Method

This is an experiment.  Participants were divided into eight experimental groups of six students each and a control group consisting of 24 participants.  Hals of the experimental participants were exposed to an aggressive model and half were exposed to a model that was subdued and non aggressive in their behaviour.  These groups were further subdivided into male and female.  Half of the participants in the aggressive condition observed same sex models, while the remaining participants in each group viewed models of the opposite sex.  The control group did not see a model at all, they were just tested for their aggressive behaviour, like the other participants were. 
The children were rated on their "normal" everyday behaviour for levels of aggression by the nursery teacher and an experimenter who knew the children well.  They were rated on four five-point rating scales,  which measured the extent to which participants displayed physical aggression, verbal  aggression, aggression toward inanimate  objects and aggressive inhibition.  This later scale, which dealt with the participants tendency to inhibit aggressive reactions in the face of high provocation, provided a measure of aggression anxiety.
51 participants were rated by both judges in order to measure inter-rater reliability.  They scored 0.89 measured by Pearson product-moment correlation.

Sample

The participants were 36 boys and 36 girls enrolled in the Stanford University Nursery School.  They ranged in age from 37 to 69 months, with an average age of 32 months.
Two adults, a male and a female, served in the of model, and one female experimenter conducted the study for all 72 children

Procedure

The first step in the procedure was for the experimenter to take each child, individually, to the experimental room.  They met the model, who was waiting for them in the hallway outside, and was invited by the experimenter to come and "join the game."  The experimenter then escorted the child to one corner of the room, which was set out as a child's play area.  The child sat at a small table and was shown how to make pictures with potato prints, in a variety of geometric forms, stickers with multi-colour pictures of animals, flowers, and Western figures that could be assembled into a scene.  These items were selected as they had proved popular in the nursery and all the children were interested in the activity.
After settling the participant in his corner, the experimenter escorted the model to the opposite corner of the  room.  This had a small table and chair, a tinker toy set, a mallet, and a five-foot inflatable Bobo doll.  The experimenter explained that these were for the model to play with and after the model was seated the experimenter left the room.
With participants in the non aggressive condition, the model assembled the tinker toys in a quiet and subdued manner totally ignoring the Bobo doll.
In contrast, with participants in the aggressive condition, the model began by assembling the tinker toys but after approximately one minute had elapsed, the model turned to the Bobo doll and spent the remainder of the time behaving aggressively toward the Bobo doll.
To prove that imitation had occurred, the model would have to behave in a way that was clearly identifiable as having been copied.  So, in addition to punching the Bobo doll, the model laid the doll on its side, sat on it and repeatedly punched the doll on the nose.  The model then picked up the Bobo doll, picked up the mallet and hit the Bobo doll on the head.  Following the assault with the mallet, the Bobo doll would be kicked around the room.  The models were encouraged to make verbally aggressive comments such as, "Sock him in the nose...," "Hit him down...," "Throw him in the air...," "Kick him...," "Pow....," and two non aggressive comments, "He keeps coming back for more," and "He sure is a tough fella."
Thus in this situation, the participants were provided with an activity that occupied their attention while at the same time ensured observation of the model's behaviour in the absence of any instructions what to do.   Since there was no opportunity to exhibit the imitated behaviour, and learning that occurred was purely on an observational or covert basis.
At the end of ten minutes, the experimenter enters the room, informed the participant the he/she would go to another room, and  said goodbye to the model.
Aggression Arousal
Prior to the test for imitation, all participants, experimental and control were subjected to mild aggression arousal to ensure that they were under some degree of encouragement to behave aggressively.  The arousal was carried out for two reasons:
The first reason is that observation of aggression exhibited by others tends to reduce the probability of aggression in the part of the observer.  Therefore, the participants in the aggressive condition, in relation to both the non aggressive and control groups, would be under weaker instigation following exposure to the models.
Second, if the participants in the non aggressive condition expressed little aggression in the face of appropriate instigation, the presence of an inhibitory process would seem to be indicated.
Following the exposure experience, therefore, the experimenter brought the participant to an anteroom that contained relatively attractive toys: a fire engine, a locomotive, a jet fighter plane, a cable care, and a baby crib.  The experimenter allowed the participant to play with these toys, but after about 2 minutes the experimenter would say that these were her very best toys, and she would not let just anyone play with them, and that she had decided to reserve these toys for other children.  However, the participant could play with any of the toys that were in the next room.
It was necessary for the experimenter to stay in the room during the experimental session, otherwise a few of the children would refuse to remain alone or would leave before the termination of the session.  However, in order to minimise and influence her presence might have on the participant's behaviour, she remained as inconspicuous as possible by busying herself with paper work at a desk in the far corned of the room and avoiding any interaction with the child.
Test for Delayed Imitation
Immediately after bing told that the good toys were reserved for "other children" the participant went into the experimental room, where they were told that they could play with any of the toys that they wanted to.  Some of these could be used in imitative or non imitative aggression and other that tended to elicit non aggressive forms of behaviour.  The aggressive toys included a three-foot Bobo doll, a mallet and peg board, two dart guns, a tether ball with a face painted on it, which hung from the ceiling.  The non aggressive toys, on the other hand, included a tea set, crayons and colouring paper, a ball, three bears, cars trucks and plastic farm animals.  In order to eliminate any variation in behaviour due to placement of the toys in the room, the play material was arranged in a fixed order for each of the sessions.
The participant spent 20 minute in this room during which time his/her behaviour was observed via a one-way mirror in a neighbouring room.  The 20 minute session was divided up into five-second intervals by means of an electric timer, thus yielding a total of 240 response units for each participant.
The male model scored the experimental sessions for all 72 children, except for the children he had seen as the model, he did not know what condition the other children were in.  In order to provide an estimate of inter-rater reliability, the performance of half of the participants were also scored independently by a second observer.  Thus neither of the two observers would have knowledge of the conditions to which the participants were placed in.  Since all but two of the participants in the aggressive condition performed the model's distinctive behaviours, the participants who had been exposed to the aggressive model were easy to spot.  Participants in the other conditions only rarely exhibited behaviour like that shown by the model.
The responses scored involved highly specific concrete classes of behaviour and yielded high inter-rater reliability score of 0.90

Response Measures

Three measures of imitation were obtained:

Imitation of physical aggression - This category included acts of striking the Bobo doll with the mallet, sitting on the doll and punching it in the nose, kicking the doll, and tossing it in the air.

Imitative verbal aggression - The participant repeats the phrases: "Sock him," "Hit him down," "Kick him," "Throw him in the air," or "Pow."
Imitative non aggressive verbal responses - The participant repeats "He keeps coming back for more," or "He sure is a tough fella."

During the pretest, a number of the participants imitated the essential components of the model's behaviour but did not perform the complete act, or they directed the imitative aggressive response to some other object than the Bobo doll.  Two responses of this type were therefore scored and interpreted as partially imitative behaviour.

Mallet aggression - Participant strikes objects other than the Bobo doll aggressively with the mallet.

Sits on Bobo doll -  Participant lays the Bobo doll on its side and sits on it, but does not behave aggressively towards it.

The following non imitative aggressive responses were also scored.

Punches Bobo doll - Participant strikes, slaps or pushes the doll aggressively.

Non imitative physical and verbal aggression - This category included physically aggressive acts directed towards objects other than the Bobo doll, and any hostile remarks except for those in the verbal imitation category; e.g. "Shoot the Bobo," "Cut him," "Stupid ball," "Knock over people," "Horses fighting, biting."

Aggressive gun play - Participant shoots darts or aims the guns and fires imaginary shots at objects in the room.

Ratings were also made of the number of the behaviour units that the participant played non aggressively or sat quietly and did not play with any of the material at all

Results

Response Category	Experimental Groups				Control groups
	Aggressive		Non aggressive
	F Model	M Model	F Model	M Model
Imitative Physical Aggression
Female Participants	5.5	7.2	2.5	0.0	1.2
Male Participants	12.4	25.8	0.2	1.5	2.0
Imitative Verbal Aggression
Female Participants	13.7	2.0	0.3	0.0	0.7
Male Participants	4.3	12.7	1.1	0.0	1.7
Mallet Aggression
Female Participants	17.2	18.7	0.5	0.5	13.1
Male Participants	15.5	28.8	18.7	6.7	13.5
Punches Bob doll
Female Participants	6.3	16.5	5.8	4.3	11.7
Male Participants	18.9	11.9	15.6	14.8	15.7
Non imitative aggression
Female Participants	21.3	8.4	7.2	1.4	6.1
Male Participants	16.2	36.7	26.1	22.3	24.6
Aggressive Gun Play
Female Participants	1.8	4.5	2.6	2.5	3.7
Male Participants	7.3	15.9	8.9	16.7	14.3

Participants in the aggression condition reproduced a good deal of physical and verbal aggressive behaviour resembling that of the models, and their mean scores differed from those participants in the non aggressive and control groups who exhibited virtually no imitative aggression (See Table 1, above).

Since there were only a few scores in the non aggressive and control conditions (approximately 70% of the participants had zero scores), and the assumption that the variance would be the same for every value could not be made, one of the only statistical tests that could be carried out was the Friedman two-way analysis of variance by ranks.  This was used to test the significance of the obtained differences.

Partial Imitation of Model's Behaviour

Individual sign tests show that both the aggressive and the control groups, relative to participants in the nonaggressive condition produced significant more mallet aggression, the difference being particularly marked with regard to female participants.  Girls who observed non aggressive models performed an average number of 0.5 mallet aggression responses compared to average values of 18.0 and 13.1 for girls in the aggressive and control groups, respectively.

Although participants who observed an aggressive model performed more mallet aggression (M=20.0) than their controls (M=13.3), the difference was not significant.

When the partially imitative response of sitting on Bobo is looked at, the over-all group differences were significantly beyond the 0.01 level (see Table 2).  Looking at the pairs of scores reveals that participants in the aggressive group reproduced this response to a greater extent than did the non aggressive or the control group, but the difference was not statistically significant.

The prediction that exposure of participants to aggressive models increases the probability of aggressive behaviour is clearly confirmed (see Table 2, below).  The main effect of treatment condition is highly significant both for physical and verbal imitative aggression.  Comparisons of pairs of scores by the sign test shows that the obtained over-all difference were due almost entirely to the aggression displayed by participants who had been exposed to the aggressive models.  Their scores were significantly higher than those of either the non aggressive or control groups, which did not differ from each other.


Non Imitative Aggression

Further comparison of the treatment pairs reveals (Table 2) that the main source of the over-all difference was the aggressive and non aggressive groups which differed significantly from each other, with participants exposed to the aggressive models displaying the greater amount of aggression.

Influence of Sex of Model and Sex of Participants on Imitation

The hypothesis that boys are more likely than girls to imitate aggression exhibited by a model was only partially confirmed.  Statistical tests revealed that for those in the aggressive condition reveal that boys reproduced more imitative physical aggression than girls (p<0.01).  The groups do not differ in their imitation of verbal aggression.
Because the distribution of the findings were so skewed, this precludes the possibility of an over-all test of the sex of the model and of the various interaction between the main effects.  Inspection of the means in Table 1 for participants in the aggression condition clearly suggests the possibility of a Sex x Model interaction.  This interaction is more consistent and pronounced for the male model than for the female model.  Male participants exhibited more physical (p<0.05) and verbal imitative aggression (p<0.05), more non imitative aggression (p<0.025) and engaged in significantly more aggressive gun play (p<0.05) following exposure to the aggressive male model than female participants.  In contrast, female participants exposed to the female model performed considerable more imitative verbal aggression and more non imitative aggression than did the boys (see Table 1).  The variance was extremely large and with only small number of results available, mean differences did not reach statistical significance.
Data for the non aggressive and control groups suggest that the behavior of the male model exerted a greater influence than the female model on the participant's behavior in general.
Except for the greater amount of mallet aggression exhibited by the control group, no significant differences were obtained between the non aggressive and control groups.  The data also indicates that the absence of significant differences may be due to the fact that participants exposed to the non aggressive female model did not differ from the controls on any measure of aggression.  With respect of the male model, the differences between the groups are striking.  Comparison of the scores in relation to the control group, participants exposed to the non aggressive male model performed significantly less imitative physical aggression (p=0.06), less imitative verbal aggression (p=0.02), less mallet aggression (p=0.003), less non imitative physical and verbal aggression (p=0.03), and they were less inclined to punch the Bob doll (p=0.07).
While some of the findings do not reach statistical significance, the consistency of the findings adds support to the interpretation in terms of influence by the model.

Non aggressive behaviour

Female participants spent more time than boys playing with dolls (p<0.001), with the tea set (p<0.001) and colouring (p<0.05).  The boys, on the other hand, devoted significantly more time than the girls on exploratory play with the guns (p<0.01).  No sex differences were found to the participants used of the other toys e.g. farm animals, cars, or the tether ball.
Two measures of non aggressive behaviour are worth mentioning.  Participants in the non aggressive condition engage in significantly more non aggressive play with dolls that either the participants in the aggressive group (p<0.02) or in the control group (p<0.02).
Even more noteworthy is the finding that participants who observed the non aggressive model spent more than twice as much time and participants in the aggressive condition (p<0.01) in simply sitting quietly without handling any of the play equipment.

Have a look at a video of some of the participants in Bandura's study.  Click here

Discussion

Much current research on social learning is focused on the shaping of new behaviour through rewarding and punishing consequences.  Unless responses are emitted, however, they cannot be influenced.  The results of this study provides strong evidence that observation of cues produced by the behaviour of others is one effective means of eliciting certain forms of responses for which the original probability is very low or zero.  Indeed, social imitation may hasten or short-cut the acquisition of new behaviours without the necessity of reinforcing successive approximations as suggested by Skinner (1953).

In this study, participants given the opportunity to observe aggressive models later produced a good deal of physical and verbal aggression (as well as non aggressive responses) substantially identical with that of the model.  In contrast participants who were exposed to the non aggressive models and those who had no previous exposure to any models only rarely performed such responses.

To the extent that observation of adult models displaying aggression communicates permissiveness for aggressive behaviour, such exposure may serve to weaken inhibitory responses and thereby to increase the probability of aggressive reaction to subsequent frustrations.  The fact that the participants expressed their aggression in ways that clearly resembled the novel patterns of the model provides striking evidence for the occurrence of learning by imitation.

An adequate theory of the mechanism underlying imitative lerning is lacking.  Some explanations that have been offered assume that the imitator performs the model's responses covertly, at first.  If it can be assumed that rewards and punishments are self administered in conjuction with the covert responses, the process of imitative learning could be accounted for in terms of the same principles as trial and error learning. 

The data provides some evidence that the male model influences the participant's behavior outside the exposure setting to a greatr extent than was true for the female model.  In the anayses of the Sex x Model interactions only the comparisons involving the male model yielded significant differences.  Paticipants exposed to the non aggressive male model performed less aggressive behaviour than controls, whereas comparisons involving the female model were consistently non significant.

In a study of learning by imitation, Rosenblith (1959) has likewise found that male experimenters are more effective than females in influencing children's behaviour.  Rosenblith advanced the tentative explanation that the scool setting may involve some social deprivation in respect to adult males which enhances the male's reward value.

The trends in the data yielded by this study suggest an alternative explanation.  In the case of a highly masculine behaviour such as physical aggression, there is a tendency for both the male and female participants to imitate the male model to a greater extent than the female model.  On the other hand, in the case of verbal aggression, which is less clearly sex linked, the greater amount of imitation occurs in relation to the same sex model.  These trends together with the finding that boys in relation to girls are in general more imitative of physical aggression but do not differ in imitation of verbal aggression, suggest that participants may be differentially affected by the sex of the model but that predictions must take into account the degree in which the behaviour in question is sex-typed.

The preceeding discussion has assumed that maleness and femaleness rather than some other personal characteristic of the particular models involved, is the significant variable, and assumption that cannot be tested directly with the available data.  It was clearly evident, particularly from the boys spontaneous remarks about the physical aggression exhibited by the female model, that they were responding in terms of what they had previously learned about appropriate female behaviour (e.g. "Who is that lady?  That's not the way for a lady to behave.  Ladies are supposed to act like ladies...." "You should have seen what that girl did in there.  She was acting like a man.  I never saw a girl act like that before.  She was punching and fighting but no swearing.")  Aggression by the male model was more likely to be seen as appropriate and approved by both the boys ("Al's a good socker, he beat up Bobo.  I want to sock like Al.") and the girls ("That man is a strong fighter, he punched and punched and he could hit Bobo right down to the floor and if Bobo got up he said "Punch your nose."  He's a good fighter like my Daddy.").

The finding that participants exposed to the quiet models were more inhibited and unresponsive than participants in the aggressive condition, together with the obtained difference on the measures of aggression, suggests that exposure to inhibited models not only decreases the probability of occrrance of aggressive behaviour but also generally restricts the range of behaviour by the participants.