Developmental Psychology.

OpeningWhat developmental psychology studies.

How a 270-day collection of cells becomes, over twenty years, a creature capable of doing calculus, falling in love, and lying about its taxes.

Developmental psychology studies the changes, across age, in the cognitive, emotional, motor, social, and moral capacities of the human animal. The discipline emerged in the early 20th century, mainly through Piaget's work in the 1920s, and has since extended past childhood into adolescence, adulthood, and old age. Some of its findings are robust over a hundred years (the broad strokes of Piaget's stages; the basic Strange Situation typology). Some have collapsed under replication scrutiny (Mischel's marshmallow, in its strong form; the predictive power of fixed mindset).

This deck covers methods, infancy, childhood, adolescence, adulthood, and aging — and the figures and debates that have organised each.

Chapter IHow to study development.

Three core designs. Cross-sectional: measure 2-year-olds, 5-year-olds, 8-year-olds at one moment. Fast and cheap; confounds age with cohort effects. Longitudinal: follow the same children from age 2 to age 8. Methodologically cleaner; takes six years and loses participants. Sequential: multiple cohorts followed for shorter periods. Compromise.

Methods specific to infancy

Infants cannot answer questions. Researchers use proxies: preferential looking (Robert Fantz, 1961 — infants look longer at novel stimuli, allowing perception studies); habituation (an infant's interest decreases with repetition; novel stimulus restores it); violation-of-expectation (Renée Baillargeon — measure surprise at impossible events to infer underlying expectations).

Famous longitudinals

The Dunedin Study (1972–) — every person born in Dunedin, NZ, in a single year (1,037 people) followed across forty-plus years. The Bristol Children of the 90s. Lewis Terman's gifted-children study (1921–, still running with descendants).

Chapter IINature and nurture.

The dichotomy used to organise the field. Twin studies (Bouchard's Minnesota Study of Twins Reared Apart, 1979–2000) established that most psychological traits are 30–70% heritable in adulthood — well above zero, well below complete determination.

The contemporary view is more nuanced: genes set reaction ranges, not outcomes. The same genotype produces different phenotypes in different environments. Avshalom Caspi and Terrie Moffitt's gene-environment-interaction work (the MAOA/maltreatment studies in 2002, the 5-HTTLPR/depression studies) was once celebrated, then largely failed to replicate, then was partly rehabilitated through better-powered analyses. The pattern is the field's: simple genetic-determinism stories are wrong, and so are simple environmental-determinism stories.

What is robustly true: shared family environment matters less for adult outcomes than parents intuit. Heritability estimates rise across the lifespan as environments become more chosen.

Chapter IIIPrenatal development.

From conception to birth: 38 weeks of an organism going from one cell to roughly 100 trillion. The first eight weeks (the embryonic period) lay down the basic body plan and are when most teratogenic damage occurs.

Teratogens

Substances that cause birth defects. Thalidomide (prescribed for morning sickness in 1957–1962; caused phocomelia in 10,000+ children before withdrawal). Alcohol (Fetal Alcohol Spectrum Disorder; first systematically described 1973). Rubella (the 1964 rubella epidemic prompted the development of the vaccine). Methylmercury (Minamata disease, 1956). Zika virus (microcephaly, 2015–16 outbreak).

Maternal stress, nutrition

The Dutch Hunger Winter (1944–45) provided a natural experiment. Children whose mothers were malnourished in early pregnancy had elevated rates of cardiovascular disease, schizophrenia, and obesity sixty years later — the foundation evidence for fetal programming (David Barker's hypothesis).

Chapter IVThe infant perceives more than was thought.

The 20th century gradually overturned the assumption that infants perceived a "blooming, buzzing confusion" (William James's phrase, 1890). What we know now: by 6 months, most infants have:

Depth perception (Gibson & Walk's visual cliff, 1960). Face preference from minutes after birth (Goren et al. 1975; the schematic-face response is innate). Phoneme discrimination across all human languages until about 10 months (Patricia Kuhl's "perceptual narrowing"). Object permanence in some form by 4 months (Renée Baillargeon's violation-of-expectation studies, against Piaget's claim that this emerged at 8 months).

Number sense — Karen Wynn (1992) showed 5-month-olds can do simple arithmetic on small numbers (1+1, 2−1) using violation-of-expectation. Elizabeth Spelke and Susan Carey have extended the work into a substantial program on infant cognition.

Chapter VPiaget's stages.

The most-taught framework in developmental psychology, despite half a century of qualifications. Piaget proposed four stages through which all children pass in order, though at variable rates:

SMSensorimotor (0–2 years). The child acquires object permanence — the recognition that objects continue to exist when out of sight. Piaget timed this to about 8 months; modern infant research finds it earlier (4 months) and in graded forms.

Preoperational (2–7 years). Symbolic thought (language, pretend play) but limited reversibility and conservation. The famous conservation tasks: pour water from a short wide glass into a tall narrow one; ask the child if there is more water now. Pre-operational children say yes.

Concrete operational (7–11). Children pass conservation tasks and can reason logically about concrete objects, but struggle with abstract or counterfactual reasoning.

Formal operational (11+). Abstract reasoning, hypothetical thinking, scientific method.

Chapter VIWhat Piaget got wrong.

Three substantive critiques of Piaget have shaped modern infant cognition.

He underestimated infants. Renée Baillargeon, Karen Wynn, Elizabeth Spelke have shown that infants demonstrate object permanence, simple arithmetic, and core understandings of physical causality at 3–6 months — far earlier than Piaget's 8-month timeline. The methods Piaget used (reach-for-hidden-object) required motor capacities the infants didn't yet have.

He overstated stage discontinuity. Modern microgenetic studies (Robert Siegler) show that children use multiple strategies simultaneously and shift gradually between them, rather than passing through hard stage transitions.

He under-counted social context. Vygotsky's emphasis on social scaffolding (see next page) addresses Piaget's main blind spot.

What survives: the broad direction of cognitive development is correct. Children become better at logical reasoning, conservation, and abstraction roughly in the order Piaget proposed.

Chapter VIIVygotsky.

Where Piaget studied the child as a solitary scientist constructing knowledge through individual exploration, Vygotsky emphasised the social origins of higher mental functions. His central concept: the Zone of Proximal Development (ZPD) — the gap between what a child can do alone and what they can do with help from a more competent partner.

"What a child can do with assistance today, she will be able to do by herself tomorrow."— Vygotsky, Mind in Society, 1934

The pedagogical concept of scaffolding (Bruner, 1976) — temporary support a teacher gives a learner that is gradually withdrawn — is a direct descendant. So is the contemporary literature on guided participation, apprenticeship learning, and dialogic teaching.

Vygotsky also emphasised private speech — children's audible self-talk during problem-solving — as the precursor to internal verbal thought.

Chapter VIIIAttachment.

Bowlby's claim — radical at the time — was that infant attachment to a caregiver is an evolved system, not a learned by-product of feeding. The monotropy hypothesis (the infant forms one primary attachment) has been softened, but the core claim is robust.

The attachment system has four functions: proximity maintenance, safe haven (return to caregiver in distress), secure base (caregiver as platform from which to explore), separation distress.

Harlow's monkeys 1958

Harry Harlow's wire-mother / cloth-mother experiments at the University of Wisconsin showed infant rhesus monkeys preferred a non-nourishing cloth surrogate over a wire surrogate that provided milk. The result undermined the prevailing behaviorist view that attachment was a secondary drive learned through feeding. It also licensed Bowlby's ethological framing.

Chapter IXThe Strange Situation.

Mary Ainsworth's structured 20-minute laboratory procedure for assessing infant attachment style at 12–18 months. The infant and mother enter an unfamiliar room; a stranger enters; the mother leaves; the mother returns. The infant's behaviour — particularly at the reunion moment — is coded into one of three styles (a fourth was added later):

Secure (B) — about 60% of US samples. Distressed at separation, comforted by reunion. Caregivers tend to be responsive and consistent.

Anxious-Resistant (C) — about 10%. Distressed at separation, ambivalent at reunion (seeking and rejecting contact). Caregivers tend to be inconsistent.

Anxious-Avoidant (A) — about 20%. Apparently unconcerned at separation, avoidant at reunion. Caregivers tend to be unresponsive.

Disorganized (D) — added by Mary Main in 1986. About 10–15%. Inconsistent and contradictory behaviour. Often associated with abuse or unresolved parental trauma.

Chapter XAdult attachment.

Cindy Hazan and Phillip Shaver's 1987 Journal of Personality and Social Psychology paper extended attachment categories to adult romantic relationships. The popular two-dimensional model has gradually replaced the three-category one: attachment anxiety (worry about partner's availability) × attachment avoidance (discomfort with closeness). Secure adults are low on both; anxious-preoccupied are high anxiety, low avoidance; dismissive-avoidant are low anxiety, high avoidance; fearful-avoidant are high on both.

The popular literature ($30 million books like Levine and Heller's Attached, 2010) has propagated a simplified version. The empirical literature is more complicated: childhood-to-adult attachment continuity is moderate (about r=0.35), not high; adult attachment is partly about current relationship quality, partly about a working model carried from childhood.

The Adult Attachment Interview (AAI) — 90-minute structured interview about childhood relationships — remains the rigorous measurement tool.

Chapter XITheory of mind.

The capacity to attribute mental states (beliefs, desires, intentions) to oneself and others. The classic Sally-Anne false-belief task: Sally puts a marble in a basket and leaves the room. Anne moves the marble to a box. Sally returns. Where will Sally look? A child who passes says "the basket" — recognising that Sally's belief about the marble's location does not match reality.

Most typically developing children pass false-belief tasks at age 4–5. Younger children fail, as do many autistic individuals. This was the empirical foundation for Simon Baron-Cohen's "mindblindness" hypothesis (1995) about autism — controversial in the autism community now (Damian Milton's "double-empathy" critique argues the deficit is in mutual understanding, not one-sided), but methodologically generative.

Implicit theory-of-mind tests (Onishi & Baillargeon 2005) using violation-of-expectation suggest some understanding of false belief in 15-month-olds — far earlier than the explicit task suggests. The difference reveals an explicit/implicit dissociation.

Chapter XIILanguage acquisition.

Children acquire language with a rapidity and uniformity that Skinner's behaviourist account (Verbal Behavior, 1957) could not explain. Chomsky's 1959 review of Skinner argued for an innate language faculty; the poverty of the stimulus argument has been a foundational claim of cognitive science since.

Critical period

Eric Lenneberg (1967) argued that language must be acquired in childhood (roughly before puberty) for full mastery. The natural experiments — Genie, the abused child discovered at 13 in 1970, who never acquired full grammar; Chelsea, a deaf woman first taught language at 32, who acquired vocabulary but not grammar — support a critical period for syntax. Second-language learners who start after about age 13 rarely achieve native-like grammar.

Bilingualism

Earlier the better. Children who are exposed to two languages from infancy generally master both. Some evidence (Bialystok) of cognitive-control advantages; the size of the effect has been disputed in recent meta-analyses.

Chapter XIIIErikson's eight stages.

Erik Erikson proposed eight psychosocial stages, each organised around a central conflict whose resolution shapes adult personality.

1. Trust vs. Mistrust (0–1). Whether the world is dependable.
2. Autonomy vs. Shame and Doubt (1–3). Self-control and willpower.
3. Initiative vs. Guilt (3–6). Acting on one's own.
4. Industry vs. Inferiority (6–11). Competence and skill-building.
5. Identity vs. Role Confusion (12–18). Who one is becoming.
6. Intimacy vs. Isolation (early adult). Capacity for close relationship.
7. Generativity vs. Stagnation (mid adult). Contributing to the next generation.
8. Integrity vs. Despair (late adult). Reflection on a life lived.

The stages are taught more often than they are tested. Empirical operationalisation — particularly via James Marcia's identity status framework (1966) — has produced a substantial research literature on the fifth stage (identity formation) at least.

Chapter XIVKohlberg and Gilligan.

Lawrence Kohlberg's six stages of moral development (1958 dissertation; published through the 1960s and 70s). Three levels — pre-conventional, conventional, post-conventional — with two stages each. Children begin with self-interest (avoiding punishment), progress to social-conformity reasoning, and (some adults) reach principled reasoning grounded in universal ethical principles.

Carol Gilligan's In a Different Voice (1982) — feminist critique. Kohlberg's research subjects were boys; the framework privileged abstract justice-based reasoning over the relational, care-based reasoning that, Gilligan argued, women were more likely to express. The "ethics of care" tradition followed.

The Kohlberg-Gilligan debate has not resolved. Empirical follow-up has not strongly supported large gender differences in the type of moral reasoning. Both frameworks have been incorporated into broader contemporary moral psychology (Jonathan Haidt's The Righteous Mind, 2012, and the moral foundations theory).

Chapter XVThe marshmallow studies.

Walter Mischel's marshmallow studies began at the Bing Nursery School at Stanford in the late 1960s. Four-year-olds were offered one marshmallow now or two if they could wait while the researcher left the room. Some held out; some did not.

The famous result: the children who delayed gratification at age 4 scored higher on the SAT a decade later, were less likely to use drugs, had lower BMIs in middle age. The marshmallow test became a celebrity in popular psychology. The 2014 popular book The Marshmallow Test codified the strong claim.

The 2018 replication (Tyler Watts and colleagues, Psychological Science) used a much larger sample (918 vs Mischel's original ~90) and crucially controlled for family socioeconomic status and home environment. The original effect dropped to near non-significance. Family resources, not innate willpower, predict both delay-of-gratification and adult outcomes.

The methodological lesson is permanent. The cultural memory of the original study has been slow to update.

Chapter XVIAdolescence.

Laurence Steinberg's Age of Opportunity (2014) and the underlying neuroscience — the dual systems model. Two brain systems mature on different timelines: the limbic/reward system (which drives sensation-seeking) matures around puberty; the prefrontal cortex (which regulates impulse control) does not finish maturing until the early twenties. The mismatch produces adolescent risk-taking.

Identity formation: James Marcia's identity-status framework (1966). Two dimensions — exploration and commitment — yield four statuses: achievement (explored, committed), moratorium (exploring, not yet committed), foreclosure (committed without exploring), diffusion (neither).

Jeffrey Arnett's concept of emerging adulthood (2000) — the period 18–29 in industrialised societies, characterised by identity exploration, instability, self-focus, feeling in-between, and a sense of possibility. The construct is now standard in lifespan textbooks.

Chapter XVIIThe smartphone debate.

Jean Twenge's iGen (2017) and Generations (2023) — generational research using the Monitoring the Future and other large datasets. Her central claim: cohorts born after 1995 (Generation Z) show sharply elevated rates of depression, anxiety, suicide, and lower rates of in-person socialising starting around 2012, coincident with the saturation of smartphones in adolescent life.

Jonathan Haidt's The Anxious Generation (2024) extended the case, attributing the change to "the great rewiring of childhood" via screens and the loss of unsupervised play.

The empirical critique: Andrew Przybylski and Amy Orben, using specification-curve analysis, find that screen-time effects on adolescent well-being are small (R² ≈ 0.4%) — comparable to eating potatoes. The cohort-level rises in adolescent mental-health problems are real, but attribution to smartphones specifically is contested.

The debate matters because it drives policy (Australia's under-16 social media ban, 2024).

Chapter XVIIICognition across adulthood.

Adult cognitive trajectories are not uniformly downhill. Different abilities follow different curves.

Processing speed peaks in the early 20s and declines steadily. Working memory peaks in the late 20s and declines slowly. Vocabulary rises until roughly 65 and remains stable. Reading comprehension tends to peak in the 50s. Wisdom-related knowledge (Baltes and Staudinger) — knowledge about the conduct of life — appears stable or slightly increasing across most of adulthood.

K. Warner Schaie's Seattle Longitudinal Study (started 1956, still running): tracked the same individuals across decades. Most cognitive abilities show modest declines beginning in the mid-50s, with significant individual variation. Education, exercise, and social engagement all predict slower decline.

The Flynn effect (rising IQ across 20th-century cohorts) — see the Psychometrics deck — complicates cross-sectional age comparisons. The grandfather may have been smarter relative to his cohort than his grandson is to his.

Chapter XIXAging.

Three findings have shaped the gerontological literature.

Cognitive reserve

Two people can have the same Alzheimer's pathology in their brain at autopsy and have shown very different clinical pictures in life. The capacity to maintain function despite pathology is cognitive reserve — built up through education, occupational complexity, and lifelong cognitive engagement.

Socioemotional selectivity

Laura Carstensen's theory (1992): as time horizons shorten, older adults prioritise emotionally meaningful goals over information-gathering ones. The empirical correlate: older adults have higher average daily positive affect than younger adults, despite worse health.

The longevity question

The "Blue Zones" literature (Buettner) makes claims about longevity hotspots. Some have replicated (Sardinia, Okinawa); others appear to be artifacts of poor record-keeping (Saul Justin Newman's 2024 Ig Nobel-winning paper). Diet, exercise, social embeddedness all predict longevity; the magnitudes are smaller than popular accounts suggest.

Chapter XXThe WEIRD problem.

Joseph Henrich, Steven Heine, and Ara Norenzayan's 2010 paper The weirdest people in the world? showed that most behavioral-science research used a slice of humanity (Western university students) that was unrepresentative on most psychological dimensions tested.

For developmental psychology specifically: parenting practices, attachment-style distributions, the timing of milestones, the amount and form of infant carrying, sleep arrangements, weaning age, and the structure of moral reasoning all vary across cultures more than Western textbooks tend to convey.

The Strange Situation, when administered cross-culturally, produces different distributional profiles. Northern German samples show higher rates of avoidant attachment than American; Israeli kibbutz samples in the 1980s showed higher rates of resistant. The classifications are stable; their interpretation as universal-vs-pathological needs cultural calibration.

The contemporary developmental field has begun systematically expanding its sample base (the ManyBabies collaborative, 2017–) but most textbook claims still rest on WEIRD evidence.