Key study: Bandura, Ross and Ross (1961)

Aim: to see whether children would imitate aggression they had seen an adult model perform, even when no longer with the model, and whether the sex of the model mattered.
Participants: 72 children (36 boys and 36 girls) aged about 3 to 5 (37 to 69 months) from the Stanford University nursery school.
Children were matched on how aggressive they already were (rated by an experimenter and a nursery teacher) and put into groups: aggressive model, non-aggressive model, or a control group with no model. There were male and female models.

Stage 1: each child played in a room while an adult model in the corner either played quietly (non-aggressive) or attacked a large inflatable Bobo doll for about 10 minutes, hitting it with a mallet and shouting things like 'Pow!' (aggressive).
Stage 2: the child was taken to a room with attractive toys, then told they could not play with them. This mild aggression arousal made all children a little frustrated.
Stage 3: the child went into a room with aggressive toys (a smaller Bobo doll, a mallet, dart guns) and non-aggressive toys (crayons, a tea set, dolls). They were observed for 20 minutes through a one-way mirror, with behaviour recorded every 5 seconds.

Results: children who saw the aggressive model copied many of the model's exact physical and verbal aggressive acts; children in the other groups rarely did. Boys imitated more physical aggression than girls. Boys who saw a male aggressive model were especially aggressive.
Conclusion: aggression can be learned through observation and imitation of a model, supporting Social Learning Theory.
Strengths: a controlled laboratory experiment with standardised procedures, so it can be repeated and cause and effect can be shown. Matching groups reduced participant variables.
Weaknesses: low ecological validity: a Bobo doll is designed to be hit and is not a person. The sample came from one university nursery, so it may not generalise. Ethical concerns: children were exposed to aggression and deliberately frustrated.