A two-year-old hides her face behind her hands and announces that you can't see her. A four-year-old watches you look for your keys in the wrong drawer and, quietly, knows you are looking in the wrong place. A sixteen-year-old does something spectacularly risky when friends are watching and something sensible when alone. A seventy-year-old forgets a name but explains the situation better than anyone else in the room.
These are not separate mysteries. They are four points on one long road, and developmental psychology is the study of the road: what changes, in what order, why, and how much it depends on the child, the family and the century. A graduate of this course stops asking "is it nature or nurture?" and starts asking "which differences, in which population, at which age, under which conditions?".
Jean Piaget (1896 to 1980) watched his own children and Swiss schoolchildren and concluded that they are not small adults with less knowledge. They think in a different way, and the way changes in steps. His four stages: sensorimotor (birth to about 2, knowing through action), preoperational (about 2 to 7, symbols and pretend but thinking tied to appearances), concrete operational (about 7 to 11, logic about real things) and formal operational (from about 11 or 12, abstract and hypothetical thought).
Two ideas have survived best: children actively build understanding rather than absorbing it, and they can be systematically wrong in ways that make sense from the inside. His best-known test is conservation. Pour water from a short wide glass into a tall thin one and a five-year-old says there is now more. Piaget read this as a lack of the logic of reversibility.
Later researchers argued he underestimated young children, because his tasks were hard to follow. In a 1974 study, James McGarrigle and Margaret Donaldson had a "naughty teddy" mess up the arrangement by accident rather than the experimenter doing it on purpose, and many more children then gave the right answer. In the same spirit, Martin Hughes's 1975 police-doll task showed that children as young as three and a half could work out where to hide a doll from a policeman, a task Piaget's "three mountains" test had said they could not do.
At the other end, researchers used looking time to ask what babies know. In the 1980s Renée Baillargeon and colleagues reported that infants as young as about three and a half to five months looked longer at an "impossible" event, such as a screen rotating through a hidden box, than at a possible one. They concluded that babies have early knowledge about objects. That claim is contested. Critics point out that looking longer may reflect novelty rather than surprise, that search tasks (reaching for a hidden toy) show later competence, and that some pupil-dilation and replication attempts, especially at 10 months, have not found the expected effect. The honest summary: infants have more early structure than Piaget thought, how much is still argued, and the stage boundaries are softer than the textbook diagram.
All constants are made up to show the logic. Looking time = 2 + 14·e−0.4n s on trial n. Test events: a possible one (the same as before) and an impossible one, which adds 6 s for an infant who tracks hidden objects, or for one who only notices that it looks different.
The toy shows why the debate is hard. Habituation shortens looking at a repeated event; a test that recovers looking is read as "this is surprising". But two different infants, one who understands objects and one who merely finds a new display interesting, can produce the same pattern. Good studies are designed to tell them apart.
Lev Vygotsky (1896 to 1934) died of tuberculosis at 37, and most of the English-speaking world met him decades later, through Mind in Society (1978). Where Piaget's child is a lone scientist, Vygotsky's child learns thinking tools by using them with others: language first used between people becomes private speech and then inner thought.
His best-known idea is the zone of proximal development: the gap between what a child can do alone and what they can do with a little help. Teaching works best inside that gap, not at the level already mastered nor far beyond reach. Wood, Bruner and Ross (1976) called the adult's changing support scaffolding: giving more help when the child struggles and less as they succeed.
Vygotsky's work is more a framework than a set of precise predictions, so it has been harder to test than Piaget's. It still shapes teaching practice across the world, from tutoring to the way many early-years programmes use guided play. The cross-cultural point: in many communities learning happens by children joining in adult work rather than in school-style lessons, a pattern Barbara Rogoff and others have documented.
John Bowlby, a British psychiatrist, argued in the 1950s and 60s that infants are built to seek closeness to a caregiver, and that this bond is not just about food. Harry Harlow's 1950s monkey experiments showed baby monkeys preferred a soft cloth surrogate to a wire one that held the milk. Mary Ainsworth, who had observed mothers and babies in Uganda, designed the Strange Situation (1970s): a short lab procedure in which a one-year-old sees a parent leave and return. Babies were sorted by their reaction at reunion: secure (upset perhaps, but comforted and soon back to play), avoidant (little visible distress, avoiding the parent on return), resistant or ambivalent (clingy and angry), and later disorganised.
In many studies, most babies are classed secure. But the rest of the picture differs by place. A 1988 meta-analysis by van IJzendoorn and Kroonenberg reported that the patterns shifted across countries; studies in northern Germany found more avoidant babies than American norms, and a study in Sapporo, Japan found more resistant ones. The disagreement is over what that means. One reading is that cultures raise babies differently (a German emphasis on early independence, Japanese babies rarely separated from their mothers, so the procedure stresses them more). Another is that the test, built in the US and the UK, does not measure the same thing everywhere. Both are taken seriously.
The strongest evidence about attachment and early care comes from the Romanian orphanages of the 1980s. The Bucharest Early Intervention Project (Nelson, Zeanah, Fox and colleagues, 2007) randomly assigned institutionalised children to foster care or to staying put; those moved earlier, especially before about two, did better on many measures. The English and Romanian Adoptees study (Michael Rutter and others) found that longer deprivation went with longer-lasting difficulties. The lesson is "sensitive period", not "critical window that slams shut": the earlier the good care, the better the recovery, and recovery is still possible later. Another caution: attachment style correlates with later outcomes, but a child's own temperament and the many things that change in life make the link far from destiny.
A theory of mind is the understanding that other people have beliefs, desires and knowledge that may differ from yours and from reality. The classic test is the false-belief task (Wimmer and Perner, 1983; the Sally–Anne version by Baron-Cohen, Leslie and Frith, 1985). Try it.
Adults answer "the basket" without a thought: Sally believes what she last saw. Many three-year-olds say "the box", because that is where the marble really is. Most children pass around four to five. Wellman, Cross and Watson's 2001 meta-analysis of 178 studies found the same shift from below chance to above chance across the many countries studied, though the age and order of steps differ: in a 2011 study (Shahaeian and colleagues) children in Iran and Australia learned some steps in different orders, an effect linked to how families talk about knowledge.
Some experiments report that babies as young as 15 months look longer at scenes that violate a false belief (Onishi and Baillargeon, 2005), but replication attempts have been mixed, and whether this reveals early understanding or something simpler is debated. It is also worth saying that theory-of-mind findings in autistic people are contested: the long-standing claim that autistic people lack a theory of mind has been challenged by researchers who describe a two-way difficulty in understanding across different minds, not a one-way deficit.
Adolescence is a long, normal rebuild. A widely cited 2018 paper (Sawyer and colleagues) argued that it now stretches from about 10 to 24, because puberty comes earlier and adult roles come later. The brain is reorganising: connections are pruned, and the prefrontal regions that help with planning and self-control keep maturing into the twenties. But "the brain isn't mature until 25" is a slogan, not a threshold; there's no age at which a switch flips, and the exact timing varies by region and by person.
One well-replicated finding is about when teenagers take risks. In a driving-game study (Gardner and Steinberg, 2005), adolescents took more risks when peers were watching than when alone, while adults did not change nearly as much. The usual reading, Laurence Steinberg's "dual-systems" account, is that a reward system matures before the control system. Critics argue the picture is more nuanced, since adolescents can reason well in calm conditions and risk-taking can also be exploration and learning. For more, see Why Teenagers Aren't Broken (02/03).
At the other end, ageing is not simply decline. Psychologists separate fluid abilities (quick reasoning, processing speed, which tend to fall from the twenties) from crystallised ones (vocabulary and knowledge, which keep growing). In 2015, Joshua Hartshorne and Laura Germine analysed large online samples and found that abilities peak at very different ages: processing speed around the late teens and early twenties, working memory in the late twenties to early thirties, and vocabulary much later, into the sixties or seventies. Their result is cross-sectional and from one research programme, so cohort differences blur it. Dementia is a disease, not normal ageing.
The bars show approximate peak ranges as summarised from Hartshorne and Germine (2015): speed 18 to 22, working memory 27 to 33, vocabulary 60 to 70. They are simplified ranges, not a curve, and the exact ages vary by task and generation.
"Is it genes or environment?" is a badly formed question, a bit like asking whether the area of a field comes from its length or its width. The better question is: in this population, how much of the difference between people goes with differences in their genes? That share is the heritability, and it has three properties people forget. It describes a population, not a person; "50% heritable" does not mean half of your height is from genes. It depends on the environment: it is higher where everyone has similar conditions, lower where conditions differ. And it is not fate: a trait can be strongly heritable and still change a lot when the environment changes, as the PKU metabolic disorder does with diet, or as average height has done over the last century (NCD-RisC's 2016 analysis found a gain of about 20 cm for South Korean women born between 1896 and 1996).
The numbers are real, though. A 2015 meta-analysis of fifty years of twin studies (Polderman and colleagues, 17,804 traits, about 14.5 million twin pairs) put the average heritability across traits at about 49%, with shared family environment usually small once genes are counted. Eric Turkheimer summarised the pattern as three laws: all human behavioural traits are heritable; the effect of being raised in the same family is smaller than the effect of genes; and a substantial part of the variation is not explained by genes or families. Critics of twin methods argue about their assumptions, and measured DNA often explains less than twin studies imply (the "missing heritability" puzzle).
Simulation of 3,000 identical-twin pairs and 3,000 fraternal pairs (identical twins share all genes, fraternal share half on average). The estimate is Falconer's: heritability = 2 × (identical correlation − fraternal correlation). A model, with the usual twin assumptions.
Slide the environment from "similar" to "very different" while leaving the genetic differences untouched. Heritability falls from about 0.9 to about 0.2. The genes did not weaken; the environment simply began to explain more of the gaps.
In 2010 Joseph Henrich, Steven Heine and Ara Norenzayan pointed out that most behavioural science samples come from people who are Western, Educated, Industrialised, Rich and Democratic (WEIRD), a small and unusual slice of humanity. In development this matters a lot. Sleeping alone, school-style questioning, "talking to" babies, and independence as the goal are cultural choices, not universals. Attachment categories vary, theory-of-mind steps vary in order, and the famous "marshmallow test", once thought to predict adult success, was shown in a larger, more diverse sample (Watts, Duncan and Quan, 2018) to be far weaker once family background was accounted for. What does appear widely is the basic outline: infants everywhere form attachments, children everywhere pass false-belief tasks in the preschool years, and adolescents everywhere seek peers. The details are cultural.
Minds are built in steps, but the steps are softer than the diagrams, and they depend on help from others and on the culture around them. Early care matters and later repair is possible. Genes and environment do not split the credit like a pie: heritability is a statement about differences within a population, and it moves when the environment does.
Try this: this week, watch a child (or remember being one) solve a problem with a little help. Notice what help worked, and how it faded as they got it. That is a zone of proximal development at work.
Sources. Piaget's stages and conservation (standard textbook accounts); McGarrigle & Donaldson 1974, Cognition 3; Hughes 1975 (doctoral thesis, Edinburgh) as reported in Donaldson 1978; Baillargeon, Spelke & Wasserman 1985; replication doubts: Pätzold & Liszkowski 2020 (PLoS ONE, pupillometry, 10-month-olds); Vygotsky 1978 (Mind in Society); Wood, Bruner & Ross 1976; Bowlby (Attachment and Loss, 1969 on); Harlow 1958; Ainsworth et al. 1978; van IJzendoorn & Kroonenberg 1988, Child Development 59; Nelson et al. 2007, Science 318; Rutter and the English and Romanian Adoptees study; Wimmer & Perner 1983; Baron-Cohen, Leslie & Frith 1985; Wellman, Cross & Watson 2001, Child Development 72 (178 studies); Shahaeian et al. 2011, Developmental Psychology 47; Onishi & Baillargeon 2005; Sawyer et al. 2018, Lancet Child & Adolescent Health 2; Gardner & Steinberg 2005, Developmental Psychology 41; Hartshorne & Germine 2015, Psychological Science 26; Polderman et al. 2015, Nature Genetics 47 (17,804 traits, about 14.5 million twin pairs, average heritability 49%); Turkheimer 2000, Current Directions in Psychological Science 9; NCD Risc 2016, eLife 5:e13410 (20.2 cm for South Korean women, 1896 to 1996 birth cohorts); Henrich, Heine & Norenzayan 2010, Behavioral and Brain Sciences 33; Watts, Duncan & Quan 2018, Psychological Science 29. Interactive numbers computed in node: twin simulation with 3,000 pairs each, genetic SD 1: similar environments (individual SD 0.3) gives heritability estimate 0.89 against a true 0.92; very different (SD 2) 0.19 against 0.20; the middle preset 0.44 against 0.44.
This describes research, not medical or parenting advice. If you are worried about a child's development, talk to a paediatrician or local service; if you are struggling yourself, findahelpline.com lists free helplines by country.