Ch 6 - Physical development

Physical growth and maturation

Child development - Increases in body size are accompanied by internal developments in muscles, bones, and the central nervous system that influence children's physical abilities.

World Health Organization (WHO) Child Growth Standards

Physical growth - International guidelines for assessing children's height and weight based on children raised in optimal environments. Deviations from standards may indicate problems such as malnutrition, overfeeding, or disease.

Growth during the first year

Physical growth - Babies often double their birth weight by 4–6 months and triple it by the end of the first year, reaching about 9.5–10 kg. Growth may occur in spurts rather than at a steady rate.

Growth by age two

Physical growth - By age two, toddlers are about half their eventual adult height and have quadrupled their birth weight to approximately 12–14 kg.

Growth from age two to puberty

Physical growth - Children typically gain about 5–8 cm in height and 3 kg in weight each year.

Middle childhood growth

Physical growth - During ages 6–11, children typically grow about 5 cm and gain 3 kg per year, so changes may be less noticeable.

Pubertal growth spurt

Adolescent physical development - A period lasting about 2–3 years when adolescents grow rapidly, gaining approximately 4.5–7 kg and 5–10 cm in height annually. Growth slows afterward until adult height is reached.

Body proportions at birth

Physical growth - A newborn's head is about 70% of its eventual adult size and accounts for roughly one-quarter of total body length.

Cephalocaudal development

Physical growth - A sequence of physical maturation and growth that proceeds from the head (cephalic region) to the tail (or caudal region). The trunk grows fastest during the first year, followed by rapid leg growth through childhood.

Proximodistal development

Physical growth - A sequence of physical maturation and growth that proceeds from the centre of the body (the proximal region) to the extremities (distal regions). Arms and legs generally grow faster than hands and feet during infancy and childhood; just before puberty, hands and feet grow rapidly first.

Body proportions in adulthood

Physical growth - By adult height, the head accounts for about 12% of total height. During adolescence, the trunk becomes the fastest-growing body segment while the legs also grow rapidly.

Ossification

Skeletal development - The process by which soft cartilage gradually hardens into bone.

Newborn bones

Skeletal development - Most bones are soft, pliable, and difficult to break at birth. Their small size and flexibility contribute to newborns' difficulty sitting up or balancing when held upright.

Fontanelles

Skeletal development - Soft spots between the newborn's skull bones that allow the skull to compress during birth and accommodate brain growth. They gradually fill with minerals as the skull develops.

Sutures

Skeletal development - Seams where the skull bones join. They allow the skull to expand as the brain grows.

Skeletal age

Physical maturation - A measure of physical maturation based on the child's level of skeletal development. It can be estimated using X-rays of the wrist and hand to assess the number of bones and their degree of ossification.

Sex differences in skeletal development

Skeletal maturation - Girls mature skeletally faster than boys. Girls are about 4–6 weeks ahead at birth, and the difference grows to approximately two years by age 12.

Completion of skeletal development

Skeletal maturation - For most practical purposes, skeletal development is complete by about age 18. The skull, hands, and wrists mature earlier, while leg bones continue developing into the mid- to late teens.

Muscle fibres at birth

Muscular development - Neonates are born with all the muscle fibres they will ever have. The existing fibres grow as protein and salts are added to muscle tissue.

Muscular development

Physical growth - Muscles mature in cephalocaudal and proximodistal directions, with the head and neck maturing before the trunk and limbs. Muscle growth is gradual during childhood and accelerates in early adolescence.

Sex differences in muscular development

Muscular development - Both sexes become stronger during adolescence, but increases in muscle mass and strength are generally greater for boys. Differences are more pronounced in upper-limb strength than in lower-limb strength.

Asynchronous physical development

Physical growth - Different bodily systems grow and mature at different rates. The brain and head reach adult proportions relatively early, reproductive organs grow slowly during childhood and rapidly during adolescence, and lymph tissues temporarily exceed adult levels before declining during adolescence.

Individual variations in physical development

Physical growth - Individuals differ considerably in their rates of growth and maturation. These differences affect visible characteristics such as height and may also contribute to differences in cognition and other areas of development.

Cultural variations in physical development

Physical growth - Groups from different cultural and geographic backgrounds may differ in average body size and rates of maturation. Such patterns vary within populations and should not be treated as universal rules for individuals.

Factors influencing physical growth

Nature and nurture - Heredity interacts with environmental factors to influence growth rate and adult height. These factors include nutrition, exposure to disease, and the emotional climate in which a child lives.

Brain growth spurt

Brain development - The period between the seventh prenatal month and 2 years of age when more than half of the child’s eventual brain weight is added. The brain grows from about 25% of adult weight at birth to 75% by age 2.

Neural development

Brain development - The development and organization of the brain and nervous system, including the formation, specialization, and connection of nerve cells.

Neuron

Neural development - A nerve cell that receives and transmits neural impulses. Neurons are the basic units of the brain and nervous system.

Synapse

Neural development - The connective space or junction between one nerve cell (neuron) and another.

Glia

Neural development - Nerve cells that serve multiple functions, including nourishing neurons, encasing them in insulating sheaths of myelin, facilitating transport, and waste removal.

Astrocytes

Neural development - A subtype of glial cells that form a network of guiding cells along which developing neurons migrate to their destinations in the brain.

Neurogenesis

Neural development - The formation of new neurons. Although most neurons form before birth, neurogenesis continues throughout life in specific brain areas, including the hippocampus, which is important for learning and memory.

Myelin

Neural development - A waxy insulating substance that encases neurons and helps support neural communication. It is produced by specialized glial cells called oligodendrocytes.

Cell differentiation

Neural development - The process through which neurons develop specialized functions based partly on where they migrate in the brain. For example, a neuron that migrates to the auditory area may become an auditory neuron.

Synaptogenesis

Neural development - The formation of synaptic connections among neurons. It proceeds rapidly during the brain growth spurt.

Synaptic pruning

Neural development - The refinement and elimination of excess neurons and synaptic connections that are not sufficiently stimulated or used. It begins near birth and continues until near the end of sexual maturation.

Brain plasticity

Neural development - The capacity for change; a developmental state that has the potential to be shaped by experience. The young brain is especially responsive to sensory stimulation and learning.

Experience-dependent brain development

Neural plasticity - The development of the brain results from both biological programs and early experiences. Frequently stimulated neural connections tend to be maintained, while less-used connections may be eliminated through pruning.

Use-it-or-lose-it principle

Neural plasticity - Neurons and neural connections that are not properly stimulated may degenerate or be eliminated. Research on dark-reared infant chimpanzees found that prolonged visual deprivation could cause irreversible damage to the visual system.

Reversible visual deprivation effects

Neural plasticity - In dark-reared infant chimpanzees, atrophy of the retina and optic-nerve neurons could be reversed if visual deprivation lasted no longer than about seven months. Deprivation lasting more than a year could cause irreversible damage and sometimes total blindness.

Enriched environment

Neural plasticity - An environment that provides varied stimulation, social interaction, and opportunities for exploration. Animals raised in enriched environments develop heavier brains and more extensive neural connections than animals raised in standard laboratory conditions.

Effects of reduced stimulation

Neural plasticity - Brains that develop in stimulating environments may lose some of their complexity if the environment later becomes less stimulating.

Early sensory experience and face recognition

Neural plasticity - Some perceptual abilities are maintained through continued exposure to relevant stimuli. Infants exposed to monkey faces early in life retained better abilities to recognize and distinguish monkey faces when they continued receiving exposure.

Socioeconomic status and brain development

Nature and nurture - Environmental conditions are associated with differences in brain growth. One study found larger head circumferences among children from higher-SES homes whose mothers had college degrees than among children from lower-SES homes whose mothers had no degrees.

Error-related negativity (ERN)

Brain development and self-regulation - A negative electrical response recorded by scalp EEG when a person makes an error during a task. It is generated by activity involving the anterior cingulate cortex and becomes larger as children mature.

Anterior cingulate cortex

Brain development and self-regulation - A structure in the medial frontal region of the brain that generates a response when a person makes an error. Its development is associated with self-regulation and monitoring one's actions.

ERN and development

Brain development and self-regulation - Young children produce smaller error-related negativities than adults, despite being similarly aware of their performance. ERN size has been associated with risk-taking, empathy, and socialized behaviour.

Childhood trauma and self-regulation

Brain development and experience - Higher levels of childhood trauma have been associated with lower self-regulation and more errors in error monitoring. This supports the role of experience in brain development and mental health.

Brain differentiation

Brain development - Different parts of the brain develop at different rates. At birth, lower subcortical centres are most developed, while higher cortical areas mature later.

Subcortical brain centres

Brain differentiation - Lower brain structures that control consciousness, inborn reflexes, and vital biological functions. These functions include digestion, respiration, and elimination.

Cerebrum

Brain structure - The highest brain centre; includes both hemispheres of the brain and the fibres that connect them.

Cerebral cortex

Brain structure - The outer layer of the brain’s cerebrum, which is involved in voluntary body movements, perception, and higher intellectual functions such as learning, thinking, and speaking.

Primary motor areas

Brain differentiation - Cortical areas that control simple motor activities, such as waving the arms. By about 6 months, they direct most of an infant’s movements.

Primary sensory areas

Brain differentiation - Cortical areas that control sensory processes such as vision, hearing, smelling, and tasting. These areas are among the first parts of the cerebrum to mature.

Early cortical development

Brain differentiation - The sensory and motor areas of the cortex are functioning well at birth, making newborns primarily reflexive, sensory-motor beings. As higher cortical centres mature, they increasingly control primitive subcortical responses.

Myelinization

Brain development - The process by which neurons are enclosed in waxy myelin sheaths that will facilitate the transmission of neural impulses.

Myelin sheath

Brain development - A waxy insulating covering around individual neurons that speeds the transmission of neural impulses. It helps the brain communicate more efficiently with different parts of the body.

Oligodendrocytes

Brain development - Cells that produce myelin, which forms insulating sheaths around neurons and improves neural communication.

Sequence of myelinization

Brain development - Myelinization follows a chronological sequence consistent with the maturation of the nervous system. Sensory pathways are reasonably well myelinated at birth or shortly afterward, while other brain areas continue developing through adolescence and early adulthood.

Motor pathway myelinization

Brain development - Myelinization of neural pathways between the brain and skeletal muscles follows cephalocaudal and proximodistal patterns, supporting increasingly complex motor skills, such as lifting the head, reaching, rolling over, sitting, standing, walking, and running.

Frontal cortex and reticular formation myelinization

Brain development - These brain areas, which help people concentrate for extended periods, are not fully myelinated until around puberty. This may help explain why infants and children have shorter attention spans than adolescents and adults.

Myelinization and emotional regulation

Brain development - Improved communication between emotive subcortical areas and regulatory prefrontal cortical areas helps children process socially important emotional cues and monitor their own emotional reactions.

Development of emotional self-regulation

Brain development - As children mature, they become better able to manage emotional reactions and delay gratification. For example, a 6-year-old may politely thank a grandparent for an unwanted gift, while a 3- or 4-year-old may immediately discard it.

Corpus callosum

Brain structure - The bundle of neural fibres that connect the two hemispheres of the brain and transmit information from one hemisphere to the other. It helps integrate the functions of both hemispheres.

Cerebral lateralization

Brain development - The specialization of brain functions in the left and the right cerebral hemispheres.

Left cerebral hemisphere

Cerebral lateralization - Controls the right side of the body and contains centres involved in speech, hearing, verbal memory, decision making, language processing, and the expression of positive emotions.

Right cerebral hemisphere

Cerebral lateralization - Controls the left side of the body and contains centres involved in visual-spatial information, nonlinguistic sounds such as music, touch sensations, and the expression of negative emotions.

Right-handedness and brain lateralization

Cerebral lateralization - About 90% of adults prefer their right hands for writing, eating, and other motor activities; these functions are generally associated with the left hemisphere. Among most left-handed people, these activities are controlled by the right hemisphere.

When does cerebral lateralization begin

Cerebral lateralization - Lateralization begins during the prenatal period and is already well underway at birth. However, specialization continues throughout childhood.

Evidence of prenatal cerebral lateralization

Cerebral lateralization - Evidence includes the tendency of many fetuses to position their right ears outward, possibly giving them a right-ear advantage for language processing, and newborns showing greater electrical activity in the left cerebral cortex in response to speech sounds.

Newborn lateral preferences

Cerebral lateralization - Most newborns turn their heads to the right rather than the left when lying on their backs, and these infants later tend to reach for objects with their right hands.

Development of lateral preferences

Cerebral lateralization - Handedness can be apparent early and is reasonably well established by age 2, but preferences for using one side of the body consistently become stronger with age.

Incomplete lateralization in young children

Cerebral lateralization - Because the brain is not fully specialized at birth, young children may use both sides of the body for different tasks more flexibly than older children and adolescents.

Brain plasticity after injury

Brain development - The brain’s ability to reorganize and recover functions after damage. Young children may recover more rapidly because neural circuits can assume functions previously performed by damaged areas before lateralization is complete. Recovery varies with the injury and other circumstances.

Motor development

Physical development - The development of control over bodily movements and the acquisition of motor skills. During the first year, infants make rapid progress from limited movement to increasingly complex actions.

Early motor milestone

Motor development - By the end of the first month, most infants can lift their chins while lying on their stomachs. They subsequently develop skills such as lifting their chests, reaching, rolling over, and sitting with support.

Motor milestone age norms

Motor development - Typical ages for acquiring motor skills can help assess an infant’s progress, but individual variation is substantial. Reaching milestones earlier does not necessarily mean a child is more intelligent or will have better future developmental outcomes.

Cephalocaudal development

Motor development - Development proceeds in a head-downward direction, with control of the head, neck, and upper body generally preceding control of the legs and lower body.

Proximodistal development

Motor development - Development proceeds from the centre of the body outward, with control of the trunk and shoulders generally preceding control of the hands and fingers.

Exception to the cephalocaudal pattern

Motor development - Research suggests that infants can intentionally control some leg movements before comparable hand movements. At around 12 weeks, infants have been observed lifting their legs to touch toys; intentional hand contact appeared around 16 weeks in the cited study.

Why can infants control leg movements early

Motor development - The hip joint is more stable and constrained than the shoulder joint, so controlling hip movement may require less coordination and experience than controlling shoulder movement.

Maturational viewpoint

Motor development - The view that motor development follows the unfolding of a genetically programmed sequence of events as nerves and muscles mature. Motor skills generally emerge in a predictable sequence as control develops downward and outward through the body.

Evidence supporting the maturational viewpoint

Motor development - Infants across different cultures tend to progress through roughly the same sequence of motor milestones. Early twin studies also suggested that practising skills did not necessarily give one twin a lasting advantage over an unpractised twin.

Role of practice in the maturational viewpoint

Motor development - Practice helps children refine motor skills that maturation has made possible, but maturation is considered the primary driver of the developmental sequence.

Experiential (or practice) hypothesis

Motor development - The view that opportunities to practise motor skills are important for motor development, in addition to maturation. Infants need experience using skills they are physically capable of performing to become proficient at them.

Evidence for the experiential hypothesis: institutionalized orphans

Motor development - Wayne Dennis found that Iranian orphans who spent much of their early lives lying in cribs and received little stimulation showed substantial motor delays. None of the 1- to 2-year-olds studied could walk, and only 15% of the 3- to 4-year-olds could walk well independently.

Maturation is necessary but not sufficient

Motor development - Physical maturation enables motor skills, but children also need opportunities to practise sitting, crawling, walking, and other movements to become proficient.

Cultural influences on motor development

Motor development - Parenting practices and opportunities for movement can influence when infants reach motor milestones. Cultural differences in motor development do not necessarily reflect genetic differences.

Kipsigis infant motor development

Motor development - Kipsigis caregivers in Kenya encourage early motor practice by supporting infants in upright positions and helping them practise walking. Kipsigis infants in the cited research sat independently about five weeks earlier and walked independently about a month earlier than Western infants.

Traditional Jamaican infant-care routines

Motor development - Practices such as massaging infants, stretching their limbs, and gently moving them while holding their arms have been associated with earlier development of sitting, crawling, and walking in research on Jamaican families.

Stepping-reflex training

Motor development - Research by Zelazo and colleagues found that infants regularly held upright and encouraged to practise stepping strengthened this response and walked earlier than infants who did not receive the training.

Upright positioning and motor development

Motor development - Repeated upright positioning and movement practice can strengthen the neck, trunk, and legs, helping infants develop skills such as sitting, standing, and walking.

Dynamical systems theory

Motor development - The view that motor development results from a complex interaction among the child’s physical capabilities, goals, and experiences. Motor skills emerge through the combined influence of maturation, practice, and the demands of particular movements.

Maturation and experience in motor development

Motor development - Both maturation and experience contribute to motor development. Maturation places limits on when a child becomes physically capable of a skill, while practice and experience influence when and how effectively the skill is performed.

Dynamical systems theory

Motor development - A theory that views motor skills as active reorganizations of previously mastered capabilities undertaken to find more effective ways of exploring the environment or satisfying other objectives. Infants combine existing abilities into new, increasingly complex and coordinated actions.

Motor skill construction

Dynamical systems theory - New motor skills emerge as infants actively reorganize existing capabilities rather than simply unfolding according to a genetic program. Early movements may be tentative and uncoordinated before becoming smoother and more efficient.

Goal-driven motor development

Dynamical systems theory - Infants acquire and refine motor skills to reach interesting objects, explore their surroundings, and achieve goals. Motivation and experience help shape how new skills emerge.

Crawling as an active reorganization

Dynamical systems theory - Crawling emerges as infants coordinate existing abilities, including turning toward interesting sights and sounds, reaching toward stimuli, and kicking with the opposite leg to propel themselves forward.

Why do infants follow a similar sequence of motor milestones

Dynamical systems theory - Human maturation prepares infants for certain abilities, and each new skill builds on component activities developed earlier. However, differences in experiences and goals lead infants to coordinate emerging skills in individual ways.

Dynamical systems theory and nature versus nurture

Motor development - Motor development results from the interaction of biological maturation, experience, motivation, and environmental opportunities. Neither nature nor nurture alone fully explains how motor skills develop.