Developmental psychology studies how people grow, think, and change from birth through old age, and it is best understood through the experiments that built the field. The peek-a-boo game that delights a six-month-old, the preschooler who insists that a taller glass holds “more” juice, the teenager who makes a risky decision despite knowing better: each of these everyday moments maps onto a well-studied line of research with specific experiments behind it. What follows is a walk through the most striking examples, from classic paradigms to recent replications that have complicated the picture.
Object Permanence and What Babies Already Know
One of the earliest concepts any developmental psychology course covers is object permanence, the understanding that something still exists even when you can no longer see it. Jean Piaget argued that infants younger than about eight months lack this understanding entirely, which is why hiding a toy under a blanket seems to make it vanish for them. But more recent research using eye-tracking and looking-time methods has pushed the timeline earlier. In one study, infants as young as three to five months were trained to expect an experimenter to retrieve an object from behind a screen. When the object was hidden behind one of two opaque screens, infants looked toward the correct location after a two-second delay, though they lost track after eight seconds. The finding suggests that some grasp of object permanence exists far earlier than Piaget proposed, even if the infant’s memory for the hidden object is brief and fragile.1PubMed. Young infants’ expectations about hidden objects
This example illustrates something important about developmental psychology as a whole: the field often revises its own milestones. A finding established decades ago gets tested with newer methods, and the age at which a skill “emerges” shifts. The underlying skill is real, but how and when it appears depends heavily on how you ask the question.
Piaget’s Conservation Tasks
Another Piagetian classic is the conservation task. Pour water from a short, wide glass into a tall, narrow one in front of a four-year-old, and the child will typically say the tall glass has more water, even though they watched you pour the same amount. Piaget interpreted this as evidence that young children cannot yet mentally reverse an action or hold two dimensions in mind at once. Children usually pass conservation tasks around age six or seven, a shift Piaget attributed to entering the “concrete operational” stage of thinking.
A more recent study tested whether children could be nudged past this hurdle by letting them physically handle the materials rather than just watching an adult pour. The results showed that active manipulation helped children recognize that the quantity stayed the same at a higher rate than passive observation did. In other words, doing rather than watching gave the children’s thinking a boost, suggesting that bodily experience plays a larger role in cognitive development than a purely abstract model would predict.2PubMed Central. Embodied Action Improves Cognition in Children: Evidence from a Study Based on Piagetian Conservation Tasks
Theory of Mind and the Sally-Anne Test
Around age four, most children gain the ability to understand that other people can hold beliefs that are different from their own, including beliefs that are wrong. Researchers call this “theory of mind,” and the most famous way to test it is the Sally-Anne task. The setup is simple: a child watches a puppet named Sally place a marble in a basket, then leave the room. While Sally is gone, another puppet named Anne moves the marble to a box. The child is then asked where Sally will look for the marble when she comes back. Children who have developed theory of mind answer “the basket,” because they understand Sally still believes the marble is there. Younger children tend to answer “the box,” because they know that is where the marble actually is and cannot separate their own knowledge from Sally’s.
The Sally-Anne test has been used extensively to study theory-of-mind understanding across populations, including children with autism, who often find the task harder or pass it later.3PubMed. The Sally-Anne test: an interactional analysis of a dyadic assessment Brain-imaging work with adults has used the same scenario to map which regions of the brain activate during false-belief reasoning compared with true-belief reasoning, showing that thinking about what someone else mistakenly believes recruits a distinct neural network.4PubMed. Neural correlates of true and false belief reasoning Theory of mind is not a one-time achievement, either. More advanced forms, like understanding sarcasm, white lies, and double bluffs, continue developing well into adolescence.
Attachment and the Strange Situation
If you have ever seen a toddler melt down when a parent leaves and then calm quickly once the parent returns, you have watched attachment behavior in real time. Mary Ainsworth formalized the study of this bond through a lab procedure called the Strange Situation. A caregiver and child enter an unfamiliar room with toys. Through a series of separations and reunions, including the introduction of a stranger, researchers observe how the child reacts. The procedure identifies four major patterns of infant-parent attachment: secure, insecure-avoidant, insecure-resistant, and insecure-disorganized.5PubMed Central. Infant-parent attachment: Definition, types, antecedents, measurement and outcome
Securely attached children tend to explore the room freely, get upset when the caregiver leaves, and greet them warmly on return. Avoidant children show little distress and may ignore the caregiver’s return. Resistant children are clingy before the separation and difficult to soothe afterward. Disorganized children show contradictory behaviors, like approaching the caregiver while looking away. Decades of follow-up research have linked these early patterns to later social functioning, peer relationships, and even romantic relationship styles in adulthood, though the links are probabilistic rather than deterministic. A secure start helps, but it does not lock in a lifelong outcome, and an insecure start does not doom anyone.
The Bobo Doll and Learning by Watching
Albert Bandura’s Bobo doll experiments from the early 1960s are probably the single most recognizable study in all of developmental psychology. Children watched an adult model either attack an inflatable Bobo doll, punching, kicking, and hitting it with a mallet, or interact calmly with other toys. When the children were then placed in a room with the same doll, those who had watched the aggressive model were far more likely to imitate the aggressive behavior. The children learned and reproduced the aggression without anyone rewarding them for it. Follow-up experiments showed that the consequences the model experienced mattered too: children who saw the model get praised for aggression imitated more, while those who saw the model punished imitated less, though they could still reproduce the behavior when asked.6PubMed Central. Albert Bandura’s experiments on aggression modeling in children: A psychoanalytic critique
The Bobo doll work gave rise to social learning theory, the idea that children acquire new behaviors not just through their own trial and error but by observing others. The implications showed up immediately in debates about children and media violence and have stayed relevant through every new screen technology since.
The Marshmallow Test, Revisited
In the original marshmallow studies from the late 1960s and 1970s, Walter Mischel offered preschoolers a choice: eat one marshmallow now, or wait and get two. Children who waited longer went on, years later, to score higher on standardized tests and were described as more socially competent. The study became a cultural touchstone, often cited as proof that self-control in early childhood predicts lifelong success.
A large-scale replication told a more complicated story. Focusing on children whose mothers had not completed college, the researchers found that an extra minute of waiting at age four predicted roughly one tenth of a standard deviation in academic achievement at age fifteen. That link was only about half the size reported in the original studies, and it shrank by two thirds once the researchers accounted for family background, early cognitive ability, and the home environment. Most of the variation in later achievement came simply from whether a child could wait at least twenty seconds, not from the full range of delay times. Associations with behavioral outcomes were even weaker and rarely reached statistical significance.7PubMed Central. Revisiting the Marshmallow Test: A Conceptual Replication Investigating Links Between Early Delay of Gratification and Later Outcomes
The takeaway is not that self-regulation is unimportant, but that a single lab task at age four captures far less of a child’s future than the popular narrative implies. Family resources, cognitive ability, and the environment a child grows up in do much of the heavy lifting.
Language Development and “Goed” Instead of “Went”
If you have ever heard a toddler say “I goed outside” or “she runned fast,” you have witnessed a developmental phenomenon called overregularization. Children initially learn a handful of irregular past-tense forms correctly, saying “went” and “ran” because they are imitating what they hear. Then, as they internalize the general rule that past tense means adding “-ed,” they start applying it everywhere, including to words that do not follow the rule. The result is errors like “goed,” “comed,” or “bringed.” After a period of these mistakes, children eventually sort out which verbs are regular and which are irregular, and the errors fade.
The pattern is called U-shaped development: performance starts correct, dips into errors, then recovers. Researchers have proposed that children use both memory, pulling the correct irregular form from what they have heard, and a rule-based process that generates a regular past tense for any verb. When memory retrieval fails, the rule fills in, and the overregularized form slips out. Young children’s memory traces are simply not robust enough to guarantee perfect retrieval every time.8PubMed. Overregularization in language acquisition
Language development also relies on social interaction. Joint attention, the moment when a parent and child are both focused on the same object or event, has long been considered a key driver of early word learning. Responding to a caregiver’s gaze or point toward something between six and eighteen months is positively linked to vocabulary growth.9Journal of Applied Developmental Psychology. Responding to Joint Attention Across the 6- Through 24-Month Age Period and Early Language Acquisition However, the relationship is not as simple as “more joint attention equals more words.” A critical review of the evidence noted that word learning can occur without joint attention in typical development, in children with autism, and in children with Williams syndrome, while joint attention can occur without commensurate vocabulary growth in children with Down syndrome.10PubMed Central. Joint Attention and Vocabulary Development: A Critical Look Joint attention matters, but calling it necessary and sufficient overstates the case.
Do Babies Know Right from Wrong?
A 2007 study by Hamlin, Wynn, and Bloom made headlines with a striking claim: infants as young as six months can evaluate other people’s actions as helpful or harmful. In their “hill paradigm,” a puppet tried to climb a steep hill. A helper puppet pushed it up, while a hinderer puppet pushed it down. After watching these scenes, both six- and ten-month-old infants preferentially reached for the helper puppet.11PubMed Central. The case for social evaluation in preverbal infants: gazing toward one’s goal drives infants’ preferences for Helpers over Hinderers in the hill paradigm
The finding was enormously influential, but recent replication attempts have struggled to reproduce it. One replication with 32 fifteen-month-olds found that exactly half reached for the helper and half for the hinderer, no preference at all.12Royal Society Open Science. Do 15-month-old infants prefer helpers? A replication of Hamlin et al. (2007) A large-scale, multi-lab coordinated replication involving many more infants found that only about 49% preferred helpers in the social condition and about 56% preferred the upward-pushing character in a nonsocial control condition; neither result differed from chance.13PubMed. Infants’ Social Evaluation of Helpers and Hinderers: A Large-Scale, Multi-Lab, Coordinated Replication Study The researchers concluded that the effect is weaker, absent, or emerges later than the original study suggested. This does not mean babies have no moral sense, but it does mean the hill paradigm may not be the clean demonstration it was once thought to be.
The Adolescent Brain and Why Teens Take Risks
Parents often wonder why a teenager can articulate exactly why texting while driving is dangerous and then do it anyway. The neuroscience of adolescent development offers a partial explanation. The prefrontal cortex, which supports planning, impulse control, and weighing consequences, is among the last brain regions to fully mature, typically not finishing development until the mid-twenties. Research using brain imaging during tasks that require cognitive control has found that adolescents show less activity in the lateral prefrontal cortex compared with adults, particularly in regions associated with proactive control, the kind of top-down regulation that lets you prepare to resist a temptation before it appears.14PLOS ONE. Cognitive Control in Adolescence: Neural Underpinnings and Relation to Self-Report Behaviors
Adolescents who showed greater activation in these prefrontal regions also reported better impulse control, more foresight, and stronger resistance to peer pressure in their daily lives. This suggests a real-world link between brain maturation and the kinds of decisions teens make. Reactive control, the ability to stop yourself after a problem has already started, develops earlier. Proactive control, planning ahead to avoid the problem, comes later. That mismatch helps explain why teens can be perfectly rational in a calm classroom discussion and then impulsive in a car full of friends.
Scaffolding and the Zone of Proximal Development
Lev Vygotsky’s concept of the zone of proximal development describes the gap between what a child can do alone and what they can do with help from someone more skilled. Scaffolding, the structured assistance a teacher or caregiver provides, is designed to bridge that gap. A parent who steadies a bicycle while a child pedals, then gradually lets go, is scaffolding. Schools use the same principle when a teacher models a problem-solving strategy, guides students through several examples, and then gradually withdraws support.
Getting the level of scaffolding right matters. Research has found that students who receive too little scaffolding may fail to acquire the target skill at all, while students who receive too much may become dependent on the support and struggle when it is removed.15Lecture Notes in Civil Engineering. The Impact of the Zone of Proximal Development Concept (Scaffolding) on the Students Problem Solving Skills and Learning Outcomes The sweet spot is temporary help that fades as the child’s own competence grows, a principle that applies from toddlerhood through adult education.
Early Social Attention and Autism
Developmental psychology also provides tools for identifying children who may follow atypical developmental paths. Research on infants who have an elevated likelihood of autism, typically because they have an older sibling on the spectrum, has found measurable differences in how these babies direct their attention to social versus nonsocial information as they grow.16PubMed Central. Infant Social Attention Associated with Elevated Likelihood for Autism Spectrum Disorder: A Multi-Method Comparison For instance, high-risk infants tend to rely more on combined head-and-eye cues when following someone’s gaze, while low-risk infants follow gaze equally well from eye movement alone.17PubMed Central. Altered gaze following during live interaction in infants at risk for autism: an eye tracking study
A meta-analysis of eye-tracking studies found that dynamic stimuli and socially relevant regions, like the target of a character’s shared focus, reliably distinguished high-risk infants who later received an autism diagnosis from those who did not.18PubMed. Evaluating the validity of eye-tracking tasks and stimuli in detecting high-risk infants later diagnosed with autism: A meta-analysis These kinds of measures could eventually contribute to earlier screening, which matters because early intervention tends to produce better outcomes. The research does not mean that unusual gaze patterns in infancy guarantee a diagnosis, but tracking these behaviors over time gives clinicians useful information.
Screen Time and Early Cognitive Development
One of the most practically relevant questions in modern developmental psychology involves screens. A longitudinal study of children in China found that children whose screen exposure increased early in life showed compromised working memory and processing speed compared with children who had lower screen-time trajectories. The researchers attributed this to the passive nature of screen-based stimulation during a period when the brain is building the networks that support cognitive control.19JAMA Pediatrics. Association Between Screen Time Trajectory and Early Childhood Development in Children in China A Brazilian study found lower cognitive scores in children exposed to two or more hours of screen time per day, though this effect existed alongside influences from maternal education and child sex.20PubMed. Screen time: Implications for early childhood cognitive development
A Canadian study of preschoolers added a useful nuance: higher total screen time was linked to lower working memory, but screen time was not associated with expressive vocabulary. Children who met the guideline of no more than one hour per day were over three times as likely to have higher working memory capacity.21Paediatrics & Child Health. Associations between screen time and cognitive development in preschoolers The emerging picture is that screens appear to affect some cognitive skills more than others, with working memory being more vulnerable than language in this age group. None of this means a toddler who watches a cartoon is damaged. But the convergence across multiple studies and countries points toward keeping screen exposure moderate during the first few years of life.
Culture Shapes the Developmental Path
Most of the classic experiments described above were designed and conducted with children from Western, middle-class families. Developmental milestones are often presented as universal, but cross-cultural research tells a more nuanced story. A systematic review found that the timing of milestones varies across cultures: some communities show precocity in motor development and self-recognition, while others show different trajectories in attachment relationships, empathy, and autobiographical memory. The same behavioral pattern can serve different developmental functions depending on the cultural context, and sometimes contradictory behaviors lead to the same outcomes across cultures.22PubMed. Culture and Development: A Systematic Relationship
Two broad developmental pathways have been described: one typical of Western middle-class families, where the focus is on individual agency and self-expression, and another characteristic of rural, traditionally living farming communities, where social connectedness and responsibility take priority. Infants in these different settings have different social experiences with caregivers and are regulated in different ways from birth. The Western middle-class pattern treats socioemotional development as something that supports the self, while the rural farming pattern treats the self as something embedded in social obligation.23Annual Review of Developmental Psychology. Children’s Socioemotional Development Across Cultures Neither pathway is better or worse. But treating one as the default and the other as a deviation distorts the science.
Why You Cannot Remember Being a Baby
Most adults have virtually no memories from before age three or so, a phenomenon called infantile amnesia. The explanation appears to be biological rather than psychological. During infancy, the hippocampus is generating new neurons at a very high rate. Research in mice has shown that this rapid neurogenesis, while essential for building the brain, actually degrades memories that have already been formed. When researchers experimentally increased neurogenesis after a memory had been created, forgetting accelerated. When they decreased neurogenesis in infant mice, forgetting slowed down. In species where most hippocampal neurons are generated before birth, like guinea pigs, infant forgetting does not occur in the same way, but artificially boosting postnatal neurogenesis induced it.24PubMed. Hippocampal neurogenesis regulates forgetting during adulthood and infancy The very process that builds a young brain also overwrites its earliest recordings.
Development Does Not Stop at Eighteen
Developmental psychology has increasingly expanded its scope beyond childhood and adolescence. Socioemotional selectivity theory describes a shift in motivation that occurs as people perceive their remaining time as limited, whether due to aging, serious illness, or an anticipated life change like a geographic move. When time feels open-ended, people prioritize exploration and information gathering. When it feels constrained, they prioritize emotionally meaningful goals, like spending time with close relationships rather than making new acquaintances.25PubMed Central. Socioemotional Selectivity Theory: The Role of Perceived Endings in Human Motivation
This shift has measurable effects on memory and attention. When people are prompted to think about a limited future, both younger and older adults show increased positivity in what they recall, remembering more positive information and less negative information compared with a neutral condition.26PubMed Central. Thinking about a limited future enhances the positivity of younger and older adults’ recall: Support for socioemotional selectivity theory The effect is not exclusive to old age. Young adults facing a deadline or a major transition show similar patterns, which is why the theory is considered a lifespan framework rather than a theory of aging alone.27PubMed. Motivational changes in response to blocked goals and foreshortened time: testing alternatives to socioemotional selectivity theory The popular image of older adults as out of touch or declining misses the fact that they are actively reprioritizing, and the research suggests this shift serves a genuine emotional function rather than reflecting a loss of capacity.