Scientific study of change & continuity across the lifespan
Developmental psychology is the scientific study of how and why people change over time. It explores systematic, age-related changes in physical, cognitive, and socio-emotional domains from conception to late adulthood, emphasising both growth and decline.
Age-based periods of psychological and physical development across the lifespan
This period consists of the germinal (first 2 weeks), embryonic (weeks 3–8), and fetal (weeks 9–40) stages. The fertilized egg ( zygote ) undergoes rapid cell division and differentiation. Major organs and body structures begin to form, and the fetus becomes responsive to environmental influences like maternal nutrition, toxins, and stress. Research focuses on teratogens (e.g., drugs, malnutrition) and their long-term effects on development 6,7.
Infants experience rapid physical and neural growth. Birth weight typically triples by age two, and the brain reaches around 80–90% of its adult size8. Motor milestones include lifting the head, crawling, and walking. Cognitive development follows Piaget’s sensorimotor stage, where infants learn object permanence and basic problem-solving. Socially, this is the stage of attachment formation and Erikson’s trust vs. mistrust.
Physical growth slows, but motor skills and independence improve (running, dressing, drawing). Language expands rapidly, and children enter Piaget’s preoperational stage, characterized by symbolic thought but limited logic. They begin developing self-concept and initiative. Focus is often on creativity, literacy, and social play9.
Steady physical growth continues. Cognitively, children enter Piaget’s concrete operational stage and grasp logical rules, conservation, and classification. Social development includes forming peer relationships and engaging in teamwork. Erikson’s industry vs. inferiority stage emphasizes building competence through school and social experiences10.
Adolescence begins with puberty and includes major physical, cognitive, and emotional transitions. Teens typically reach Piaget’s formal operational stage allowing for abstract reasoning. Erikson’s identity vs. role confusion stage highlights the search for values, beliefs, and identity. Emotional regulation fluctuates due to hormonal shifts and peer pressures11.
Physical peak is reached, though fertility and metabolism begin to decline. Cognitive capacity stabilizes, and some propose the development of postformal thought. Erikson’s stage is intimacy vs. isolation, as individuals pursue romantic partnerships, careers, and parenthood. Life achievements such as higher education and employment are emphasized12.
Signs of aging appear, and health issues like menopause may emerge. Cognitive abilities are often maintained with increased expertise and practical intelligence. Erikson’s stage is generativity vs. stagnation, involving the nurturing of others and societal contributions. Midlife often involves reevaluation of goals and taking on new social roles like caregiving for parents13.
This stage varies widely between individuals. Physical decline may occur, including sensory loss and chronic illness. Cognitive aging includes some memory reduction, though crystallized intelligence often remains stable. Erikson’s stage is integrity vs. despair as individuals reflect on life. Research explores dementia, successful aging, and emotional well-being in this stage14.
Development is cumulative — earlier experiences like secure attachment in infancy can shape outcomes in adolescence and adulthood. Psychologists study both normative changes (common to most individuals) and individual differences at each stage of life6,15.
In-depth tables of age ranges, core conflicts, & hallmark achievements
Big idea : Children are little scientists who actively build mental models of the world. Development proceeds through four qualitatively different stages, each unlocking new logical operations that let the child think in increasingly abstract ways.
| Stage | Approx. Age | Key Attainments | Hallmark Tasks |
|---|---|---|---|
| Sensorimotor | 0 – 2 yrs | Object permanence, goal-directed actions | A-not-B task |
| Pre-operational | 2 – 7 yrs | Symbolic play, language explosion, egocentrism | Three-mountain task |
| Concrete Operational | 7 – 11 yrs | Conservation, decentration, class inclusion | Liquid & clay conservation tasks |
| Formal Operational | 12 yrs + | Abstract reasoning, hypothetico-deductive logic | Pendulum problem |
Children progress via assimilation() & accommodation(), constructing increasingly complex schemas.
Big idea : Personality evolves through eight lifespan “crises.” At each age we must balance two opposing forces (e.g. Trust ↔ Mistrust); successful resolution leaves us with a virtue (hope, will, purpose…) that fuels later growth.
| # | Conflict | Virtue Gained | Age Span |
|---|---|---|---|
| 1 | Trust vs Mistrust | Hope | 0 – 18 m |
| 2 | Autonomy vs Shame | Will | 18 m – 3 yrs |
| 3 | Initiative vs Guilt | Purpose | 3 – 5 yrs |
| 4 | Industry vs Inferiority | Competence | 6 – 11 yrs |
| 5 | Identity vs Role Confusion | Fidelity | 12 – 18 yrs |
| 6 | Intimacy vs Isolation | Love | 18 – 40 yrs |
| 7 | Generativity vs Stagnation | Care | 40 – 65 yrs |
| 8 | Integrity vs Despair | Wisdom | 65 yrs + |
Successful resolution produces a psychosocial virtue that scaffolds the next stage.
Big idea : Behaviour is driven by unconscious instinctual energy (libido). As this energy shifts to different body zones across childhood, we face conflicts; how we resolve them shapes adult personality traits.
| Stage | Age | Erogenous Focus | Fixation Outcome (if unresolved) |
|---|---|---|---|
| Oral | 0 – 18 m | Mouth, sucking | Oral-dependent habits (smoking, overeating) |
| Anal | 18 m – 3 yrs | Bladder & bowel control | Anal-retentive or expulsive traits |
| Phallic | 3 – 6 yrs | Genitals (Oedipal focus) | Vanity, recklessness |
| Latency | 6 – 12 yrs | Dormant libido | Difficulties with intimacy |
| Genital | 12 yrs + | Mature sexuality | Balanced personality |
Freud framed development as resolution of instinctual energy (libido) conflicts; modern views treat it as historical context.
Big idea : Moral judgment matures in three levels (pre-, conventional, post-) that reflect widening concern—from self-interest to societal rules to universal principles of justice. Progress depends on cognitive growth and social dialogue.
| Level | Stage | Reasoning Focus |
|---|---|---|
| Pre-conventional | 1. Obedience / Punishment | Avoid punishment |
| 2. Instrumental Relativist | Self-interest, rewards | |
| Conventional | 3. Interpersonal Concordance | “Good-boy/girl” approval |
| 4. Law & Order | Maintain social order | |
| Post-conventional | 5. Social Contract | Mutual benefit, rights |
| 6. Universal Ethical Principles | Abstract justice / conscience |
Progression is not strictly age-bound; few adults reach Stage 6. Moral judgment ≠ moral behaviour.
Big idea : Development is the product of ever-changing interactions (proximal processes) between a person and nested environmental systems—from family to culture to historical era.
Later PPCT revision emphasises Proximal Processes, Person, Context, Time. No discrete stages but shifts as nested contexts evolve.
Big idea : Learning is fundamentally social. Adults and peers “scaffold” the child within the Zone of Proximal Development, and external cultural tools (language, symbols) gradually become internal mental functions.
Development occurs through internalisation() of culturally mediated tools (language, symbols). The Zone of Proximal Development defines the gap between solo ability & guided potential. Staged change is fluid rather than discrete.
How developmental scientists generate evidence
// pseudocode – accelerated longitudinal design
participants = recruit_birth_cohorts(years=[2015,2017,2019])
for wave in range(6):
measure(participants, variables=["cognition","pubertyHormones"])
Sequential designs combine the strengths of cross-sectional and longitudinal approaches to separate age, cohort, and period effects.
High-frequency observations capture moment-to-moment change (e.g., videotaping soilution of a conservation task or smartphone ESM of adolescent mood).
Special protections include assent, parental consent, minimal risk protocols, and mandated reporting for vulnerable populations.
Key physical, cognitive & socio-emotional changes
| Domain | 6 mo | 12 mo | 24 mo |
|---|---|---|---|
| Motor | Sits unaided | Crawls / cruises | Runs, climbs |
| Cognitive | Object permanence emerging | First words | Symbolic play |
| Social | Social smile | Joint attention | Self-recognition |
Milestones are averages—individual variability is expected.
Cross-cutting issues shaping modern development
OECD data show adolescents averaging 49 hrs/week on screens, linked to mental-health risks and cognitive inflexibility; policy recommends family media plans and tech-company accountability.
Developmental psychopathology frames ADHD, ASD, and learning disorders as variations in brain networks, advocating early intervention and strength-based supports.
WHO emphasises nurturing care (nutrition, security, responsive caregiving) during the first 1000 days to avert inter-generational inequality.