Human Dev. Chapter 4
Growth and Stability
Rapid growth occurs during the first two years of a child’s life.
Weight triples by the first birthday: W(12 ext{ months}) = 3 imes W_0
Regular height and weight measurements are taken at doctor’s appointments to spot potential problems.
Head and body do not grow at the same rate across early development:
Newborn head size is about frac{1}{4} of body size.
By age 2, head size is about frac{1}{5} of body size.
In adulthood, head size is about frac{1}{8} of body size.
Average sizes differ among males and females, and among different races.
Four major principles govern human growth:
1) Cephalocaudal: development begins with the head and upper body, then proceeds to the lower body;
2) Proximodistal: development proceeds from the center of the body outward;
3) Hierarchical Integration: simple skills develop separately and then are integrated into more complex skills;
4) Independence of Systems: different body systems grow at different rates.
The Nervous System and Neural Development
The nervous system comprises the brain, the spinal cord, and the nerves that extend throughout the body.
Neurons are the nerve cells that make up the nervous system.
Neuron structure:
Cell body (soma): contains the nucleus; processes messages
Dendrites: receive messages from other neurons
Axon: carries messages for other neurons
Myelin: insulation for the axon; increases efficiency and keeps message intact
Terminal buttons: releases neurotransmitters
Synapse: small gap between neurons where neurotransmitters carry messages from one neuron to another
At birth, humans have between 100 and 200 billion neurons.
Prenatal development can create approximately 250{,}000 additional neurons per minute.
Despite the large number of neurons at birth, there are very few synaptic connections and few myelinated axons initially.
Neural Development in the Early Years
In the first two years, as learning occurs, billions of neural connections are created; the neural network becomes increasingly complex throughout childhood and life.
Neurons increase in size and myelination occurs, coating the axons with myelin.
Myelin provides insulation and protection for the axons and enables faster, more efficient transmission.
The baby’s brain triples in weight by age 2: W{ ext{brain}}(2 ext{ years}) oughly 3 imes W{ ext{birth}}
Early development involves neural repositioning and functional organization:
Some neurons move into the cerebral cortex (upper brain) while others move to subcortical levels below the cortex.
Subcortical levels are most fully developed at birth and control primitive abilities (breathing, heart rate, reflexes).
Neurons in the cerebral cortex become more developed and interconnected over time.
Specific developmental spurts:
A synaptic/myelination growth spurt around 3 ext{--}4 ext{ months} in auditory and visual cortexes rapidly improves auditory and visual skills; motor skills also improve.
Synaptic Pruning and Efficiency
Babies are born with more neurons than they need; the synaptic connections formed in the first 2 years are also in excess.
Synaptic pruning is the process of eliminating unused or under-stimulated neurons and synaptic connections to create a more efficient and elaborate nervous system.
This is one of the few aspects of growth where loss of cells is beneficial, leading to more efficient neural networks.
Visual/Auditory Development and Brain Organization
During development, neurons reposition based on function, with some moving to the cerebral cortex and others remaining subcortical.
Subcortical areas are mature at birth and govern reflexive, primitive functions; cortical areas mature later, supporting complex processing and integration.
Visual and auditory development show dramatic improvements in early months due to cortical maturation and synaptogenesis.
Visual cliff experiments by Gibson and Walk contribute to understanding depth perception development.
Environmental Influences on Brain Development
Shaken Baby Syndrome (SBS): physical abuse causing violent shaking of an infant.
Consequences can include brain rotation and vascular tears resulting in lost neural connections.
In the US, about 20 ext{ per } 100{,}000 babies are injured by SBS each year.
About 25 ext{%} of those babies die; about 80 ext{%} of survivors suffer permanent brain damage (e.g., blindness, hearing impairment, speech disabilities, learning and behavior disorders).
Environmental influence: recognizing and preventing SBS is a major public health concern.
Brain plasticity:
Brain development is influenced by genetic patterns but is highly shaped by environment.
Plasticity is greatest in the early years; if one brain region is injured, another can often take over (functional reorganization).
Infants with brain injury can recover more fully than adults with similar injuries.
Sensory experiences:
Enriched environments influence neuron size and neural connections; restricted environments lead to different brain structure and weight.
Much evidence on sensory stimulation comes from animal studies (rats, kittens).
Early caregiving behaviors (cuddling, talking, singing, reading, holding on the lap) provide multisensory stimulation that promotes development.
Rhythms and states:
Infants develop rhythmic, repetitive patterns of behavior that help integrate behavior.
An infant’s state reflects their degree of wakefulness/awareness in response to stimulation.
Infants’ Rhythms and States
Babies develop rhythms—repetitive cyclical patterns of behavior that help integrate behavior.
A baby’s state is a major body rhythm; a state is the degree of awareness an infant displays to stimulation.
Video-based observations (e.g., Owen) illustrate different states in the first months.
Sleep in Infancy
Newborns sleep a lot: about 16{,}–{,}17 hours per day; sleep patterns vary, some more or less.
Newborns do not sleep through the night initially and often wake for feeding or comfort.
Night vs day: early days may mix day and night; exposure to daylight helps differentiate day from night.
By around the 4th month, sleep patterns become more predictable; some parents choose sleep training.
Infant sleep includes active sleep (REM-like):
Heart rate increases, blood pressure rises, breathing is more rapid during active sleep.
Initially, about half of infant sleep is active; by 6 months this drops to about one third.
Active sleep may serve autostimulation to keep the brain active when alert time is limited.
Do infants dream? Likely not, given limited life experience and different brain wave patterns compared to adults.
Environmental factors can influence infant sleep.
Sudden Infant Death and Safety
Sudden Unexpected Infant Death (SUID): the unexpected death of an infant under 1 year old with unclear cause after investigation.
Possible causes include SIDS, accidental suffocation or strangulation, or unknown health conditions.
Sudden Infant Death Syndrome (SIDS): unexplained death of a seemingly healthy infant during sleep; about 1{,}250 cases per year in the US.
Occurs when normal breathing is interrupted; exact cause remains unknown.
American Academy of Pediatrics (AAP) guidelines to reduce SIDS risk:
Place baby on back to sleep.
Share a bedroom with parents for at least 6 months.
Do not use soft bedding in the crib; use a fitted sheet only.
Possible causes of SIDS (not definitive):
Brain abnormality in the hippocampus, child abuse, undiagnosed sleep disorders, nutritional deficiencies, problems with reflexes, undiagnosed illness.
Risk factors for SIDS: males, Black Americans, low birth weight, low Apgar scores.
Nutrition, Growth, Malnutrition, and Obesity
Proper nutrition is vital for achieving physical potential and also affects cognitive and social development.
General guidance: about 50 ext{ calories per pound per day}; counting calories is not usually necessary because infants regulate intake.
Expressed as: ext{Calories per day} = 50 imes ( ext{weight in pounds})
Malnutrition: improper amount or balance of nutrients; more common in underdeveloped nations (about 10 ext{%} of infants) and in poverty-stricken US populations (about 20 ext{%} of children).
Consequences: slower growth and lower IQ scores.
Undernutrition: nutrient deficiency; examples include:
Marasmus: severe deficiency of proteins and calories; results in stunted growth and often death.
Kwashiorkor: significant protein deficiency; causes swelling of the abdomen, arms, legs, and face as the body struggles to use available nutrients.
Nonorganic Failure to Thrive: growth failure not due to malnutrition but due to lack of stimulation and attention; child may appear underdeveloped and apathetic despite adequate nutrition; often reversible with parent training or new caregiving environment.
Obesity in infancy:
Defined as a BMI at or above the 95th percentile for age and sex.
Not a simple direct link to adult obesity, but overfeeding in infancy can contribute to excess fat cell development that predisposes weight gain later.
Infants delivered by cesarean section are about twice as likely to become obese later.
Breastfeeding, Bottle Feeding, and Solids
Discussion prompts: pros and cons of breastfeeding vs formula feeding; is one superior?
Benefits of breast milk:
Contains all nutrients necessary for growth
Provides some immunity against childhood diseases
May enhance cognitive growth
Easily digested
Possible emotional advantages for mother and child
Barriers to breastfeeding: lactation difficulties (pain, low supply, baby not satisfied); medical issues for baby (slow weight gain, NICU) or mother (returning to work/school, personal conflicts).
Breastfeeding alternatives:
Combination feeding: breast milk when available and formula otherwise
Exclusive pumping: pump breast milk and feed via bottle
Exclusive formula feeding: formula only via bottle
Formulas are regulated and nutritionally complete.
Introducing solids (AAP recommendation): after 6 months
Best to introduce one new food at a time to identify allergies or preferences
Solid foods are a step toward weaning (ending breast/bottle feeding gradually)
Methods: purees (home-made/stocked baby foods) and Baby-Led Weaning (BLW)
The Development of the Senses
Babies understand the world through sensation (Sensation: physical stimulation of sense organs) and perception (Perception: mental processing of sensory information).
Vision:
Newborn visual acuity is roughly 20/200 to 20/600; by about 6 months, most infants achieve 20/20 vision.
Binocular vision is achieved around 14 ext{ weeks}.
Depth perception develops early; Gibson and Walk’s visual cliff study contributes to understanding.
Visual preferences (Fantz): infants show preferences for complex stimuli over simple ones; curved lines over straight lines; 3D shapes over 2D; human faces over nonfaces; mother’s face over other faces.
Auditory perception:
Hearing is functional before birth; infants have robust auditory perception after birth.
Sound localization is achieved by age 1.
Infants can discriminate between different groups of sounds.
Olfactory and gustatory senses:
Smell: well-developed at birth; breastfed babies recognize their mother’s scent within about 12–18 days after birth.
Taste: infants have an innate preference for sweetness and a negative reaction to bitter tastes; they develop taste preferences influenced by what their mother drank during pregnancy.
Touch:
One of the most highly developed senses at birth; the sense of touch is well established by 32 weeks after conception.
The youngest infants respond to gentle touches; many basic reflexes (rooting, sucking, grasping) require touch.
Pain perception:
Infants are capable of feeling pain from birth; pain response becomes quicker as they age.
Early exposure to pain may lead to permanent changes in neural wiring and greater pain sensitivity in adulthood.
Multimodal perception:
The idea that sensations from different senses are integrated and coordinated.
It is unclear whether perception is initially integrated or whether integration develops over time, but evidence suggests integration occurs early.
Motor Development: Reflexes, Gross, and Fine Motor Skills
The infant body is not optimized for mobility: large/heavy head, short limbs, high fat composition; nonetheless mobility develops rapidly.
Reflexes:
Reflexes are unlearned, involuntary responses that occur in the presence of stimuli and promote infant survival.
Some reflexes persist into adulthood; others disappear as voluntary control increases; pediatricians use reflex checks to assess development.
Classic reflexes and ages of disappearance (examples):
Rooting: 3 ext{ weeks}; turns head toward cheek touch; aids feeding.
Stepping: 2 ext{ months}; legs move when held upright in a standing position; prepares for independent locomotion.
Swimming: 4 ext{--}6 ext{ months}; infant tends to paddle/kick when face down; aids survival in water.
Grasping: 5 ext{--}6 ext{ months}; fingers close around an object; facilitates interaction.
Moro: 6 ext{ months}; arms thrust outward when head/neck support is removed; primitive protection from falls.
Startle: persists in different form; infant flings out arms, arches back, and spreads fingers in response to loud sounds; protective function.
Babinski: 8 ext{--}12 ext{ months}; toes fan out when the sole is stroked; unknown exact function.
Eye-Blink: remains; rapid blinking in direct light; protective of the eyes.
Sucking: remains; reflex to suck on lips or something touching lips; food intake mechanism.
Gag: remains; reflex to clear throat; prevents choking.
Gross motor skills (typical progression):
Rolling over (belly to back: 3 ext{--}4 ext{ months}; back to belly: 5 ext{ months})
Sitting up: 6 ext{--}8 ext{ months}
Crawling, pulling up, cruising: 9 ext{--}11 ext{ months}
Walking: 12 ext{--}14 ext{ months}
Fine motor skills:
Reaching for objects: 4 ext{--}6 ext{ months}
Pincer grasp: 10 ext{ months}
Scoop and eat with a fork/spoon: 18 ext{ months}
Holding a crayon and scribbling: 1 ext{--}2 ext{ years}
Dynamic Systems Theory (Esther Thelen): motor skills are assembled through interactions among multiple systems (brain, body, environment, and motivation).
Crawling is not simply “the brain deciding to move”; it requires physical strength, coordination, cognitive awareness that movement is possible, and motivation to explore.
Emphasizes motivation and environmental exploration as major contributors to motor development.
Milestone norms:
Milestone timing is based on norms (average performance of a large sample by age) but should be used cautiously because they don’t account for individual differences.
Norms largely reflect predominantly white, middle- to upper-class populations and may not be culturally diverse.
Nutrition, Hunger, and Growth: Practical Implications
Nutrition is essential for reaching physical potential and supports cognitive and social development.
Caloric guidance: about 50 ext{ calories per pound per day}; infants typically regulate intake without constant counting of calories.
Malnutrition and health outcomes:
Higher prevalence in poorer regions and populations; associated with slower growth and reduced cognitive performance.
Forms of malnutrition:
Marasmus: protein-calorie deficiency leading to growth arrest and potential death.
Kwashiorkor: protein deficiency causing edema and other serious symptoms.
Nonorganic Failure to Thrive: growth failure due to lack of stimulation/attention rather than nutrition; reversible with parent training and improved environment.
Obesity risk:
Obesity defined as ext{BMI} ext{ at or above the } 95^{ ext{th}} ext{ percentile} for age/sex.
While not a direct one-to-one predictor of adult obesity, early overfeeding can contribute to lifelong obesity risk via adipocyte development.
Cesarean delivery is associated with about twice the risk of obesity later in life.
Feeding Practices and Weaning
Discussion prompts on breastfeeding vs formula feeding consider:
Nutritional completeness, immunity, bonding, convenience, and potential challenges.
Solid foods introduction:
Recommended after 6 months; introduce new foods one at a time to identify allergies and preferences; solid foods facilitate gradual weaning from breast/bottle.
Methods include purees and Baby-Led Weaning (BLW).
The Sense of Touch, Pain, and Multisensory Integration
Touch is foundational in early exploration and learning; early tactile experiences influence other senses and development.
Pain sensitivity begins in infancy and increases with age; early painful experiences can alter neural pathways, potentially influencing pain sensitivity later in life.
Multimodal perception recognizes that senses interact and are integrated to form a coherent understanding of the world;
The degree and timing of integration are subjects of ongoing research, but there is evidence for early multimodal integration.
Connections to Broader Concepts and Real-World Relevance
Early brain development is shaped by both biology and environment; interventions in infancy (e.g., enriched environments, responsive parenting) can optimize outcomes.
Ethical considerations include:
Preventing harm (e.g., SBS prevention via caregiver education and social support).
Equitable nutrition access to reduce malnutrition and obesity disparities.
Responsible guidance on sleep practices to reduce SIDS risk while supporting family routines.
Practical implications for caregivers and educators:
Promote multisensory engagement (talking, reading, singing, holding, gentle touch).
Support healthy sleep routines and safe sleep environments.
Monitor developmental milestones with awareness of cultural and individual variability; use norms as general guidelines rather than strict requirements.
Foundational principles connect to broader psychology and neuroscience concepts:
Cephalocaudal, proximodistal growth patterns mirror the ongoing development from head/upper body toward limbs.
Hierarchical integration aligns with how simple motor and cognitive tasks build into complex skills.
Independence of systems highlights that different bodily systems (nervous, muscular, sensory) mature at different rates.
Key formulas and numerical references summarized:
Head/Body proportions: ext{Head}/ ext{Body} = frac{1}{4} ext{ (birth)}, frac{1}{5} ext{ (age 2)}, frac{1}{8} ext{ (adult)}
Weight growth: W(12 ext{ months}) = 3 imes W_0
Caloric guidance: ext{Calories per day} = 50 imes ( ext{weight in pounds})
SUID/SIDS: about 1{,}250 cases per year in the US; specific risk factors include male sex, Black American ethnicity, low birth weight, and low Apgar scores.
Vision milestones: binocular vision by 14 ext{ weeks}; vision acuity improves from roughly 20/200–20/600 at birth to 20/20 by about 6 months.
Obesity criterion: BMI at or above the 95^{ ext{th}} percentile for age/sex.
Terminology Quick Reference
Cephalocaudal, Proximodistal, Hierarchical Integration, Independence of Systems: four growth principles.
Synaptic pruning: elimination of unused synapses to increase neural efficiency.
Multimodal perception: integration of sensory information across modalities.
SIDS: unexplained sudden death during sleep; opposite concerns include Safe Sleep practices per AAP guidance.
BLW: Baby-Led Weaning; approach to introducing solids based on infant self-feeding.
Dynamic Systems Theory: motor development emerges from interacting systems (brain, body, environment, motivation).
Ethical and Practical Implications Highlight
Prevention of abusive situations (e.g., SBS) through caregiver education and social support.
Accurate interpretation of developmental norms to avoid pathologizing typical variation across cultures and individuals.
Support for families to choose feeding methods that align with health, cultural values, and life circumstances, while ensuring infant nutrition and well-being.
Recognition of early environmental enrichment as a modulator of brain development, with implications for education and early intervention programs.