, families were revisited when children were 13-14, 17-18, and 22-24 months of age, and certain cohorts were seen again when children were 3 years of age.

Mother-child and sometimes father-child dyads were observed during free play at each assessment. Children and parents were provided with standard sets of toys and were videotaped for 10 to 20 minutes. Parents were instructed to remain with their children and to interact with them in whatever way was most natural. They were told they could use any or all of the toys provided, but not to introduce other toys into the play session.

Assessments of Children's Language

Measures of children's language and other forms of communication were based on maternal report and/or coding of children's language from verbatim transcriptions of the videotaped free-play sessions. Transcripts were coded for children's word types and tokens, semantic usage, and syntactic complexity (e.g., MLU) using the Systematic Analysis of Language Transcripts (SALT; Miller & Chapman, 1993) as well as special-purpose coding systems.

Maternal reports of children's language were based on the MacArthur Communicative Development Inventories (MCDI; Fenson et al., 1994), which were modified in various studies to access more detailed information about the situations in which children used specific words and phrases. In one longitudinal study of children's language growth from 9 to 21 months, which is later discussed in detail, data on children's language were obtained through biweekly interviews with mothers (Tamis-LeMonda, Bornstein, Kahana-Kalman, Baumwell, & Cyphers, 1998; Tamis-LeMonda, Bornstein, & Baumwell, 2001). To facilitate these interviews, we provided mothers with a packet of language inventories at the end of the 9-month home visit and scheduled weekly times to discuss children's language progress. Packets included versions of the Early Language Inventory (ELI; Bates, Bretherton, & Snyder, 1988) and MacArthur CDI (Fenson et al., 1994) as well as checklists exemplifying various semantic and syntactic uses of words and phrases (Tamis-LeMonda & Bornstein, 1994). At younger ages (i.e., from the start of the study to around 13 months), a subset of items from the MCDI was used (specifically, the ELI on which the MCDI was based), as children at these ages expressed few words and their receptive language was still limited. Early interviews lasted approximately 15 to 20 minutes, whereas interviews at later ages, which utilized the full MCDI, took up to 2 hours.

During telephone interviews, we asked the mother whether her child understood and/or expressed each word/phrase on the list, and in the case of an affirmative response, further probed as to whether the child's understanding and/or production of the word was context "restricted" (e.g., understanding "dog" only as referring to the child's dog; saying "dog" only in imitation or to the child's dog) or "flexible" (e.g., understanding and/or saying "dog" in reference to all dogs).

After probing for specific words/phrases, we asked about various other language milestones, including children's combination of words into simple sentences (e.g., see dog), and whether children used specific grammatical and semantic speech acts. From these interviews, children's receptive and productive vocabulary sizes were computed at each age, as was the timing of various language milestones (e.g., onset of understanding; first words in production; the timing of 50 flexible words in productive language; combinatorial speech).

In certain of these studies, children were also assessed using standardized tests of performance, including the Peabody Picture Vocabulary Test, Third Edition (PPVT-III; Dunn & Dunn, 1997) as well as the Mental Development Index (MDI) of the Bayley Scales of Infant Development—Second Edition (BSID-II; Bayley, 1993). The PPVT, a widely used measure of receptive vocabulary, assesses children's listening comprehension at various ages (for our purposes, focus was on children of 36 months). Children are presented with four pictures per testing plate and asked to point to the one that corresponds to the assessor's spoken word. Items are administered in sets of 12 and testing continues until children reach a ceiling, defined as 8 or more errors within a set of 12. Children in many of our studies were also administered the MDI at 14, 24, and 36 months of age as a measure of their mental status. The MDI assesses abilities in cognitive, language, and personal-social areas of development through items that code for children's vocalizations and language, memory, classification abilities, etc. Both the PPVT and MDI, although standardized tests, capture children's language abilities relative to normative samples.

Coding of Parents' Responsiveness

Parents' responsiveness was coded at either a macro-level (i.e., global ratings) or micro-level (i.e., coded for each parent and child behavioral turn). For macro-level coding systems, coders viewed videotaped free play sessions and coded maternal (or paternal) responsiveness on 5- or 7-point scales ranging from never responsive to always responsive.

For micro-level coding schemes, the play sessions were transcribed verbatim and included notes about target actions of the child and parent. From these transcripts (along with viewing of the videotapes), the frequencies of children's vocalizing, bidding toward mother, looking at objects, and playing were obtained. These child behaviors provided a starting point for coding whether or not the parent responded to each child behavior. Responsiveness to target behaviors was defined as a positive and meaningful change in the parent's behavior that was temporally contiguous (i.e., occurring within 5 seconds), contingent on a change in the child's behavior, and appropriate. We adopted a 5-second window for responsiveness based on research demonstrating the duration for various basic temporal events (e.g., lines of poetry, spoken sentences, breath cycles, and communicative movements) falls between 2 and 7 seconds (see Jaffe, Beebe, Feldstein, Crown, & Jasnow, 2001, for discussion; Rovee-Collier, 1995). As an example, if the child looked at a cup (target act = exploring) and the mother said, "cup," the mother was credited with being responsive; similarly, if the child said, "cup" (target act = vocalizing) and mother responded, "Yes, that's a cup," she would be credited with responding. For each instance of responsiveness, coders noted what the mother was responding to (response target) and the precise nature of her response (response type). The categories of child behaviors, as well as maternal responses to these individual behaviors, were mutually exclusive. From these data, frequency counts were obtained on infant target acts, mothers' responses to each target act, and various types of responses (e.g., descriptions, questions).

CHILDREN'S EARLY LANGUAGE ACHIEVEMENTS

In this section, findings from the various studies that we have conducted are presented in developmental order. Presentation begins with research on children's early understanding of words (receptive language), followed by children's first words in production, vocabulary spurt, and first sentences (combinatorial speech). Each of these subsections opens with a description of the target language achievement, followed by research on the role of parents in supporting the specific achievement.

Understanding Words: The Dawning of Language

The first months of life can be conceptualized as a protracted period of social-communicative "lessons." People have long been captivated by the rhythmic mutuality that characterizes the earliest infant-caregiver interactions (e.g., Bateson, 1975, 1979; Bloom, 1998a; Fogel, Messinger, Dickson, & Hsu, 1999). Caregivers and babies take turns gazing, vocalizing, pausing, smiling, and moving in exquisite synchrony, as though engaged in a communicative "dance" (Lock, 2001; Stern, 1985). Through their participation in these moment-to-moment exchanges, babies are socialized into the communication system (e.g., Hsu & Fogel, 2001). Already by 6 months of age, infants take an active lead in initiating interactions with familiar people, and they exhibit heightened responsiveness to their partners' cues (e.g., Lock, 2001). These early protoconversations (Bateson, 1975; Bloom, 1998a) lay the foundation for later conversations and verbal exchanges (e.g., Ginsburg & Kilbourne, 1988; Locke, 1995), which retain many of the temporal and affective qualities of earlier social communications, yet are marked by the infant's increasing appreciation of the meanings of the words that others use during social interactions.

It is somewhere around 9-10 months that the transition from prelinguistic to linguistic communications occurs, a milestone that is rooted in infants' growing social-cognitive understandings of others (Tamis-LeMonda & Adolph, in press). Through daily participation in reciprocal exchanges, infants acquire an understanding of agency and intentionality (e.g., Lock, 2001; Tomasello, 1995). By the end of the first year, infants are thought to appreciate that people can share attention toward objects and events in the outside world, an understanding that has been referred to as "secondary intersubjectivity" (Stern, 1985; Trevarthen, 1993). This recognition enables children to benefit from social interactions in new ways, and expands their opportunities for learning enormously (Baldwin & Moses, 1996; Tomasello, 1993). Children look where adults look, imitate adults' actions with novel objects, and reference adults in ambiguous situations. At this same time, infants begin to exhibit deictic gestures, such as pointing and showing, suggesting that they are engaged in purposeful communication and wish to convey their intentions or emotions to others (e.g., Bates, Benigni, Bretherton, Camaioni, & Volterra, 1979; Camaioni, Aureli, Bellagamba, & Fogel, 2003). These episodes of joint attention and shared understanding are central to children's social cognitive and language development (e.g., Baumwell, Tamis-LeMonda, & Bornstein, 1997; Bornstein & Tamis-LeMonda, 2001; Camaioni, 2001; Homer & Tamis-LeMonda, in press).

In light of these social-cognitive achievements, it is unsurprising that the 9-10 month period has also been referred to as the "dawning of language" as reflected in infants' rudimentary understanding of simple words and phrases (Bates et al., 1979; Volterra, Bates, Benigni, Bretherton, & Camaioni, 1979). These initial developments in language comprehension can be distinguished from the more sophisticated understanding of words and phrases that is exhibited by slightly older children. Specifically, infants' early understanding of words is routinized and depends on the contextual cues that accompany speech acts; infants appear to have a very rudimentary grasp of the meanings of words, and early word learning is often restricted to a specific context and specific actions (e.g., Barrett, 1995; Bloom, 1973; Camaioni, 2001; Camaioni et al., 2003; Tomasello, 1992; Snyder, Bates, & Bretherton, 1981).

We have documented the transition from restricted to flexible understanding over the period of 9 to 13 months based on data obtained from biweekly language interviews conducted with mothers (as described above). Virtually all infants displayed restricted understanding at the onset of language, with words becoming increasingly flexible with development. In the initial 2 months of interviews, the majority of infants demonstrated at least rudimentary understanding of their name and the names of familiar people (e.g., caregivers and siblings; mama, dada), basic foods (bottle, Cheerios), simple commands (e.g., "No!"), and performatives and games (e.g., "bye-bye," "clap hands," and "where's baby?"). Although mothers reported that their infants understood these simple words/phrases, further probing revealed that infants' understanding depended on the cues that accompanied these verbal expressions. For example, in several instances, infants responded to phrases such as "pattycake" or "clap hands"

with the appropriate clapping behavior; however, their responses were cued by parents' own clapping, nodding of the head, smiles, and melodic chanting of "pattycake." Parents' facial expressions, vocal intonations and gestures cajoled infants to join in on the fun, and lent familiarity to the continued repetition of the words associated with "pattycake."

In other instances, infants responded to words/phrases in generalized, diffuse ways, again suggesting only a tenuous grasp of the word's true meaning. For example, several infants were reported to laugh, flail their arms, and kick their legs in response to hearing phrases that contained the word "out" or "outside" (e.g., "We're going out. Out? Out? Do you want to go out?"), as though anticipating an impending excursion. However, these verbal phrases were typically accompanied by the assemblage of a stroller, packing a bag, and walking towards the door. Again, infants' reactions appeared to be context-dependent, making it questionable whether their emotional reactions of enthusiasm signaled true understanding of the meaning of the word "out." (To illustrate this limited nature of early understanding, Table 4.1 presents a diary excerpt taken from the first language interview conducted with a mother of a girl who was 9.5 months at the time of the call.)

On the other hand, these early glimmers of understanding were meaningful precursors to more reliable forms of receptive and productive language, illustrating the ways in which infants' experiences with repeated and familiar events provide the structure for interpreting others' speech. Infants map words onto existing concepts or cognitive structures, which form the building blocks to language (e.g., Bloom, 1998a; Hoff-Ginsberg, 1997). By participating in familiar social and cultural routines, children develop scripts, or generalized event representations of their life experiences that are comprised of specific information about actors, actions, and the objects or recipients of their actions (e.g., Nelson, 1981, 1986).

Moreover, parents' reports about their infants' receptive vocabularies were neither arbitrary nor overly inclusive. Rather, mothers were very selective about the words they claimed their infants "understood." Each week mothers reported a select handful of words that elicited consistent behavioral responses from their infants. A typical pattern was for infants' restricted understanding of a specific word to shift to flexible understanding within 1 to 2 weeks, and many of the first words "understood" were those that appeared in children's productive language weeks or months later.

As expected, across the developmental period of 9 to 13 months, an increasing proportion of the words that children understood were flexible or independent of contextual cues. Thus, the word "pattycake" would eventually induce infant clapping in the absence of parents' clapping, and "Let's go out" would impel infants to toddle to the door before strollers appeared. Moreover, context-flexible words functioned to "bootstrap" the acquisition of new words. As children came to understand the meaning of specific verbs, for example, the nouns and noun phrases to those actions were more readily understood (e.g., "eat" facilitates mapping new food terms—"Eat your peas!", "Eat your Cheerios!", Eat your apple!").

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