Lifespan Development. Tara L. Kuther

words together over time based on their experiences as well on as trial and error (Tomasello, 2012). Finally, it appears that language learning does not occur as quickly or effortlessly as Chomsky described (Miller, 2016).

      Interactionist Perspective on Language Development

      From an interactionist view, language development is a complex process reflecting the dynamic interplay of two factors: children’s biological capacities and the social context in which they are reared. A newborn’s ability to discriminate a wide variety of speech sounds and to prefer human speech over recorded sounds—as well as to prefer the sounds and patterns of their native language over those of other languages—suggests an inborn sensitivity to language. Yet the language that an infant learns and the pace of learning are influenced by environmental factors. Let’s explore biological and contextual contributors to language.

      Biological Contributions to Language Development

      Evolutionary theorists explain language as having evolved as a function of natural selection. Language gave some of our early human ancestors an advantage in survival and reproduction over those who did not have language (Berwick & Chomsky, 2016; Hauser et al., 2014). Specifically, language evolved as an adaptation that fulfilled early humans’ need to communicate information that was more complex than could be conveyed by simple calls and hoots (Tamariz & Kirby, 2016). Language may have emerged with increases in the size of human communities and the corresponding complexity of social dynamics, as well as humans’ increasingly large, more sophisticated brains (Aiello & Dunbar, 1993; Turnbull & Justice, 2016).

      The brain specifically is wired for language at birth. Speech sounds produce more activity in the left hemisphere of newborns’ brains, while nonspeech sounds elicit more activity in the right hemisphere (Vannasing et al., 2016). Three-month-old infants show functional neural activity in response to language that is similar but less refined, focused, and organized than that of adults (Dehaene-Lambertz, 2017). Adult language, too, is largely governed by the left hemisphere, and cortical activity in language areas increases from infancy through adulthood (Paquette et al., 2015). Two areas in the left hemisphere of the brain are vital for language and distinguish humans from other primates: Broca’s and Wernicke’s areas (Friederici, 2017). Broca’s area controls the ability to use language for expression. Damage to this area inhibits the ability to speak fluently, leading to errors in the production of language. Wernicke’s area is responsible for language comprehension. Damage to Wernicke’s area impairs the ability to understand the speech of others and sometimes affects the ability to speak coherently.

      Through infant-directed speech, adults attract infants’ attention by using shorter words and sentences, higher and more varied pitch, repetition, and a slower rate. Infants prefer listening to infant-directed speech, and infant-directed speech appears cross culturally.

      Ali Russell/Alamy

      Although the brain plays a crucial role in language capacities, it cannot completely account for language development. For example, recent research has identified multiple genes associated with language development that work together in an epigenetic fashion, influenced by environmental factors (Dediu & Christiansen, 2016; Fisher, 2017). In addition, experience influences the brain architecture that supports language development (Westermann, 2016). For example, we have seen that infants’ ability to detect sounds not used in their native language declines throughout the first year of life, suggesting that contextual factors—specifically, exposure to the native languag—influence older infants’ sensitivity to speech sounds (Posner, 2001; Sansavini, Bertoncini, & Giovanelli, 1997). At the same time, information processing factors largely dependent on neurological development, such as attention and memory, affect how infants comprehend and respond to social interaction and other contextual influences on language development (Perszyk & Waxman, 2018). For example, the statistical learning abilities that enable infants to see patterns and learn quickly are also associated with rapid language learning (Lany, Shoaib, Thompson, & Estes, 2018).

      Contextual Contributions to Language Development

      Language development occurs in a social context. Most adults naturally speak to young infants in a sing-song way that attracts their attention. Infant-directed speech, or “motherese,” uses repetition, short words and sentences, high and varied pitch, and long pauses (Thiessen, Hill, & Saffran, 2005). Infants prefer listening to infant-directed speech than to typical adult speech, and they prefer adults who use infant-directed speech (Schachner & Hannon, 2011). EEG recordings show that babies demonstrate more neural activity in response to infant-directed speech than adult speech, suggesting that they are better able to attend to it and distinguish the sounds (Peter, Kalashnikova, Santos, & Burnham, 2016). Infant-directed speech exaggerates sounds, helping infants hear and distinguish sounds, and enables them to map sounds to meanings (Estes & Hurley, 2012; Kitamura & Burnham, 2003; Thiessen et al., 2005). In one study, 7- and 8-month-old infants were more likely to learn words presented by infant-directed speech than those presented through adult-directed speech (Singh, Nestor, Parikh, & Yull, 2009).

      When babies begin to engage in canonical babbling, a type of babbling with well-formed syllables that sounds remarkably like language, parents, regardless of socioeconomic status, ethnicity, and home environment, tune in and treat the vocalizations in a new way (Oller, Eilers, & Basinger, 2001). Because the utterances sound like words, parents help infants to associate the word-like utterances with objects and events, encouraging vocabulary development.

      Parental responsiveness to infants’ vocalizations predicts the size of infants’ vocabularies, the diversity of infants’ communications, and the timing of language milestones (Tamis-LeMonda et al., 2014). One study showed that infants of highly responsive mothers achieved language milestones such as first words, vocabulary spurt, and telegraphic speech at 9 to 13 months of age, which was 4 to 6 months earlier than infants of low-responsive mothers (Tamis-LeMonda, Bornstein, & Baumwell, 2001). Fathers’ responsiveness to their 2- and 3-year-olds predicted toddlers’ cognitive and language abilities (Tamis-LeMonda, Shannon, Cabrera, & Lamb, 2004). Parental responsiveness is also associated with the language skills of adopted children, supporting the contextual influence of parents.

      Babies learn language by interacting with more mature, expert speakers who can speak at their developmental level. Parents often adjust their infant-directed speech to match infants’ linguistic needs by, for example, using longer and more complicated words and sentences as infants’ comprehension increases (Englund & Behne, 2006; Sundberg, 1998). Even as infants learn speech, they continue to display preferences for some features of infant-directed speech. A study of 12- and 16-month-old infants indicated that they preferred the high pitch and pitch variability of infant-directed speech but not the shorter utterances or simplified syntax (Segal & Newman, 2015).

      The quality of language input from parents and the number of words children hear is related to their vocabulary size at age 2 (Hoff et al., 2002). Children whose mothers address a great deal of speech to them develop vocabulary more rapidly, are faster at processing words they know, and are faster at producing speech than children whose mothers speak to them less often (Hurtado, Marchman, & Fernald, 2008; Weisleder & Fernald, 2013). The number of words and different grammatical structures used in maternal speech, as well as grammatical complexity, predict the size of children’s vocabulary and understanding of grammar (Hadley, Rispoli, Fitzgerald, & Bahnsen, 2011; Huttenlocher, Waterfall, Vasilyeva, Vevea, & Hedges, 2010).

      Although parents do not reliably reinforce correct grammar, they tend to communicate in ways that tell young children when they have made errors and show how to correct them (Saxton, 1997). Adults often respond to children’s utterances with expansions, which are enriched versions of the children’s statements. For example, if a child says, “bottle fall,” the parent might respond, “Yes, the bottle fell off the table.” Adults also tend to recast