Developmental Psychology — Research Methods, Cognitive, Socioemotional and Language Development, Adolescence and Adulthood

Three questions developmental psychology asks

Developmental psychology studies how and why people change from birth to death. The field has long revolved around three questions.

• Nature and nurture: which matters more, genes or environment? A meta-analysis of some 2,700 twin studies spanning 50 years found an average heritability of about 49% across human traits (Polderman et al., 2015). Genes and environment contribute roughly equally, and they influence each other.
• Continuity and discontinuity: does development accumulate gradually, or jump in stages like a staircase?
• Critical and sensitive periods: are there abilities that can never be learned if a certain window is missed? Today researchers more often speak of sensitive periods, windows when learning comes most easily, rather than strict “critical periods.”

How development is studied

To see change with age, research design matters.

Design	Method	Strength	Limitation
Cross-sectional	Compare several age groups at once	Fast and cheap	Age effects mix with cohort effects (generational differences)
Longitudinal	Follow the same people over time	Shows individual change directly	Cost, dropout, practice effects
Sequential	Follow several cohorts at once	Separates age and cohort effects	Most complex and expensive

For example, if you compare intelligence test scores of people in their 20s and 70s at a single point in time, the older group tends to score lower. But part of that gap may come not from age but from generational differences in schooling. Schaie’s Seattle Longitudinal Study used a sequential design to separate the two and showed that many intellectual abilities hold up relatively well into the 60s (Schaie, 2005).

Cognitive development: Piaget and beyond

Piaget’s four stages

Piaget held that children grow by building schemas through interaction with the world, fitting new experiences into existing schemas (assimilation) and changing schemas to fit experience (accommodation).

Stage	Approx. age	Key achievements
Sensorimotor	0–2	Object permanence; the beginnings of representation
Preoperational	2–7	Explosion of symbols and language; egocentrism; lack of conservation
Concrete operational	7–11	Conservation, classification, logic about concrete objects
Formal operational	11+	Hypothetico-deductive reasoning, abstract thought

Critiques of Piaget

• He underestimated infants: using the violation-of-expectation paradigm — infants look longer at surprising events — Baillargeon (1987) reported that 3- to 4-month-olds seem to expect hidden objects to continue to exist. Whether looking time can be read as “understanding” remains debated.
• Stages are less clear-cut than proposed: the same child can behave as if in different stages depending on the task, so modern research tends to see development as domain-specific and gradual.
• The role of culture and schooling: formal operations are not a stage every adult reaches automatically; they depend heavily on educational experience.

Vygotsky: thinking that grows in society

Vygotsky argued that cognitive development begins in social interaction. He called the range of tasks a child cannot do alone but can do with help the zone of proximal development (ZPD). Wood, Bruner and Ross (1976) named the process of adults fitting their help to the child’s level and gradually withdrawing it scaffolding.

Theory of mind: others think differently from me

Theory of mind is the understanding that others can hold beliefs, desires and intentions different from one’s own. In Wimmer and Perner’s (1983) false-belief task, many children around ages 4–5 became able to answer, “They don’t know, so they’ll look in the wrong place.” Baron-Cohen, Leslie and Frith (1985) used the “Sally-Anne task” to study theory of mind in autistic children.

Socioemotional development: attachment and temperament

In Harlow’s (1958) experiments with infant monkeys, the monkeys clung far longer to a soft cloth mother than to a wire mother that provided milk — evidence that attachment is not explained by feeding alone. Bowlby’s and Ainsworth’s attachment theory and research on adult attachment are covered in detail in the article on attachment theory.

Babies differ in temperament from birth. In the New York Longitudinal Study, Thomas and Chess (1977) grouped infants as “easy,” “difficult” and “slow-to-warm-up.” Kagan showed that behavioural inhibition — shrinking back from the unfamiliar — persists to a degree but changes with parenting and experience. What matters is the goodness of fit between temperament and environment.

Language development

Babies progress from babbling (around 6 months) to first words (around 12 months) and two-word combinations (18–24 months), after which vocabulary grows explosively.

• Nativism: seeing children master complex grammar from limited input, Chomsky argued that humans have an innate capacity for language.
• Usage-based theory: Tomasello (2003) explained that children build grammar by reading others’ intentions and extracting patterns from expressions they hear often.
• Evidence for sensitive periods: Johnson and Newport (1989) found that the younger immigrants were when they arrived in the US, the higher their English grammar judgement scores; those who arrived after puberty scored lower and varied more.

Adolescence: identity and the changing brain

Erikson saw the central task of adolescence as identity versus role confusion. Marcia (1966) classified identity status into four types depending on the presence of exploration and commitment.

	Committed	Not committed
Explored	Identity achievement	Moratorium
Not explored	Foreclosure (a path set by others)	Identity diffusion

The dual systems model is often invoked to explain why adolescents take more risks: a reward-sensitive system becomes highly active around puberty, while the prefrontal system that regulates impulses matures slowly into the mid-20s (Steinberg, 2008). Risky choices rise especially when peers are watching. The model has been criticised as a simplification, though, so interpretations that treat teenagers as “helpless because their brains aren’t finished” call for caution.

Adulthood and old age

Does intelligence decline with age?

Horn and Cattell (1967) divided intelligence into two kinds.

• Fluid intelligence: the ability to solve novel problems quickly. It peaks in the early-to-mid 20s and declines gradually.
• Crystallised intelligence: accumulated knowledge and vocabulary. It is maintained or even grows into later middle age.

Why people get happier with age

According to Carstensen’s (Carstensen et al., 1999) socioemotional selectivity theory, the shorter people feel their remaining time is, the more they focus on emotionally meaningful relationships rather than gathering new information. Older adults may therefore show a positivity effect, attending to and remembering positive more than negative information (Mather & Carstensen, 2005). This is considered one reason emotional well-being in later life is higher than many expect.

Check questions

Question 1. In 2026, a researcher gathers 20-, 50- and 80-year-olds and compares their digital skills; the older the group, the lower the score. What is wrong with concluding that “digital skills decline with age”?

Answer. This is a cross-sectional study, so cohort effects are mixed in. The 80-year-olds grew up without digital devices, so the gap may reflect differences in experience rather than ageing. A longitudinal or sequential design is needed.

Question 2. Four-year-old Minsu watched his mother move chocolate from a drawer to a box. Asked where his younger brother, who didn’t see this, will look for the chocolate, Minsu answers “the drawer.” What ability has he shown?

Answer. He understood that his brother’s belief can differ from reality. This false-belief understanding is a key marker of theory of mind.

Question 3. Which kind of task is a translator in their 70s more likely to do better than someone in their 20s: one relying on fluid or crystallised intelligence?

Answer. A task drawing on vocabulary and background knowledge. That depends on crystallised intelligence, which is maintained or can grow with age.

---

References

• Polderman, T. J. C., et al. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics, 47(7), 702–709.
• Schaie, K. W. (2005). Developmental Influences on Adult Intelligence: The Seattle Longitudinal Study. Oxford University Press.
• Piaget, J. (1952). The Origins of Intelligence in Children. International Universities Press.
• Baillargeon, R. (1987). Object permanence in 3½- and 4½-month-old infants. Developmental Psychology, 23(5), 655–664.
• Vygotsky, L. S. (1978). Mind in Society: The Development of Higher Psychological Processes. Harvard University Press.
• Wood, D., Bruner, J. S., & Ross, G. (1976). The role of tutoring in problem solving. Journal of Child Psychology and Psychiatry, 17(2), 89–100.
• Wimmer, H., & Perner, J. (1983). Beliefs about beliefs: Representation and constraining function of wrong beliefs in young children’s understanding of deception. Cognition, 13(1), 103–128.
• Baron-Cohen, S., Leslie, A. M., & Frith, U. (1985). Does the autistic child have a “theory of mind”? Cognition, 21(1), 37–46.
• Harlow, H. F. (1958). The nature of love. American Psychologist, 13(12), 673–685.
• Thomas, A., & Chess, S. (1977). Temperament and Development. Brunner/Mazel.
• Kagan, J. (1994). Galen’s Prophecy: Temperament in Human Nature. Basic Books.
• Tomasello, M. (2003). Constructing a Language: A Usage-Based Theory of Language Acquisition. Harvard University Press.
• Johnson, J. S., & Newport, E. L. (1989). Critical period effects in second language learning. Cognitive Psychology, 21(1), 60–99.
• Erikson, E. H. (1968). Identity: Youth and Crisis. Norton.
• Marcia, J. E. (1966). Development and validation of ego-identity status. Journal of Personality and Social Psychology, 3(5), 551–558.
• Steinberg, L. (2008). A social neuroscience perspective on adolescent risk-taking. Developmental Review, 28(1), 78–106.
• Horn, J. L., & Cattell, R. B. (1967). Age differences in fluid and crystallized intelligence. Acta Psychologica, 26, 107–129.
• Carstensen, L. L., Isaacowitz, D. M., & Charles, S. T. (1999). Taking time seriously: A theory of socioemotional selectivity. American Psychologist, 54(3), 165–181.
• Mather, M., & Carstensen, L. L. (2005). Aging and motivated cognition: The positivity effect in attention and memory. Trends in Cognitive Sciences, 9(10), 496–502.