Infancy ranges in age from the moment of birth to about 2 years of age when the young child begins to use words to make sentences. In fact, the word “infant” means literally “one who is unable to speak.” Infancy is unique in that it is the earliest stage of life outside of the womb and is viewed as the most important formative period of development. During this early stage of life, infants are remarkably dependent on their caregivers for all their needs. Prior to the emergence of single words, which occurs as the first year of life comes to an end, infants convey their needs to their caregivers nonverbally through facial and body expressions and verbally such as crying and cooing.

Very young infants spend a great deal of their time sleeping and when awake are occupied with bodily sensations and sensory experiences. As they get older, infants become more interested in exploring the objects in their environment and establishing relationships with the significant people in their world.

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The single most significant aspect of human development is the impressive brain growth and plasticity that takes place during the first 2 years of life. The neural pathways of the human brain undergo major changes in response to environmental stimulation during infancy. Importantly, these changes in brain maturation along with environmental events contribute to the development of the ability to use and understand single words, which signals the emergence of language in the second year of an infant’s life. As infants develop the ability to use language to communicate, it also enables them to think and conceptualize in the ways that we commonly identify as characteristic of only human beings. Play becomes central to an infant’s exploration of his or her world. Infancy is also a period of marked changes in body size and motor skills. As the infant comes to the end of the second year of life, independent locomotion becomes firmly established and further characterizes the demarcation of infancy from later childhood stages. In spite of the uniqueness and importance of the infancy stage in human development, only recently are infants being considered worthy of systematic, scientific pursuit.

Historical Perspectives On The Study Of The Infant

Over the last few decades, the scientific study of the infant world has had a tremendous growth spurt likely due to innovative investigative techniques applied to study a broad range of infant abilities. We currently have more information on early perceptual, sensory, cognitive, language, motor, sensorimotor, social,  emotional,  and  self-regulatory  abilities  than had been imagined 50 years earlier. However, regardless of the fact that new information on infant abilities has been amassing exponentially for the past 50 years, the contributions of the early theorists remain among the most influential in the field of infancy. By deeming children, including infants, worthy of systematic study, John Locke (1632–1704) and Jean Jacques Rousseau (1712–1778) laid the groundwork for modern developmental psychology even though they expressed different outlooks on what goes on in the mind of the infant.

Locke  stressed  the  role  of  the  environment  as the main determinant of infant behavior, a precursor of modern behaviorism. This empiricist view has frequently been interpreted as support for “controlling” children, which many, including Rousseau, disagreed with. Rousseau who represented the romanticist view, recommended  “understanding”  infants  rather  than   seeking control of them. Infants, as depicted by Rousseau, are born with a natural ability to learn, although, in his view, they are born as “idiots automaton,” knowing nothing.

As the 19th century began, the ideas of Charles Darwin (1809–1888) fueled the romanticist viewpoint. In particular, Darwin’s assertion that human development was duplicating human evolution although at an accelerated pace furthered the viewing of infants as objects to be studied. Interestingly, Darwin was also considered a pioneer infancy researcher in that he kept a detailed diary of his sons’ first 2 years of development, one of the first known “baby biographies.” The early romanticism versus empiricism controversy was replaced in the 19th century by one of the most widely debated topics in the field of developmental psychology: Which contributes more to development, environment or nature?

An early proponent of the nature theory, Arnold Gesell (1880–1961) claimed the primacy of genetics in determining development. To capture what he considered the genetically determined unfolding of development, Gesell created scales to assess behavioral change in infants and young children. Contrary to Gesell, John B. Watson (1878–1958) believed that children could be taught just about anything, given the right kind of nurturing environment. To highlight the impact of environmental contribution, Watson emphasized parental responsibility for infant outcome; if the outcome was negative, it was the parents’ fault. Both Gesell and Watson made a lasting imprint on how infants are viewed. In fact, heated discussion of the nature-nurture viewpoints continued throughout the 1800s, 1900s, and still is being debated today. While many researchers have sought to provide evidence that only one of these viewpoints is correct, the most up-to-date interpretation from years of systematic study by many developmental psychologists is that environment and genetics together contribute to infant development.

Sigmund Freud (1856–1939) believed that the value of understanding infant development lay in what it might reveal about the adult mind. While psychoanalytic theory stresses the importance of biologically determined  aspects  of  psychological  functioning, the importance of the environment, particularly as it encouraged the child’s nature to unfold, was acknowledged as well. Freud’s theories countered the extremist views of nature or nurture. Freud never used direct infant observation to understand the mind of the infant.

Rather, he used reconstructive techniques. Like Freud, Jean Piaget (1896–1980) was ultimately interested in the working of the human mind throughout the life span, but in contrast to Freud, Piaget directly observed and experimented on infants to understand the progression of cognitive ability. Piaget’s detailed descriptions of his own children led to his conceptualization of infant skills as inherited capabilities emerging as substages in an invariant order with each subsequent stage representing a stable reorganization of the infant in relation to his or her environment.

Throughout the last century, the focus of infant development has been to document the emergence and extent of a wide range of infant abilities. Theorists of the earlier centuries, such as Rousseau, claimed that infants were capable of very little. However, current scientific findings support the idea that infants have a myriad of capabilities evident from birth and even before. Contemporary infancy researchers have created  innovative  experimental  paradigms  that  tap into the behavioral repertoire of infants in order to discover what goes on in the mind of infants. Dramatic findings from the past 50 years of systematic, scientific inquiry not only determined that infants are capable of learning more and learning earlier than previously believed, but also were dispensed readily to consumers because of the rise of mass media and electronic communication. Once in the public domain, research findings quickly became topics of major interest for parents, shaping the way babies were being raised. Producing intellectually precocious infants became somewhat of an obsession for many parents, resulting in less emphasis on rearing mentally healthy infants. The emphasis of infancy research has shifted somewhat in the past few years, however, to include the identification of factors associated with producing optimal psychological outcomes for infants and their families In addition to this recent interest in infant social, emotional, and behavioral development, contemporary infancy researchers continue to investigate a broad range of infant abilities.

The Scientific Study Of Infant Development

Overview of Infant Abilities Studied

Using the scientific approach, researchers have investigated  various  areas  of  infant  development. To describe infancy, psychologists traditionally divide development into separate domains of perception and sensation, motor skills, cognition and language, social behavior, and emotional and behavioral development. However, despite the depiction of infant development as discrete areas, research on infant development would suggest that there is much interdependence of all of the domains.

Perception and sensation involves the development and use of the infant’s senses, specifically vision, touch, hearing, smell, vestibular-proprioceptive movement, and taste. Although not fully developed at birth, perception and sensation are critical experiences for the infant; in fact, until infants can engage in active exploration of the environment, which occurs toward the latter part of the first year of life, they spend most of their time looking at, listening to, touching, smelling, and tasting their environment. The development of motor skills enables infants to become independent explorers of their environment. Investigating the development of perception and sensation in infants includes not only the study of the emergence of gross motor skills such as crawling but also fine motor skills such as hand-eye coordination. Motor development progresses from simple acts, such as turning over, to more complex motor skills, such as walking, to fine motor coordination of objects, such as little pieces of food, crayons, or toys.

Cognitive development refers to the study of mental processing skills used by infants to understand their world, with the development of representational and symbolic thinking critical markers for later optimal development. Although findings from scientific investigations prove that even newborns are not in a state of “blooming, buzzing confusion,” as characterized by William James in the 1800s, it is more toward the end of the second year that infants behave in a way that indicates they are actually reasoning and planning actions mentally. Prior to that, infants do show the ability to form conceptual categories but do not have the cognitive skills to mentally represent a plan of action in thoughts or words in order to choose appropriate plans of action and eliminate less worthy plans. Processing of information becomes more sophisticated with the emergence of symbolic thinking, and social relationships are enriched as infants develop shared meaning with others through language.

Infants process and express social cues long before they ever use language to form relationships. Descriptions of mother-infant relationships by Daniel Stern in the 1980s depicted the nonverbal and verbal “dances” between caretakers and infants, which Stern described in terms of eye gaze, touch, and positive verbalizations such as cooing. Tiffany Field studied how mothers and infants develop shared experiences by analyzing face-to-face interactions during which infants  were  seen  matching  their  own  experiences with that of their caretakers. The interchange between social and emotional development becomes clear when considering the studies by Mary Ainsworth that showed that infants who are secure in their relations with their caretakers show good self-regulation of emotions upon separations and reunions.

Investigators studying emotional development during infancy focused on the emergence of feeling states, how emotions are expressed, and how infants interpret the emotions of others. Michael Lewis and colleagues evaluated infants’ facial expressions and found that the emergence of basic primary emotions, such as joy, interest, sadness, and anger, is apparent from  birth  but  secondary  emotions  such  as  shame and guilt emerge in the second year of life. Studying behavioral skills of infants involves the measure of inherited temperament and later learned patterns of behavior. A number of researchers have paid particular attention to the emergence of self-regulatory skills and how infants use these skills to cope with everyday stresses, such as separation and frustration, sleeping through the night, adjusting to new food tastes, and inhibiting one’s impulses when told “no.” Jerome Kagan and colleagues engaged in extensive longitudinal studies and determined that infant’s feelings, affects, and emotions are moderated by individual, inherited temperament traits, such as behavioral inhibition, and that these temperament traits persist in one form or another into later childhood, adolescence, and adulthood.

Recent Scientific Advancements

The dramatic increase in our understanding of infant development over the past 50 years can likely be attributed to the growth of scientific approaches utilized to study infants as indicated earlier. It is generally agreed that the best way to investigate the world of infants is to observe them directly in a controlled environment using systematic experimentation. Use of a scientific approach yields better validity and reliability and can control for the effects of experimenter bias and subjectivity. Much of the research prior to the second half of the 20th century used less scientific means to study individual infants. For example, Piaget, among others, documented qualitative case studies from baby biographies. While many researchers agreed that Piaget ushered in a direct, rather than retrospective, approach to studying infant development, his investigative style was not scientifically rigorous, which may account for some of the discrepancies found by later researchers regarding the emergence of early cognitive skills. It is well acknowledged, however, that infants are not as easy to study as older children or adults.

The mode of communication relied on most frequently by researchers in the field of psychology to collect information is language expression, typically verbal and sometimes written. To systematically study infants, researchers had to determine how to develop methods that compensated for the infants’ lack of mature expressive ability. Infants cannot describe their feelings, fill out self-report measures, complete standardized written tests, or respond to interview questions. Because it is so difficult to utilize typical methodology with infants, infancy researchers have deduced what is going on in an infant’s mind by developing creative procedures for systematically measuring responses using behaviors within an infant’s repertoire, such as crying, sucking, tracking objects visually, kicking, or orienting the head.


The habituation paradigm measures infant response to repeated exposure of the same visual or auditory stimulus. This paradigm is based on the premise that infants, like older humans and animals, will show reductions in attention as they encounter the same stimulus repeatedly, which is determined by assessing length of visual gaze to the familiar stimulus. Eventually infants will fail to attend when they have “habituated” to the stimulus as if they were bored. The gradual decline in looking time over repeated presentations or trials due to increasing familiarity with the same stimulus is known as habituation. Researchers also assess how quickly infants recover or “dishabituate” when changes are made to the original stimulus and infer from changes in looking behavior whether infants can detect differences. Typically, a criterion for initial looking time duration is determined and habituation occurs when subsequent presentations consistently yield durations shorter than the habituation criterion. Following habituation, the stimulus is changed. If the infant’s looking time increases from the habituation criterion, signaling dishabituation, or recovery, it is inferred that the infant can tell the difference between the two stimuli. If the infant cannot detect a difference, looking time will be shorter than the habituation criterion.

Using this paradigm, Bennett Bertenthal and colleagues found that 5-month-old infants when presented with point lights of a person can discriminate when that person is moving from a random, disorderly pattern of the same person’s point lights. Bertenthal interpreted this as suggesting that young infants have stored knowledge of the human form and its movements. Tiffany Field and colleagues used the habituation/dishabituation technique and found that even newborns could differentiate between happy, sad, and surprised facial expressions. This paradigm has also been used to investigate many other aspects of infant development including recognition of the mother’s face, object categorization, and the detection of emotions and speech sounds.

Preferential Looking and Violation of Expectancies

Another creative approach to understanding infant behavior relies on head turning to determine preferences and is called the paired-preference approach. For example, in a number of preferential looking studies, infants  are  required  to  show  preference  between two visual stimuli presented side by side or two sounds presented to one or the other ear. Investigators try to determine which of the two stimuli is most preferred by the infant by judging where the infant is looking. In the 1960s when infancy researchers first began to use innovative experimental techniques, Robert Fantz and his colleagues used the preferential looking paradigm to document that infants could distinguish visual patterns and, in fact, demonstrated marked visual preferences by infants when presented with two different patterns side by side. To do this infants were introduced to competing patterns while in an infant seat or on their mother’s lap and in a darkened room. Patterns were mounted as a display of flashing lights on a screen directly in front of the infant. In earlier studies, observers gazed through a small peephole in a screen to allow them to look directly at the infant’s face and judge where the infant was looking.

As the preferential looking methodology became more  sophisticated,  researchers  such  as  Marshall Haith detected the precise position of the infant’s gaze by using an electronic sensor to measure the angle of reflection of an invisible infrared light reflected off the infant’s cornea. By using the preferential looking method, researchers have found that young infants prefer the sound of their mother’s voice and the odor of their mother’s breast milk compared to that of another woman. Recently researchers have used variations on preferential looking to cleverly detect cognitive abilities in infants not documented or even believed to exist earlier because of limits in the methodology of prior time periods. For example, in the 1990s Karen Wynn used preferential looking as part of a violation of expectancy paradigm to show infants could detect the outcome of the addition or subtraction of one discrete object from a small collection of two or three objects. Wynn interpreted this as evidence of precocious number concept and concluded that 5-montholds discriminated between quantitative outcomes when their expectancy of addition or subtraction solutions was violated.

Classical  and Operant Conditioning

The interest in behaviorism during the 20th century led a number of infancy researchers to try conditioning young infants to determine aspects of conditioned learning. Hans Papousek, for example, demonstrated classical conditioning in newborns by pairing a neutral stimulus (a sound) with an unconditioned response (head turning). The sound eventually became the conditioned stimulus triggering the head turning, which became the conditioned response. Specifically, Papousek paired a sound with the turning of an infant’s head by touching the corner of the infant’s mouth with a bottle. After repeated exposure, as soon as the sound was heard the newborns turned their heads in anticipation of contact with the bottle. Thus, the neutral stimulus, the sound, became a conditioned stimulus and the head turn became the conditioned response.

While the classical conditioning of an infant demonstrated that infants have the potential to learn, infancy researchers sought to go further and investigate whether infants can be instrumental in changing their own behavior to secure reinforcement, a technique referred to as operant or instrumental conditioning. Operant conditioning has been used by many infant researchers to assess whether infants can learn that their own behavior controls environmental stimuli, thus  actively  engaging  in  the  learning  process.  In the 1970s, James B. Watson, one of the first infancy researchers to utilize this approach, conditioned infants to turn their heads to produce movement in an overhead crib mobile. In the 1980s Anthony DeCasper and William Fifer used newborn infants’ nonnutritive sucking  responses  to  activate  a  tape  recording  of either the mother’s or an unfamiliar female’s voice reading from a book. DeCasper and Fifer found that the newborns learned that different pause lengths between bursts of sucking would differentiate which recording would be turned on and as a function of what was learned infants consistently sucked in such a way that would turn on the mother’s voice.

A series of studies by Carolyn Rovee-Collier and colleagues investigated infant learning and memory utilizing a paradigm termed mobile conjugate reinforcement, a variant of operant conditioning. Infants learned to produce movement in a crib mobile when placed supine in their cribs with a ribbon extending from one ankle to the bar of a mobile stand. Movement in a mobile attached to the mobile stand was contingent on the kicking of the infant’s ankle. Infants readily  learned  to  increase  their  ankle  movements over an initial baseline and if presented with the same mobile at later increments of time demonstrated good short and long-term retention. Rovee-Collier and colleagues used mobile conjugate reinforcement to demonstrate memory for color, number of components, and even contextual determinants of memory, such as odor and music. Investigation of infant response to changes in the mobile suggested that infants differ in how they react to violations of the expected type of stimulation with some infants demonstrating extreme negative reactivity and others learning to compensate for the change.

Physiological Responses

Infancy researchers have also used physiological indices to expand on or substantiate behavioral measures of infant ability and from this infer what is going on inside the infant’s mind. Two common ways that have been used to assess levels of physiological arousal under a variety of circumstances are by using instruments that measure heart rate and galvanic skin response. Recent technological advances have also made possible physiological measurements of electrical brain activities recorded by surface electrodes or brain scans. Using physiological measures has been particularly useful in providing researchers with a way to investigate infant abilities prior to the infant’s capability of expressing the ability behaviorally. For example, depth perception was assessed in infancy using a visual cliff paradigm that required crawling. Based on this research by James and Eleanor Gibson and colleagues, it was concluded that at 6 months of age infants are capable of perceiving the drop-off between themselves and their mother. However, does depth perception emerge at 6 months of age or was research on the emergence of depth perception limited because the visual cliff paradigm could not be used with young infants who were not yet crawling? To study depth perception in younger infants, researchers cleverly attached heart monitors to the infants and looked at changes when young infants were merely placed next to the visual cliff. Moderate arousal signified by changes in heart rate was found in infants as young as 2 months, suggesting depth perception occurs earlier than previously determined.

A number of infancy researchers have studied emotional development using physiological measures to corroborate findings originally yielded by behavioral measures. Jerome Kagan and colleagues initially used behavioral responses to investigate how infants respond to unexpected or novel situations. They found that infants who respond with crying and high motor activity, a response dyad he termed “inhibited temperament,” tend to develop into children who avoid people, objects, and situations that are novel or unfamiliar, whereas uninhibited infants tend to develop into children who spontaneously approach novel persons, objects, and situations. As he continued investigating inhibited temperament longitudinally, Kagan developed an interest in determining if physiological differences could further distinguish between inhibited and uninhibited temperament types. His next step was to utilize the latest developed technology and he measured functional MRI signal response within the amygdala, finding, as he suspected, variations in amygdalar responses to novelty. Adults who as infants were categorized as inhibited showed greater functional MRI signal response within the amygdala to novel  versus  familiar  faces  compared  with  those adults categorized as uninhibited during infancy. Kagan interpreted these findings as evidence of early appearing manifestations of brain functions relating to temperament that are preserved into early adulthood.

Physiological findings also broadened our understanding  of  how  self-control  of  emotion  develops during infancy. Stephen Porges developed a procedure called vagal tone to measure heart rate variability that occurs  at  the  frequency  of  breathing.  Suppression of vagal tone has been found to be associated with the activation of coping strategies in children. Susan Calkins used this measure to investigate relations between mother-infant interactions during which the mother’s face remained still and infants’ physiological responses. Infants showed suppression of vagal tone during the still-face interaction indicating the need for physiological regulation of distress. Infants who did not suppress vagal tone (did not activate coping strategies) during the still-face interaction showed less positive affect and higher reactivity and lower mother infant synchrony in normal play.

Infancy In Perspective

Learning about infant development is becoming so sophisticated that the questions that can be answered about infant ability seem limitless. Infants are clearly not “idiots automaton” as proposed by Rousseau centuries earlier, and new scientific inquiry confirms that Piaget, among others, underestimated the emergence of infant skills abilities. Researchers have even found novel ways to document perceptual and sensory abilities in the developing fetus. It has become clear that many recent research findings have had a dramatic impact on how parents regard their infants. Our recent history has shown that many parents influenced by findings of early skill development became determined to produce “smarter” babies as early in life as possible. To produce “superbabies,” many parents enrolled their infants in programs of structured education before the age of 3 years. This also led to the phenomenon of the “supermom,” the woman who raises superbabies while holding down a job and performing as a “perfect” wife. Perhaps as a reaction to the overemphasis on achievement for both infant and mother, the early 21st century has seen a rise in concern for the psychological outcomes of infants and parents. The current research trend focuses on topics such as emotional intelligence and the goodness of relationships between parents and children. From an applied standpoint there is also growing emphasis on early detection of developmental problems and interventions that may alleviate or even prevent delays. Knowing when certain skills emerge during infancy can help clinicians determine mature or immature developmental  status  of  infants  and  identify  those infants at risk for future difficulty. In spite of centuries of study about infant development, it seems like our knowledge about this first stage of life outside the womb is still in its infancy, but it is clear that continued scientific research is valuable for our society.


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