Let’s discuss them shortly. Before we explore the big experiments, let’s clear up two small but powerful ideas in Developmental Psychology: Growth & Change.

Growth: The natural changes that happen as your body and brain mature.
Example – A baby learning to crawl, then walk, then run.

Change: The shifts in how we think, feel, or relate to others.
Example – A teenager who once feared speaking up in class becomes confident in college.

Here’s the twist: growth happens automatically (like getting taller), but change depends on experiences and environment (like learning to share because you have siblings).

Growth example: Your voice deepens in adolescence, whether you like it or not.
Change example: You become more patient after taking care of a younger cousin.

Take Piaget’s kids – they weren’t just “getting older,” their thinking moved from simple (believing the sun follows them) to complex (understanding science).
Or Erikson’s stages – at each age, life throws us a challenge, and how we deal with it shapes our identity.

So next time you see someone acting their age or not, ask yourself: is it just growth, or is it change shaped by their experiences?

Must-Know Developmental Psychology Experiments

1. Harlow – Attachment in Rhesus Monkeys
2. Ainsworth – Strange Situation
3. Baillargeon – Object Permanence in Infants
4. Piaget – Conservation Tasks
5. Piaget – Three-Mountains Task
6. Konrad Lorenz’s Imprinting Experiment
7. Kohlberg – Moral Development (Heinz Dilemma)
8. Thomas & Chess – Temperament Study

Experiments discussed

A relevent video has been added to the heading of all experiments to allow understanding of concepts visually.

1. Harlow – Attachment in Rhesus Monkey

Harry Harlow separated baby rhesus monkeys from their mothers and gave them two “surrogate mothers”

• One was made of wire but gave milk.
• The other was covered in a soft cloth but gave no milk.
  The idea was to test: do infants bond because of food, or because of comfort?

Findings:
The monkeys spent most of their time clinging to the cloth mother, only going to the wire mother briefly for feeding. When scared, they ran to the soft mother for comfort, not the feeding one.

Concepts :

2. Ainsworth – Strange Situation

Mary Ainsworth observed infants (1–2 years old) in a lab “playroom.” The baby experienced different situations:

• Playing with mother present.
• Mother is leaving the room.
• Stranger entering.
• Mother returning.
  Researchers watched how the baby reacted to separation and reunion.

Findings (Attachment Types):

• Secure attachment: Babies cried when mom left, but were easily comforted when she returned.
• Insecure-avoidant: Babies showed little distress and avoided mom upon return.
• Insecure-resistant: Babies were very distressed when mom left and wanted comfort when she returned.
• Insecure-ambivalent: Babies both clung to mom and resisted her (seeking closeness but also pushing away).

Concepts (What it shows):

• Attachment styles shape how children relate to caregivers and later relationships.
• A “secure base” (like mom) helps babies explore the world safely.
• Early interactions with caregivers influence trust, security, and emotional regulation.

3. Baillargeon – Object Permanence in Infants

Baillargeon ne test kiya ki chhote babies ko samajh hai ya nahi ki cheezein chupne par bhi exist karti hain. Unhone 3–4 mahine ke babies ko do scenes dikhaye: ek normal (car wall se takra ke ruk gayi) aur ek “impossible” (car wall ke paar nikal gayi jaise wall hai hi nahi). Babies ne impossible scene zyada der tak dekha, matlab unhe surprise hua. Ye dikhata hai ki chhote babies ko bhi basic idea hota hai ki objects chupne par gaayab nahi hote, woh fir bhi exist karte hain.

Findings:
Using Violation-of-Expectation experiments in the 1980s, Baillargeon showed that 3-4-month-old infants look longer at “impossible” events (e.g., a screen seeming to pass through a hidden box), demonstrating that they expect objects to persist and obey solidity/continuity, i.e., early object permanence. (No ethics were violated; the “violation” is of the baby’s expectations.)

Comparison with Piaget: In the 1950s, Piaget (observational/reach-based tasks) concluded object permanence appears at 8–12 months. Baillargeon (controlled, looking-time experiments) found it earlier at 3–4 months. Both agree the milestone is crucial; the ~40-year gap and better methods explain why Baillargeon’s results refined and updated Piaget’s estimate.

Concepts:

• Object Permanence: Understanding that objects continue to exist even when out of sight (earlier than Piaget suggested).
• Violation-of-Expectation Method: Infants look longer at things that defy their expectations, revealing what they understand.

4. Piaget – Conservation Tasks

Jean Piaget showed children two glasses with equal amounts of water. Then he poured the same water into a taller, thinner glass. He asked the child which glass had more water. He did similar tasks with clay (rolled into different shapes) and coins (spread out vs. close together).

Findings

• Younger children (4–6 years) thought the taller glass, longer clay, or spread-out coins had “more.”
• They judged only by appearance, not by logic.
• Children around 7 years and older understood that the amount/quantity stays the same even if the shape or arrangement changes.

Concepts

• Conservation: Understanding that quantity stays the same despite changes in appearance.
• Centration: Younger kids focus on one feature (like the height of the glass).
• Decentration: Older kids can consider multiple features (height + width).
• Stage of Cognitive Development: Marks the shift from preoperational (intuitive, fooled by looks) to concrete operational (logical, rule-based thinking).

5.  Piaget – Three-Mountains Task

In Piaget’s Three-Mountains Task, children looked at a model with three mountains and were asked what a doll on the other side could see. Little kids (4–7 years) usually said what they could see, not the doll, showing they couldn’t imagine someone else’s view. Older kids could see things from the doll’s point of view, so egocentrism goes away as kids grow.

Findings: Young children (4–7 years) usually described only what they could see, not the doll’s view, showing cognitive egocentrism. Older children (7–12 years) could take the doll’s perspective, showing that egocentrism decreases with age.

Concepts:

• Egocentrism in children – difficulty seeing things from others’ perspectives.
• It’s a key part of the preoperational stage in Piaget’s theory.

Piaget studied egocentrism in children and teenagers with different methods. An experiment was done for children, while teenagers were assessed through interviews and observations. For a simpler comparison of the findings between the research, please refer to the illustration below.

6. Konrad Lorenz’s Imprinting Experiment

Findings:
The goslings that hatched with their mother followed her, while those that first saw Lorenz followed him everywhere, even preferring him over their real mother. This showed that goslings form an immediate attachment during a short critical period after birth, a process Lorenz called imprinting. This study strongly influenced later human attachment theories (e.g., Bowlby).

Concepts Drawn:

7. Kohlberg – Moral Development (Heinz Dilemma)

Kohlberg presented people (children, adolescents, and adults) with moral dilemmas, the most famous being the Heinz Dilemma. In this story, a man named Heinz considers stealing a drug he cannot afford to save his dying wife. Participants were asked whether Heinz should steal the drug and, more importantly, why they thought so.

Findings:
Kohlberg discovered that the reasoning behind the answers mattered more than whether participants said Heinz should or shouldn’t steal. From this, he outlined three levels of moral development (each with two stages):

• Preconventional (obedience, self-interest)
• Conventional (law/order, social approval)
• Postconventional (social contract, universal principles)

Concepts Drawn:

• Moral reasoning develops in stages linked to cognitive growth.
• Justice and fairness are central to higher levels of moral reasoning.
• Not everyone reaches the highest (postconventional) stage.

8. Still-Face Experiment – Tronick

Edward Tronick asked mothers to interact normally with their babies and then suddenly hold a completely still, expressionless face. While mothers stayed unresponsive, researchers observed the infants’ reactions.

Findings:
At first, babies tried to re-engage their mothers with smiles, coos, and gestures. When no response came, they became upset, cried, and looked away. This showed that infants expect emotional connection and responsiveness in social interaction.

Concepts Drawn:

• Early social and emotional development depends on caregiver responsiveness.
• Babies are active participants in relationships, not passive.
• Lack of response can cause stress and emotional distress even in very young infants.

So, after exploring these experiments, how much of human behaviour do you think is shaped by biology alone and how much is shaped by experience, environment, and social interaction?

Let’s test your learning with CUET-style MCQs.

1. In Harlow’s experiment, infant rhesus monkeys showed the strongest attachment to:
A. Wire mother with food
B. Cloth mother with comfort
C. Both equally
D. No preference

2. The main finding of Ainsworth’s Strange Situation is related to:
A. Moral development
B. Attachment styles
C. Memory development
D. Intelligence testing

3. Baillargeon’s study mainly challenged Piaget’s idea of:
A. Egocentrism
B. Object permanence timing
C. Conservation
D. Classical conditioning

4. In Piaget’s conservation task, younger children fail mainly due to:
A. Reversibility
B. Decentration
C. Centration
D. Accommodation

5. Egocentrism in Piaget’s Three-Mountains Task is most common in:
A. Sensorimotor stage
B. Preoperational stage
C. Concrete operational stage
D. Formal operational stage

6. Konrad Lorenz’s imprinting shows that attachment is:
A. Gradual and learned
B. Reward-based
C. Rapid and occurs in a critical period
D. Dependent on punishment

7. In Kohlberg’s theory, moral reasoning is based primarily on:
A. Age
B. Emotion
C. Reasoning behind decisions
D. Intelligence quotient

8. Heinz dilemma is used to study:
A. Moral development
B. Social perception
C. Emotional regulation
D. Language acquisition

9. The Still-Face experiment shows that infants:
A. Prefer objects over people
B. Do not notice facial changes
C. Expect responsive social interaction
D. Only respond to food-based cues

10. In Ainsworth’s Strange Situation, secure attachment is characterised by:
A. No reaction to separation
B. Avoidance of the caregiver on return
C. Severe aggression toward caregiver
D. Distress on separation but comfort on reunion

Answers

1. B
2. B
3. B
4. C
5. B
6. C
7. C
8. A
9. C
10. D