How do infants connect and make sense of what they are learning? Remember that Piaget believed that we are continuously trying to maintain cognitive equilibrium, or balance, between what we see and what we know. Children have much more of a challenge in maintaining this balance because they are constantly being confronted with new situations, new words, new objects, etc. All this new information needs to be organized, and a framework for organizing information is referred to as a schema. Children develop schemas through the processes of assimilation and accommodation.
For example, 2-year-old Deja learned the schema for dogs because her family has a Poodle. When Deja sees other dogs in her picture books, she says, “Look mommy, dog!” Thus, she has assimilated them into her schema for dogs. One day, Deja sees a sheep for the first time and says, “Look mommy, dog!” Having a basic schema that a dog is an animal with four legs and fur, Deja thinks all furry, four-legged creatures are dogs. When Deja’s mom tells her that the animal she sees is a sheep, not a dog, Deja must accommodate her schema for dogs to include more information based on her new experiences. Deja’s schema for dog was too broad since not all furry, four-legged creatures are dogs. She now modifies her schema for dogs and forms a new one for sheep.
Let’s examine the transition that infants make from responding to the external world reflexively as newborns, to solving problems using mental strategies as two-year-olds. Piaget called this first stage of cognitive development sensorimotor intelligence (the sensorimotor period) because infants learn through their senses and motor skills. He subdivided this period into six substages:
Table 1. Sensorimotor substages
Stage 1 – Reflexes
Birth to 6 weeks
Stage 2 – Primary Circular Reactions
6 weeks to 4 months
Stage 3 – Secondary Circular Reactions
4 months to 8 months
Stage 4 – Coordination of Secondary Circular Reactions
8 months to 12 months
Stage 5 – Tertiary Circular Reactions
12 months to 18 months
Stage 6 – Mental Representation
18 months to 24 months
Substages of Sensorimotor Intelligence
For an overview of the substages of sensorimotor thought, it helps to group the six substages into pairs. The first two substages involve the infant’s responses to its own body, call primary circular reactions. During the first month first (substage one), the infant’s senses, as well motor reflexes are the foundation of thought.
Substage One: Reflexive Action (Birth through 1st month)
This active learning begins with automatic movements or reflexes (sucking, grasping, staring, listening). A ball comes into contact with an infant’s cheek and is automatically sucked on and licked. But this is also what happens with a sour lemon, much to the infant’s surprise! The baby’s first challenge is to learn to adapt the sucking reflex to bottles or breasts, pacifiers or fingers, each acquiring specific types of tongue movements to latch, suck, breathe, and repeat. This adaptation demonstrates that infants have begun to make sense of sensations. Eventually, the use of these reflexes becomes more deliberate and purposeful as they move onto substage two.
Substage Two: First Adaptations to the Environment (1st through 4th months)
Fortunately, within a few days or weeks, the infant begins to discriminate between objects and adjust responses accordingly as reflexes are replaced with voluntary movements. An infant may accidentally engage in a behavior and find it interesting, such as making a vocalization. This interest motivates trying to do it again and helps the infant learn a new behavior that originally occurred by chance. The behavior is identified as circular and primary because it centers on the infant’s own body. At first, most actions have to do with the body, but in months to come, will be directed more toward objects. For example, the infant may have different sucking motions for hunger and others for comfort (i.e. sucking a pacifier differently from a nipple or attempting to hold a bottle to suck it).
The next two substages (3 and 4), involve the infant’s responses to objects and people, called secondary circular reactions. Reactions are no longer confined to the infant’s body and are now interactions between the baby and something else.
Substage Three: Repetition (4th through 8th months)
During the next few months, the infant becomes more and more actively engaged in the outside world and takes delight in being able to make things happen by responding to people and objects. Babies try to continue any pleasing event. Repeated motion brings particular interest as the infant is able to bang two lids together or shake a rattle and laugh. Another example might be to clap their hands when a caregiver says “patty-cake.” Any sight of something delightful will trigger efforts for interaction.
Substage Four: New Adaptations and Goal-Directed Behavior (8th through 12th months)
Now the infant becomes more deliberate and purposeful in responding to people and objects and can engage in behaviors that others perform and anticipate upcoming events. Babies may ask for help by fussing, pointing, or reaching up to accomplish tasks, and work hard to get what they want. Perhaps because of continued maturation of the prefrontal cortex, the infant becomes capable of having a thought and carrying out a planned, goal-directed activity such as seeking a toy that has rolled under the couch or indicating that they are hungry. The infant is coordinating both internal and external activities to achieve a planned goal and begins to get a sense of social understanding. Piaget believed that at about 8 months (during substage 4), babies first understood the concept of object permanence, which is the realization that objects or people continue to exist when they are no longer in sight.
The last two stages (5 and 6), called tertiary circular reactions, consist of actions (stage 5) and ideas (stage 6) where infants become more creative in their thinking.
Substage Five: Active Experimentation of “Little Scientists” (12th through 18th months)
The toddler is considered a “little scientist” and begins exploring the world in a trial-and-error manner, using motor skills and planning abilities. For example, the child might throw their ball down the stairs to see what happens or delight in squeezing all of the toothpaste out of the tube. The toddler’s active engagement in experimentation helps them learn about their world. Gravity is learned by pouring water from a cup or pushing bowls from high chairs. The caregiver tries to help the child by picking it up again and placing it on the tray. And what happens? Another experiment! The child pushes it off the tray again causing it to fall and the caregiver to pick it up again! A closer examination of this stage causes us to really appreciate how much learning is going on at this time and how many things we come to take for granted must actually be learned. This is a wonderful and messy time of experimentation and most learning occurs by trial and error.
The child is now able to solve problems using mental strategies, to remember something heard days before and repeat it, to engage in pretend play, and to find objects that have been moved even when out of sight. Take, for instance, the child who is upstairs in a room with the door closed, supposedly taking a nap. The doorknob has a safety device on it that makes it impossible for the child to turn the knob. After trying several times to push the door or turn the doorknob, the child carries out a mental strategy to get the door opened – he knocks on the door! Obviously, this is a technique learned from the past experience of hearing a knock on the door and observing someone opening the door. The child is now better equipped with mental strategies for problem-solving. Part of this stage also involves learning to use language. This initial movement from the “hands-on” approach to knowing about the world to the more mental world of stage six marked the transition to preoperational thinking, which you’ll learn more about in a later module.
Memory requires a certain degree of brain maturation, so it should not be surprising that infant memory is rather fleeting and fragile. As a result, older children and adults experience infantile amnesia, the inability to recall memories from the first few years of life. Several hypotheses have been proposed for this amnesia. From the biological perspective, it has been suggested that infantile amnesia is due to the immaturity of the infant brain, especially those areas that are crucial to the formation of autobiographical memory, such as the hippocampus. From the cognitive perspective, it has been suggested that the lack of linguistic skills of babies and toddlers limit their ability to mentally represent events; thereby, reducing their ability to encode memory. Moreover, even if infants do form such early memories, older children and adults may not be able to access them because they may be employing very different, more linguistically based, retrieval cues than infants used when forming the memory. Finally, social theorists argue that episodic memories of personal experiences may hinge on an understanding of “self”, something that is clearly lacking in infants and young toddlers.
However, in a series of clever studies Carolyn Rovee-Collier and her colleagues have demonstrated that infants can remember events from their life, even if these memories are short- lived. Three-month-old infants were taught that they could make a mobile hung over their crib shake by kicking their legs. The infants were placed in their crib, on their backs. A ribbon was tied to one foot and the other end to a mobile. At first infants made random movements, but then came to realize that by kicking they could make the mobile shake. After two 9 minute sessions with the mobile, the mobile was removed. One week later the mobile was reintroduced to one group of infants and most of the babies immediately started kicking their legs, indicating that they remembered their prior experience with the mobile. A second group of infants was shown the mobile two weeks later and the babies made only random movements. The memory had faded. Rovee-Collier and Hayne (1987) found that 3-month-olds could remember the mobile after two weeks if they were shown the mobile and watched it move, even though they were not tied to it. This reminder helped most infants to remember the connection between their kicking and the movement of the mobile. Like many researchers of infant memory, Rovee-Collier (1990) found infant memory to be very context dependent. In other words, the sessions with the mobile and the later retrieval sessions had to be conducted under very similar circumstances or else the babies would not remember their prior experiences with the mobile. For instance, if the first mobile had had yellow blocks with blue letters, but at the later retrieval session the blocks were blue with yellow letters, the babies would not kick.
Infants older than 6 months of age can retain information for longer periods of time; they also need less reminding to retrieve information in memory. Studies of deferred imitation, that is, the imitation of actions after a time delay, can occur as early as six-months of age, but only if infants are allowed to practice the behavior they were shown. By 12 months of age, infants no longer need to practice the behavior in order to retain the memory for four weeks.
Our vast intelligence also allows us to have language, a system of communication that uses symbols in a regular way to create meaning. Language gives us the ability to communicate our intelligence to others by talking, reading, and writing. There are many components of language that will now be reviewed.
Components of Language
Phoneme: A phoneme is the smallest unit of sound that makes a meaningful difference in a language. The word “bit” has three phonemes. In spoken languages, phonemes are produced by the positions and movements of the vocal tract, including our lips, teeth, tongue, vocal cords, and throat, whereas in sign languages phonemes are defined by the shapes and movement of the hands.
There are hundreds of unique phonemes that can be made by human speakers, but most languages only use a small subset of the possibilities. English contains about 45 phonemes, whereas other languages have as few as 15 and others more than 60. The Hawaiian language contains less phonemes as it includes only 5 vowels (a, e, i, o, and u) and 7 consonants (h, k, l, m, n, p, and w).
Infants are born able to detect all phonemes, but they lose their ability to do so as they get older; by 10 months of age a child’s ability to recognize phonemes becomes very similar to that of the adult speakers of the native language. Phonemes that were initially differentiated come to be treated as equivalent.
Morpheme: Whereas phonemes are the smallest units of sound in language, a morpheme is a string of one or more phonemes that makes up the smallest units of meaning in a language. Some morphemes are prefixes and suffixes used to modify other words. For example, the syllable “re-” as in “rewrite” or “repay” means “to do again,” and the suffix “-est” as in “happiest” or “coolest” means “to the maximum.”
Semantics: Semantics refers to the set of rules we use to obtain meaning from morphemes. For example, adding “ed” to the end of a verb makes it past tense.
Syntax: Syntax is the set of rules of a language by which we construct sentences. Each language has a different syntax. The syntax of the English language requires that each sentence have a noun and a verb, each of which may be modified by adjectives and adverbs. Some syntaxes make use of the order in which words appear. For example, in English “The man bites the dog” is different from “The dog bites the man.”
Pragmatics: The social side of language is expressed through pragmatics, or how we communicate effectively and appropriately with others. Examples of pragmatics include turn- taking, staying on topic, volume and tone of voice, and appropriate eye contact.
Lastly, words do not possess fixed meanings but change their interpretation as a function of the context in which they are spoken. We use contextual information, the information surrounding language, to help us interpret it. Examples of contextual information include our knowledge and nonverbal expressions such as facial expressions, postures, and gestures. Misunderstandings can easily arise if people are not attentive to contextual information or if some of it is missing, such as it may be in newspaper headlines or in text messages.
Language Development Progression
An important aspect of cognitive development is language acquisition. The order in which children learn language structures is consistent across children and cultures. Starting before birth, babies begin to develop language and communication skills.
Do newborns communicate? Of course they do. They do not, however, communicate with the use of oral language. Instead, they communicate their thoughts and needs with body posture (being relaxed or still), gestures, cries, and facial expressions. A person who spends adequate time with an infant can learn which cries indicate pain and which ones indicate hunger, discomfort, or frustration.
Intentional Vocalizations: In terms of producing spoken language, babies begin to coo almost immediately. Cooing is a one-syllable combination of a consonant and a vowel sound (e.g., coo or ba). Interestingly, babies replicate sounds from their own languages. A baby whose parents speak French will coo in a different tone than a baby whose parents speak Spanish or Urdu. These gurgling, musical vocalizations can serve as a source of entertainment to an infant who has been laid down for a nap or seated in a carrier on a car ride. Cooing serves as practice for vocalization, as well as the infant hears the sound of his or her own voice and tries to repeat sounds that are entertaining. Infants also begin to learn the pace and pause of conversation as they alternate their vocalization with that of someone else and then take their turn again when the other person’s vocalization has stopped.
At about four to six months of age, infants begin making even more elaborate vocalizations that include the sounds required for any language. Guttural sounds, clicks, consonants, and vowel sounds stand ready to equip the child with the ability to repeat whatever sounds are characteristic of the language heard. Eventually, these sounds will no longer be used as the infant grows more accustomed to a particular language.
At about 7 months, infants begin babbling, engaging in intentional vocalizations that lack specific meaning and comprise a consonant-vowel repeated sequence, such as ma-ma-ma, da-da- da. Children babble as practice in creating specific sounds, and by the time they are 1 year old, the babbling uses primarily the sounds of the language that they are learning. These vocalizations have a conversational tone that sounds meaningful even though it isn’t. Babbling also helps children understand the social, communicative function of language. Children who are exposed to sign language babble in sign by making hand movements that represent real language.
Gesturing: Children communicate information through gesturing long before they speak, and there is some evidence that gesture usage predicts subsequent language development. Deaf babies also use gestures to communicate wants, reactions, and feelings. Because gesturing seems to be easier than vocalization for some toddlers, sign language is sometimes taught to enhance one’s ability to communicate by making use of the ease of gesturing. The rhythm and pattern of language is used when deaf babies sign just as it is when hearing babies babble.
Understanding: At around ten months of age, infants can understand more than they can say, which is referred to as receptive language. You may have experienced this phenomenon as well if you have ever tried to learn a second language. You may have been able to follow a conversation more easily than contribute to it. One of the first words that children understand is their own name, usually by about 6 months, followed by commonly used words like “bottle,” “mama,” and “doggie” by 10 to 12 months.
Infants shake their head “no” around 6–9 months, and they respond to verbal requests to do things like “wave bye-bye” or “blow a kiss” around 9–12 months. Children also use contextual information, particularly the cues that parents provide, to help them learn language. Children learn that people are usually referring to things that they are looking at when they are speaking, and that that the speaker’s emotional expressions are related to the content of their speech.
Holophrasic Speech: Children begin using their first words at about 12 or 13 months of age and may use partial words to convey thoughts at even younger ages. These one word expressions are referred to as holophrasic speech. For example, the child may say “ju” for the word “juice” and use this sound when referring to a bottle. The listener must interpret the meaning of the holophrase, and when this is someone who has spent time with the child, interpretation is not too difficult. But, someone who has not been around the child will have trouble knowing what is meant. Imagine the parent who to a friend exclaims, “Ezra’s talking all the time now!” The friend hears only “ju da ga” to which the parent explains means, “I want some milk when I go with Daddy.”
Language Errors: The early utterances of children contain many errors, for instance, confusing /b/ and /d/, or /c/ and /z/. The words children create are often simplified, in part because they are not yet able to make the more complex sounds of the real language. Children may say “keekee” for kitty, “nana” for banana, and “vesketti” for spaghetti because it is easier. Often these early words are accompanied by gestures that may also be easier to produce than the words themselves. Children’s pronunciations become increasingly accurate between 1 and 3 years, but some problems may persist until school age.
A child who learns that a word stands for an object may initially think that the word can be used for only that particular object, which is referred to as underextension. Only the family’s Irish Setter is a “doggie”, for example. More often, however, a child may think that a label applies to all objects that are similar to the original object, which is called overextension. For example, all animals become “doggies”.
First Words and Cultural Influences: First words if the child is using English tend to be nouns. The child labels objects such as cup, ball, or other items that they regularly interact with. In a verb-friendly language such as Chinese, however, children may learn more verbs. This may also be due to the different emphasis given to objects based on culture. Chinese children may be taught to notice action and relationships between objects, while children from the United States may be taught to name an object and its qualities (color, texture, size, etc.). These differences can be seen when comparing interpretations of art by older students from China and the United States.
Telegraphic (Text Message) Speech: By the time they become toddlers, children have a vocabulary of about 50-200 words and begin putting those words together in telegraphic speech, such as “baby bye-bye” or “doggie pretty”. Words needed to convey messages are used, but the articles and other parts of speech necessary for grammatical correctness are not yet used. These expressions sound like a telegraph, or perhaps a better analogy today would be that they read like a text message. Telegraphic speech/text message speech occurs when unnecessary words are not used, typically thought of as using only two words together. “Give ball” is used rather than “Give the baby the ball.”
Infant-directed Speech: Have you ever wondered why adults tend to use that sing-song type of intonation and exaggeration used when talking to children? This represents a universal tendency and is known as infant-directed speech. It involves exaggerating the vowel and consonant sounds, using a high-pitched voice, and delivering the phrase with great facial expression. Why is this done? Infants are frequently more attuned to the tone of voice of the person speaking than to the content of the words themselves, and are aware of the target of speech.
Theories of Language Development
Psychological theories of language learning differ in terms of the importance they place on nature and nurture. Remember that we are a product of both nature and nurture. Researchers now believe that language acquisition is partially inborn and partially learned through our interactions with our linguistic environment.
Learning Theory: Perhaps the most straightforward explanation of language development is that it occurs through the principles of learning, including association and reinforcemen Additionally, Bandura (1977) described the importance of observation and imitation of others in learning language. There must be at least some truth to the idea that language is learned through environmental interactions or nurture. Children learn the language that they hear spoken around them rather than some other language. Also supporting this idea is the gradual improvement of language skills with time. It seems that children modify their language through imitation and reinforcement, such as parental praise and being understood. For example, when a two-year-old child asks for juice, he might say, “me juice,” to which his mother might respond by giving him a cup of apple juice.
However, language cannot be entirely learned. For one, children learn words too fast for them to be learned through reinforcement. Between the ages of 18 months and 5 years, children learn up to 10 new words every day. More importantly, language is more generative than it is imitative. Language is not a predefined set of ideas and sentences that we choose when we need them, but rather a system of rules and procedures that allows us to create an infinite number of statements, thoughts, and ideas, including those that have never previously occurred. When a child says that she “swimmed” in the pool, for instance, she is showing generativity because it easily generated from the normal system of producing language.
Other evidence that refutes the idea that all language is learned through experience comes from the observation that children may learn languages better than they ever hear them. Children who are deaf whose parents do not speak American Sign Language very well nevertheless are able to learn it perfectly on their own, and may even make up their own language if they need to. A group of children who could not hear in a school in Nicaragua, whose teachers could not sign, invented a way to communicate through made-up signs. The development of this new Nicaraguan Sign Language has continued and changed as new generations of students have come to the school and started using the language. Although the original system was not a real language, it is becoming closer and closer every year, showing the development of a new language in modern times.
Chomsky and Nativism: The linguist Noam Chomsky is a believer in the nature approach to language, arguing that human brains contain a language acquisition device that includes a universal grammar that underlies all human language. According to this approach, each of the many languages spoken around the world (there are between 6,000 and 8,000) is an individual example of the same underlying set of procedures that are hardwired into human brains. Chomsky’s account proposes that children are born with a knowledge of general rules of syntax that determine how sentences are constructed. Language develops as long as the infant is exposed to it. No teaching, training, or reinforcement is required for language to develop as proposed by Skinner.
Critical Periods: Anyone who has tried to master a second language as an adult knows the difficulty of language learning. Yet children learn languages easily and naturally. Children who are not exposed to language early in their lives will likely never learn one. Case studies, including Victor the “Wild Child,” who was abandoned as a baby in France and not discovered until he was 12 year old, and Genie, a child whose parents kept her locked in a closet from 18 months until 13 years of age, are two examples of the child deprivation. Both of these children made some progress in socialization after they were rescued, but neither of them ever developed language. This is also why it is important to determine quickly if a child is deaf, and to communicate in sign language immediately. Children who cannot hear who are not exposed to sign language during their early years may never learn it. The concept of critical periods highlights the importance of both nature and nurture for language development.
Critical Period vs. Sensitive Period
A sensitive period (as seen in Chapter 1) is similar to a critical period in which the brain is relatively more plastic and more sensitive to the influence of experience in forming new synapses. New synapses can still form for an extended period of time outside of this optimal period despite being more difficult. Critical or sensitive periods in the life of an organism during which certain experiences or conditions may exert disproportionate influence (either for harm or benefit) on long-term developmental outcomes have been the subject of investigation for over a century.
Brain Areas for Language: For the 90% of people who are right-handed, language is stored and controlled by the left cerebral cortex, although for some left-handers this pattern is reversed. These differences can easily be seen in the results of neuroimaging studies that show that listening to and producing language creates greater activity in the left hemisphere than in the right. Broca’s area, an area in front of the left hemisphere near the motor cortex, is responsible for language production (Figure 1). This area was first localized in the 1860s by the French physician Paul Broca, who studied patients with lesions to various parts of the brain. Wernicke’s area, an area of the brain next to the auditory cortex, is responsible for language comprehension.
- Piaget, J. (1954). The construction of reality in the child. New York: Basic Books. ↵
- Rovee-Collier, C. (1987). Learning and memory in infancy. In J. D. Osofsky (Ed.), Handbook of infant development, (2nd, ed., pp.98-148). New York: Wiley. ↵
- Giles, A., & Rovee-Collier, C. (2011). Infant long-term memory for associations formed during mere exposure. Infant Behavior and Development, 34(2), 327-338. ↵
- Rovee-Collier, C., & Hayne, H. (1987). Reactivation of infant memory: Implications for cognitive development. In H. W. Reese (Ed.), Advances in child development and behavior. (Vol. 20, pp. 185-238). London, UK: Academic Press. ↵
- Rovee-Collier, C. (1990). The “memory system” of prelinguistic infants. Annuals of the New York Academy of Sciences, 608, 517-542. https://doi.org/10.1111/j.1749-66231990.tb48908. ↵
- Campanella, J., & Rovee-Collier, C. (2005). Latent learning and deferred imitation at 3 months. Infancy, 7(3), 243-262. ↵
- Klein, P. J., & Meltzoff, A. N. (1999). Long-term memory, forgetting, and deferred imitation in 12-month-old infants. Developmental Science, 2(1), 102-113. ↵
- Werker, J. F., & Tees, R. C. (2002). Cross-language speech perception: Evidence for perceptual reorganization during the first year of life. Infant Behavior and Development, 25, 121-133. ↵
- Hatch, E. M. (1983). Psycholinguistics: A second language perspective. Rowley, MA: Newbury House Publishers. ↵
- de Boysson-Bardies, B., Sagart, L., & Durand, C. (1984). Discernible differences in the babbling of infants according to target language. Journal of Child Language, 11(1), 1–15. ↵
- Petitto, L. A., & Marentette, P. F. (1991). Babbling in the manual mode: Evidence for the ontogeny of language. Science, 251(5000), 1493–1496. ↵
- verson, J. M., & Goldin-Meadow, S. (2005). Gesture paves the way for language development. Psychological science, 16(5), 367-371. ↵
- Mandel, D. R., Jusczyk, P. W., & Pisoni, D. B. (1995). Infants’ recognition of the sound patterns of their own names. Psychological Science, 6(5), 314–317. ↵
- Baldwin, D. A. (1993). Early referential understanding: Infants’ ability to recognize referential acts for what they are. Developmental Psychology, 29(5), 832–843. ↵
- Dobrich, W., & Scarborough, H. S. (1992). Phonological characteristics of words young children try to say. Journal of Child Language, 19(3), 597–616. ↵
- Clark, E. V. (2009). What shapes children’s language? Child-directed speech and the process of acquisition. In V. C. M. Gathercole (Ed.), Routes to language: Essays in honor of Melissa Bowerman. NY: Psychology Press. ↵
- Gleitman, L. R., & Newport, E. L. (1995). The invention of language by children: Environmental and biological influences on the acquisition of language. An invitation to cognitive science, 1, 1-24. ↵
- Stork, F. & Widdowson, J. (1974). Learning About Linguistics. London: Hutchinson. ↵
- Skinner, B. F. (1953). Science and human behavior. NY: Free Press. ↵
- Bandura, A. (1977). Social learning theory. Englewood Cliffs, NJ: Prentice Hall. ↵
- Anglin, J. M. (1993). Vocabulary development: A morphological analysis. Monographs of the Society for Research in Child Development, 58(10), v–165. ↵
- Goldin-Meadow, S., & Mylander, C. (1998). Spontaneous sign systems created by deaf children in two cultures. Nature, 391(6664), 279–281. ↵
- Senghas, R. J., Senghas, A., & Pyers, J. E. (2005). The emergence of Nicaraguan Sign Language: Questions of development, acquisition, and evolution. In S. T. Parker, J. Langer, & C. Milbrath (Eds.), Biology and knowledge revisited: From neurogenesis to psychogenesis (pp. 287–306). Mahwah, NJ: Lawrence Erlbaum Associates. ↵
- Chomsky, N. (1965). Aspects of the theory of syntax. Cambridge, MA: MIT Press. ↵
- Chomsky, N. (1972). Language and mind. NY: Harcourt Brace. ↵
- Rymer, R. (1993). Genie: A scientific tragedy. Harmondsworth: Penguin. ↵
- Mayberry, R. I., Lock, E., & Kazmi, H. (2002). Development: Linguistic ability and early language exposure. Nature, 417(6884), 38. ↵
- Colombo, J., Gustafson, K. M., & Carlson, S. E. (2019). Critical and sensitive periods in development and nutrition. Annals of Nutrition & Metabolism, 75 Suppl 1(1), 34–42. https://doi.org/10.1159/000508053 ↵