The Physical Milestones in Development (College Board AP® Psychology): Study Guide

Prenatal development

• Physical development begins at conception and follows a broadly predictable sequence across the lifespan, although the timing of individual milestones can vary

• Prenatal development is the period of development from conception to birth

Factors influencing prenatal development

• Several factors can influence the major physical and psychological milestones that occur during prenatal development:

  • Teratogens are harmful substances or agents in the environment that can cross the placenta and negatively affect fetal development

    • E.g. alcohol is one of the most common teratogens. Even small amounts can alter fetal brain development

      • Children of mothers who drink heavily during pregnancy are at risk of fetal alcohol spectrum disorder (FASD), which can cause physical abnormalities, cognitive deficits, and learning difficulties

    • Other teratogens include cigarettes, certain prescription and over-the-counter medications, illegal drugs, environmental toxins, and some infections

  • Maternal illness: certain diseases and infections contracted by the mother during pregnancy can pass through the placenta and harm the developing fetus

    • E.g. some bacteria and viruses are not filtered by the placenta and can be contracted by the fetus

  • Genetic mutations: errors in genetic coding can affect physical and psychological development regardless of environmental conditions

  • Hormonal factors: hormonal imbalances during prenatal development can influence physical and psychological outcomes

  • Environmental factors: the broader prenatal environment, including maternal nutrition, stress levels, and exposure to pollutants, can shape development

Development in infancy and childhood

• Physical development in infancy and childhood follows the same general sequence across all typically developing children

  • The timing of each milestone can vary considerably between individuals

• Physical development proceeds in two directions:

  • Cephalocaudal (head to tail): development proceeds from the head downward

    • Infants gain control of their head and neck before their trunk and legs

  • Proximodistal (center outward): development proceeds from the center of the body outward

    • Trunk control develops before arm and hand control

Motor development

• All humans develop the same basic motor skills in the same sequence:

  • Gross motor skills involve large muscle movements, e.g. rolling, crawling, sitting, standing, walking, jumping

  • Fine motor skills involve smaller, more precise movements, e.g. drawing, writing, picking up small objects

• Most babies around the world can:

  • roll over when they are about 3–4 months old

  • sit at around 6–8 months

  • walk by themselves at about 12 months

• Motor development depends on neural development

  • As the brain develops and neurons become myelinated, motor control improves and movements become more refined, coordinated, and intentional

    • Environmental encouragement and stimulation may have some effect on motor skills, but the effect is relatively small compared to biological readiness

Reflexes

• Infants are born with a set of automatic reflexes

  • These are innate, involuntary responses to specific stimuli that indicate on-track physical and psychological milestone development

• The presence of these reflexes at birth indicates healthy neurological development

  • Their absence or asymmetry can signal developmental problems

The visual cliff

• Research using the visual cliff apparatus, conducted by Gibson & Walk (1960), demonstrates that infants have an early ability to perceive depth

  • The study also provides an example of how researchers can assess responses in participants who cannot verbally communicate

• Gibson & Walk constructed a raised glass-topped table with two sides:

  • a shallow side, where a patterned surface was directly beneath the glass

  • a deep side, where the patterned surface dropped away beneath the glass, creating the visual impression of a steep cliff edge, even though the glass made it physically safe to cross

• Infants were placed on the shallow side and their mothers called to them from the deep side

• Gibson & Walk found that:

  • 92% of infants refused to crawl over the apparent cliff edge, even when encouraged by their mothers

  • very young infants who were too young to crawl showed an increased heart rate when placed over the deep side, indicating physiological awareness of depth even before motor avoidance was possible

• Gibson & Walk concluded that this provides evidence that depth perception develops very early in infancy

  • This develops before babies have had sufficient experience to have learned a fear of heights

Critical and sensitive periods

• Critical periods are specific windows of time during which certain skills or abilities must develop

  • If the necessary experience does not occur during the critical period, the ability may never fully develop

    • E.g. experiments with animals have shown that depriving an eye of visual stimulation at the very beginning of life leads to underdevelopment of the occipital lobe responsible for processing vision in that eye

• A sensitive period is where development is most easily influenced during this window but can still occur, with more difficulty, outside it

  • E.g. learning to play a musical instrument is easier during childhood than in adulthood, as the brain is more plastic and receptive during this window. Musical skill can still be acquired later in life, unlike abilities tied to strict critical periods

Imprinting

• Imprinting is a phenomenon observed in non-human animals in which a newborn bonds with and follows the first moving object it encounters after birth

  • It is an example of behavior that must occur within a critical period

• Imprinting functions as a survival mechanism, ensuring that a newborn bonds with its caregiver

• Konrad Lorenz (1935) demonstrated imprinting in a key study with goslings:

  • Lorenz randomly divided a clutch of goose eggs into two groups

    • One group hatched in their natural environment with their biological mother (control group)

    • The other group hatched in an incubator where the first moving thing they saw was Lorenz (experimental group)

  • When all goslings were mixed together, the control group followed their natural mother while the experimental group followed Lorenz

    • They had imprinted on him

  • Lorenz identified that imprinting in goslings occurs within a critical period of approximately 13–16 hours after hatching

    • If exposure to a moving object does not occur within this window, imprinting does not take place and the animal fails to form a normal attachment to a caregiver

  • Lorenz concluded that imprinting, and by extension, early attachment is innate rather than learned

Development in adolescence

• Adolescence is the transitional period between childhood and adulthood, characterized by rapid and significant physical change

• The primary physical milestones of adolescence are the adolescent growth spurt and puberty:

  • The adolescent growth spurt is a rapid increase in height and weight

    • This typically occurs earlier in females than males

  • Puberty is the process through which reproductive ability develops, involving the maturation of primary and secondary sex characteristics:

    • Primary sex characteristics: the reproductive organs themselves (e.g., the testes and ovaries)

    • Secondary sex characteristics: physical features that signal sexual maturity but are not directly involved in reproduction (e.g., breast development, facial hair, pubic hair, voice changes)

  • Key milestones include:

    • menarche: the first menstrual period in females, typically occurring between ages 11 and 14

    • spermarche: the first ejaculation in males, typically occurring between ages 12 and 14

• The adolescent brain continues to develop significantly during this period

  • The prefrontal cortex, responsible for planning, impulse control, and decision-making, is not fully developed until the mid-20s

• This helps explain why adolescents are more prone to risk-taking, impulsive behavior, and emotional reactivity

Development in adulthood

• Adulthood spans the majority of the human lifespan and is characterized by a gradual leveling off and then varying decline across multiple physical domains

• Key areas of physical change in adulthood:

  • Reproductive ability, which declines gradually in both sexes

    • In females, menopause marks the end of the menstrual cycle and reproductive ability, typically occurring around age 50

  • Mobility and flexibility: joints become less flexible and muscle mass decreases with age

    • Physical peak is typically reached in early adulthood

  • Reaction time slows progressively from the mid-20s onward, affecting performance on tasks requiring rapid responses

  • Sensory acuity: both visual and auditory acuity decline with age

    • E.g. the lens of the eye becomes less flexible, making it harder to focus on close objects (presbyopia); high-frequency hearing sensitivity declines (presbycusis)

• In the older population, physical decline becomes more pronounced:

  • Approximately half of those over age 80 have lost significant height

  • The senses lose much of their efficacy, affecting memory, cognition, and personality

  • Diseases such as Alzheimer's can affect memory, cognition, and personality

• Despite physical decline, many cognitive abilities remain stable or improve in adulthood

  • Crystallized intelligence tends to remain stable or increase while fluid intelligence declines