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Physical Development
Across the Lifespan

Explore how our bodies grow, mature, and change from prenatal development through late adulthood: brain growth, motor milestones, puberty, and aging.

AP Psychology

What Is Physical Development?

Physical development refers to the biological changes in body structure, brain architecture, sensory capacities, and motor skills that occur throughout the entire human lifespan: from the moment of conception through the final years of life.

Lifespan Stages at a Glance

Prenatal
Conception to birth: rapid cell division, organ formation, and neural tube development
Infancy
Birth to ~2 years: explosive brain growth, reflexes, and motor milestones
Childhood
~2 to 11 years: continued brain maturation, fine motor skills, and physical coordination
Adolescence
~12 to 18: puberty, growth spurts, and prefrontal cortex pruning
Adulthood
~18 to 65: peak physical capacity, then gradual sensory and muscular decline
Late Adulthood
65+: accelerated decline in reaction time, bone density, and immune function

Prenatal Development: Three Stages

Germinal Weeks 1-2 Zygote divides Implants in uterine wall Embryonic Weeks 3-8 Organs form Heart beats by week 4 Fetal Week 9 - Birth Organs mature Movement, brain develops rapidly

Brain Growth in Infancy

At birth, the brain has nearly all its neurons (roughly 100 billion). What changes dramatically is the number of synaptic connections between them. The infant brain forms new synapses at an astonishing rate: up to \(700\) new connections per second in the first years of life.

Synaptic Density Age Birth 1 yr 3 yr 10 yr 20 yr 60 yr Peak: ~Age 2-3 Synaptic Pruning

Motor Milestones in Infancy

Motor development follows two key principles: cephalocaudal (head to toe) and proximodistal (center to extremities). Infants gain head control before they can walk, and control their torso before their fingers.

1
~2 months: Lifts head while lying on stomach (cephalocaudal: head first)
2
~6 months: Sits without support; begins reaching and grasping objects
3
~9 months: Crawls; develops pincer grasp (proximodistal: fine motor control)
4
~12 months: Stands alone and begins walking (cephalocaudal: legs last)

Maturation vs. Experience

Maturation is the biologically driven sequence of growth and change that is relatively independent of experience. Experience refers to the environmental inputs and practice opportunities that can accelerate, delay, or shape development.

Maturation
Genetically programmed sequence: all healthy children sit before they stand, stand before they walk. No amount of practice can make a 2-month-old walk.
Experience
Enriched environments accelerate skill refinement. Deprivation (e.g., orphanages with minimal stimulation) can delay motor and cognitive milestones.

Puberty and Adolescence

Puberty is the period of rapid physical maturation involving hormonal and bodily changes that occur primarily during early adolescence. It is triggered by the hypothalamus signaling the pituitary gland to release growth and sex hormones.

Primary Sex Characteristics
Reproductive organs and structures directly involved in reproduction (ovaries, testes, uterus)
Secondary Sex Characteristics
Non-reproductive physical features: breast development, voice deepening, facial hair, body hair

The Adolescent Brain

Prefrontal Cortex Last to fully mature (~age 25) Limbic System Emotional Center Matures earlier than prefrontal cortex Myelination Speeds up neural signals (ongoing)

Adulthood: Peak and Decline

Physical abilities generally peak in the mid-20s. After that, a slow, gradual decline begins. The rate of decline depends on genetics, lifestyle, exercise, and nutrition.

1
Muscular strength peaks around age 25, then declines \(\sim 1\%\) per year after 30
2
Sensory acuity: vision and hearing begin subtle decline in the 40s; presbyopia (farsightedness) is common
3
Reaction time slows gradually; processing speed decreases but crystallized knowledge compensates
4
Fertility: gradual decline for both sexes; menopause in women occurs around age 50

Aging in Late Adulthood

Physical Capacity (%) Age (years) 0 50 100 10 25 40 55 70 85 Muscle Strength Sensory Acuity Immune Function Peak ~25

Critical vs. Sensitive Periods

Both concepts describe windows of time when development is especially responsive to experience, but they differ in how rigid that window is.

Critical Period Sensitive Period
Definition A fixed window when certain experiences must occur for normal development An optimal window when the brain is especially responsive, but not the only time
Flexibility Rigid: if missed, development may be permanently impaired Flexible: development can still occur later, just with more difficulty
Example Imprinting in birds; prenatal organ formation Language acquisition; social bonding in humans
AP Exam Focus Teratogen effects; Harlow's monkeys Language learning; neural plasticity

Teratogens and Prenatal Risk

A teratogen is any agent that can cause birth defects or disrupt prenatal development. The impact depends on the timing, dose, and genetic vulnerability of the organism.

Alcohol
Fetal Alcohol Spectrum Disorders (FASD): cognitive deficits, facial abnormalities, growth restriction
Nicotine
Low birth weight, premature birth, increased risk of SIDS and later behavioral problems
Viruses
Rubella, Zika, and other infections can cause intellectual disability, blindness, or microcephaly

How to Analyze an AP Question

1
Identify the life stage: Is the question about prenatal, infancy, childhood, adolescence, or adulthood?
2
Determine the system: Is it asking about brain development, motor skills, sensory changes, or hormonal processes?
3
Check for nature vs. nurture: Is the correct answer about maturation (biology) or experience (environment)? Often it is both.
4
Watch for developmental direction: Remember cephalocaudal (head to toe) and proximodistal (center to extremities).
5
Eliminate absolutes: If an answer says development "never" or "always" happens a certain way, it is likely wrong. Development varies.

Example: Infant Motor Sequence

1
Life stage: Infancy (~8 months)
2
System: Gross motor development
3
Principle: Cephalocaudal development: control progresses from head → torso → legs. Sitting (torso) before walking (legs) is perfectly normal.
4
Conclusion: Mia is on track. Walking typically begins around 12 months (with wide normal range from 9-16 months).

Example: Adolescent Risk-Taking

1
Life stage: Adolescence (15 years)
2
System: Brain maturation (prefrontal cortex vs. limbic system)
3
Explanation: The limbic system (emotion, reward, peer influence) is fully active, but the prefrontal cortex (impulse control, judgment) will not fully mature until ~25.
4
Conclusion: This "dual-systems" imbalance explains why teens can know something is risky yet still choose to do it: especially with peer presence amplifying reward signals.

Common Mistakes on the AP Exam

✕ Confusing Critical and Sensitive
Critical periods are rigid (miss it, lose it). Sensitive periods are optimal but flexible. Many students use them interchangeably.
✕ Thinking Development Stops
Physical development continues into late adulthood. The brain retains plasticity throughout life, and exercise can slow physical decline at any age.
✕ Mixing Up Directions
Cephalocaudal = head to tail (top-down). Proximodistal = near to far (center to extremities). Students reverse these under time pressure.

Summary Across the Lifespan

Stage Key Physical Change Brain Focus
Prenatal Organ formation; neural tube Neuron production
Infancy Motor milestones; rapid growth Synaptogenesis (\(\text{700/sec}\))
Childhood Fine motor skills; coordination Myelination; pruning
Adolescence Puberty; growth spurts Prefrontal cortex matures last
Early Adulthood Peak strength and senses Full prefrontal maturation (~25)
Late Adulthood Sensory and muscular decline Slower processing; plasticity remains

Question 1 of 5

The principle that motor development proceeds from the head downward to the feet is called:

Question 2 of 5

During which prenatal stage are developing organs most vulnerable to teratogens?

Question 3 of 5

Why do adolescents tend to engage in riskier behavior than adults, even when they understand the dangers?

Question 4 of 5

A researcher finds that children raised in severely deprived orphanages show delayed language development, but those adopted before age 6 eventually catch up. This best illustrates the concept of a:

Question 5 of 5

Synaptic density in the human brain reaches its peak during which period?