Source: The Developing Mind, pp. 81–84 (3-3), pp. 415–418 (11-5a), pp. 421–422 (11-6a)
Tags: behavioural genetics, heritability, twin studies, adoption studies, gene-environment interaction, Piaget, cognitive development, sensorimotor, preoperational, concrete operational, formal operational, object permanence, conservation, egocentrism, temperament, easy child, difficult child, slow-to-warm-up, goodness of fit, Psychology 1100, Ohio State
Difficulty: Intermediate Prerequisites: General understanding of genetics (genes, chromosomes, DNA) is helpful but not strictly required. The intelligence section in the Thinking Mind chapter overlaps with heritability concepts covered here.
Three frequently missed topics from the Developing Mind chapter are grouped here: the field of behavioural genetics (how researchers disentangle genetic and environmental contributions to behaviour), Piaget's stage theory of cognitive development (how children's thinking changes qualitatively as they grow), and temperament (the innate, biologically based emotional and behavioural style that appears early in life). Together, they address the nature-nurture question from multiple angles. Behavioural genetics provides the methods; Piaget provides a framework for cognitive growth; temperament illustrates how biology shapes personality from birth.
Behavioural genetics uses twin and adoption studies to estimate how much genes and environment contribute to psychological traits. Piaget proposed four stages of cognitive development (sensorimotor, preoperational, concrete operational, formal operational), each with distinct thinking abilities and limitations. Temperament is a child's inborn behavioural style (easy, difficult, or slow-to-warm-up), and the "goodness of fit" between temperament and environment predicts adjustment.
Behavioural genetics
The field that studies the relative contributions of genetic and environmental factors to individual differences in behaviour and psychological traits. Think of it as the science of "how much is nature and how much is nurture?"
Heritability
The proportion of variation in a trait within a population that is attributable to genetic differences among individuals. A heritability of 0.50 means half the observed variation in that trait, within that population, is associated with genetic variation.
Twin study
A research design that compares the similarity of identical (monozygotic, MZ) twins and fraternal (dizygotic, DZ) twins on a trait. If MZ twins are more similar than DZ twins, this suggests a genetic contribution, since MZ twins share nearly 100% of their DNA while DZ twins share about 50%.
Adoption study
A research design that compares adopted children to both their biological parents (with whom they share genes) and their adoptive parents (with whom they share an environment). If adopted children resemble their biological parents more than their adoptive parents on a trait, this points to genetic influence.
Shared environment
Environmental factors that make siblings in the same family more similar to each other. Includes family income, parenting style (to some degree), neighbourhood, and home resources.
Non-shared environment
Environmental factors that make siblings in the same family different from each other. Includes different friend groups, different teachers, unique experiences, and differential parental treatment. Non-shared environment often accounts for more variation than shared environment.
Gene-environment interaction
The principle that the effect of a gene depends on the environment, and the effect of the environment depends on the genotype. A child with a genetic predisposition for anxiety may or may not develop an anxiety disorder depending on their life circumstances.
Gene-environment correlation
The tendency for genetically influenced traits to shape the environments a person encounters. A child who is genetically predisposed to be sociable may seek out social situations, which then further develop their social skills.
Jean Piaget
Swiss developmental psychologist (1896–1980) who proposed that children move through four qualitatively distinct stages of cognitive development. His theory emphasises that children are active learners who construct their understanding of the world.
Schema
A mental framework or concept that organises and interprets information. As children encounter new experiences, they modify their schemas through assimilation and accommodation.
Assimilation
Incorporating new information into an existing schema. A child who has a schema for "dog" sees a horse and calls it a "big dog."
Accommodation
Modifying an existing schema (or creating a new one) to fit new information that does not match. The child learns that a horse is not a dog and creates a new "horse" schema.
Sensorimotor stage (birth to ~2 years)
Piaget's first stage, in which infants learn about the world through sensory experiences and motor actions. The major achievement is object permanence.
Object permanence
The understanding that objects continue to exist even when they cannot be seen, heard, or touched. Develops around 8 to 12 months. Before this, "out of sight" is literally "out of mind."
Preoperational stage (~2 to 7 years)
Piaget's second stage, marked by the development of language and symbolic thinking but limited by egocentrism and the inability to perform mental operations like conservation.
Egocentrism
The inability to take another person's perspective. A preoperational child assumes everyone sees, thinks, and feels the same way they do. This is not selfishness; it is a cognitive limitation.
Conservation
The understanding that certain properties of an object (quantity, volume, number) remain the same even when the object's appearance changes. Preoperational children fail conservation tasks: they think a tall, thin glass holds more water than a short, wide glass, even when both hold the same amount.
Concrete operational stage (~7 to 11 years)
Piaget's third stage, in which children can perform mental operations on concrete (tangible, visible) objects and understand conservation, classification, and seriation. Thinking is logical but limited to concrete, real-world situations.
Formal operational stage (~12 years and up)
Piaget's fourth stage, in which adolescents and adults can think abstractly, reason hypothetically, and engage in systematic problem-solving. Not everyone reaches this stage fully.
Temperament
An individual's characteristic pattern of emotional reactivity and regulation, observable from early infancy and believed to be largely biologically based. Think of it as a child's "factory settings" for how they respond to the world.
Easy temperament
A child who is generally cheerful, adapts quickly to new situations, and has regular biological rhythms (eating, sleeping). Approximately 40% of children in Thomas and Chess's research.
Difficult temperament
A child who is often irritable, reacts intensely to stimulation, has irregular biological rhythms, and adapts slowly to change. Approximately 10% of children.
Slow-to-warm-up temperament
A child who is initially cautious and withdrawn in new situations but gradually adapts with time and gentle exposure. Approximately 15% of children. The remaining 35% did not fit neatly into any category.
Goodness of fit
The match between a child's temperament and the demands of their environment (including parenting style). A good fit promotes healthy development; a poor fit increases the risk of behavioural problems. A highly active child in a structured, patient household has a better fit than the same child in a rigid, impatient one.
Behavioural genetics addresses the nature-nurture question using quantitative methods.
The core tools are twin studies and adoption studies, each of which isolates genetic and environmental influences differently:
Twin studies compare MZ (identical) and DZ (fraternal) twins. If a trait is more similar in MZ twins, genetics is contributing.
Adoption studies separate genetic from environmental influence by examining whether adopted children resemble their biological parents (genes) or adoptive parents (environment).
Key findings across many traits:
Most psychological traits (intelligence, personality, psychopathology risk) show moderate heritability (typically 0.30 to 0.60).
Shared environment (growing up in the same home) matters in childhood but often has diminishing influence by adulthood.
Non-shared environment (unique experiences that differ between siblings) is a major source of individual differences.
Gene-environment interactions and correlations make the picture more complex than a simple "nature vs. nurture" split.
Important nuances:
Heritability is specific to a population and an environment. It can differ between groups and change over time.
A high heritability does not mean a trait is unchangeable. Environmental interventions can still have large effects.
Genes do not code for behaviours directly. They influence the development of brain structures, neurotransmitter systems, and hormonal patterns, which in turn influence behavioural tendencies.
Piaget proposed that children progress through four invariant stages in a fixed order, each involving a qualitative shift in how they think.
Children are not passive recipients of knowledge; they actively construct understanding through interaction with their environment.
Two processes drive development:
Assimilation: fitting new experiences into existing schemas.
Accommodation: adjusting schemas to fit new experiences.
Infants learn through direct sensory and motor interaction with the world.
Early in the stage, behaviour is reflexive. By the end, infants engage in purposeful, goal-directed actions.
Key milestone: object permanence (around 8–12 months).
Before this, an infant who watches a toy being hidden under a blanket will not search for it.
After achieving object permanence, the infant will actively search for the hidden object.
The A-not-B error: even after achieving basic object permanence, infants may search in the first hiding location (A) even after watching the object being moved to a second location (B).
Children develop language, symbolic play (pretending a stick is a sword), and mental imagery.
Limitations:
Egocentrism: difficulty taking another's perspective. In the classic three-mountains task, a child cannot describe what the scene looks like from someone else's viewpoint.
Centration: focusing on only one aspect of a situation while ignoring others. This underlies the failure at conservation tasks.
Lack of conservation: the child does not understand that quantity is unchanged when appearance changes. Pour water from a short, wide glass into a tall, thin one, and the preoperational child will say the tall glass has "more."
Animism: attributing life and intentions to inanimate objects ("the sun goes to sleep").
Irreversibility: inability to mentally reverse an action or process.
Children can perform logical operations, but only on concrete, tangible materials.
Achievements:
Conservation: the child now understands that pouring water into a different-shaped glass does not change the amount.
Classification: the ability to group objects by shared features and understand hierarchies (a poodle is a dog, and a dog is an animal).
Seriation: the ability to arrange objects in a logical order (shortest to tallest).
Reversibility: the child can mentally reverse an operation (if 3 + 4 = 7, then 7 - 4 = 3).
Limitation: difficulty with abstract or hypothetical reasoning. A concrete operational child can solve a logic problem about real objects but struggles with "if A is taller than B, and B is taller than C, is A taller than C?" when no objects are visible.
Abstract thinking, hypothetical reasoning, and systematic experimentation become possible.
Adolescents can think about possibilities that do not exist, reason from premises that may be false, and engage in scientific reasoning (testing hypotheses systematically).
Not everyone reaches full formal operational thinking. Cultural and educational factors influence whether this stage is fully attained.
Piaget also described adolescent egocentrism at this stage, including the "imaginary audience" (believing everyone is watching and judging you) and the "personal fable" (believing your experiences are unique and that you are invulnerable).
Piaget underestimated children's abilities. More recent research (using simplified tasks) has shown that infants demonstrate some understanding of object permanence earlier than Piaget suggested, and preschoolers can take others' perspectives in simpler tasks.
Development may be more continuous and less stage-like than Piaget proposed.
Piaget underemphasised the role of social and cultural factors (a gap addressed by Vygotsky's sociocultural theory).
Despite these criticisms, Piaget's framework remains the most influential stage theory in developmental psychology.
Thomas and Chess (1977) identified three broad temperament categories based on a longitudinal study of infants:
Easy (~40%): positive mood, regular routines, adaptable, mild reactions.
Difficult (~10%): negative mood, irregular routines, slow to adapt, intense reactions.
Slow-to-warm-up (~15%): initially withdrawn, low activity level, mild reactions, gradually adapts.
About 35% of children did not fit neatly into any of these three types.
Temperament is largely biologically based:
It appears early (observable in newborns) and shows moderate stability across the lifespan.
Twin studies show higher concordance for temperament in MZ twins than DZ twins, suggesting genetic influence.
Kagan's research on behavioural inhibition showed that infants who were highly reactive (easily startled, cried readily) at 4 months were more likely to be shy and cautious as toddlers and children.
Goodness of fit is the critical concept:
The outcome for a child depends on the match between their temperament and their environment.
A "difficult" child in a patient, consistent, flexible environment may develop well. The same child in a harsh or rigid environment faces higher risk of behavioural problems.
This means temperament does not determine destiny. Parenting, culture, and context all interact with the child's inborn style.
Behavioural genetics research informs public health (knowing that a trait is partly environmental motivates interventions) and clinical practice (genetic risk for a disorder like depression does not mean it will develop; environment matters). Piaget's stages have directly shaped early childhood education: hands-on, age-appropriate learning materials in preschools are a Piagetian legacy. Temperament research has practical implications for parenting: understanding a child's temperament helps caregivers set realistic expectations and create environments that support the child rather than fighting their natural tendencies.
"Heritability of 0.50 means genes account for 50% of an individual's trait." Heritability is a population statistic about variation, not a statement about any single person.
"Piaget's stages mean children suddenly jump from one type of thinking to another." The transitions are gradual, and children may show abilities from adjacent stages during transitional periods.
"Egocentrism in Piaget's theory means the child is selfish." Egocentrism is a cognitive limitation (the inability to take another's perspective), not a moral failing.
"A difficult temperament means a child will have behavioural problems." Goodness of fit, not temperament alone, predicts outcomes. A difficult temperament in a well-matched environment can lead to positive development.
⚠️ Understand what heritability is and is not. Be prepared for a question testing whether you know heritability is a population statistic, not an individual one.
⚠️ Know the twin study logic: why MZ/DZ comparisons tell us about genetic influence.
⚠️ Be able to place a child's described behaviour into the correct Piagetian stage. The most common question format describes a child failing or passing a specific task (conservation, object permanence, abstract reasoning).
⚠️ Conservation and egocentrism are the two most tested Piagetian concepts. Know what they are and which stage they characterise.
⚠️ Know the three temperament types and the concept of goodness of fit. A scenario-based question may describe a parent-child interaction and ask about the fit.
True or False: Adoption studies compare identical and fraternal twins to estimate heritability.
A: False. That describes twin studies. Adoption studies compare adopted children to their biological and adoptive parents.
Fill in the blank: The understanding that objects continue to exist even when they are out of sight is called ________.
A: Object permanence.
True or False: A child in the concrete operational stage can think abstractly about hypothetical scenarios.
A: False. Abstract and hypothetical thinking characterise the formal operational stage. Concrete operational children can reason logically only about tangible, real-world problems.
Fill in the blank: The match between a child's temperament and the demands of their environment is called ________.
A: Goodness of fit.
Q: Identical twins raised in the same home are more similar on a personality trait than fraternal twins raised in the same home. What does this suggest about the trait?
A: It suggests a genetic contribution to the trait. MZ twins share nearly 100% of their DNA while DZ twins share about 50%, so greater similarity in MZ twins points to genetic influence beyond what shared environment alone would produce.
Q: A 4-year-old watches you pour water from a short, wide glass into a tall, thin glass and says the tall glass now has "more water." Which Piagetian stage is this child in, and what concept have they not yet acquired?
A: The preoperational stage. The child has not yet acquired conservation, the understanding that the quantity of liquid remains the same regardless of the shape of the container. The child is focused on the height of the water (centration) and cannot mentally reverse the pouring (irreversibility).
Q: Explain the difference between gene-environment interaction and gene-environment correlation.
A: Gene-environment interaction means that the effect of a gene depends on the environment (and vice versa). A genetic risk for depression may only manifest in a stressful environment. Gene-environment correlation means that genetically influenced traits lead people to encounter certain environments. A genetically sociable child seeks social settings, which further reinforce their sociability.
Q: A parent describes their newborn as having irregular eating and sleeping patterns, crying intensely, and being slow to adjust to new situations. What temperament category does this describe, and what parenting approach does the "goodness of fit" model suggest?
A: This describes a difficult temperament. The goodness of fit model suggests the parent should be patient, consistent, and flexible, adjusting their expectations and routines to accommodate the child's needs rather than forcing rigid schedules or reacting with frustration.
Q: Name and briefly describe Piaget's four stages of cognitive development in order.
A: (1) Sensorimotor (birth to ~2): learning through senses and motor actions; achieves object permanence. (2) Preoperational (~2 to 7): symbolic thinking and language develop, but limited by egocentrism and lack of conservation. (3) Concrete operational (~7 to 11): logical thinking about concrete objects; masters conservation, classification, and seriation. (4) Formal operational (~12+): abstract, hypothetical, and systematic reasoning becomes possible.
Behavioural genetics connects to the intelligence material in the Thinking Mind chapter (same methods, same heritability concepts applied to IQ). Piaget's theory connects to the memory chapter (schemas are central to how information is encoded and retrieved) and to education and learning (the Adaptive Mind chapter). Temperament connects to the personality unit (temperament is considered a precursor to adult personality traits) and to clinical psychology (temperament interacts with environment to predict psychopathology risk).
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