Another way could be infection - if an infection is caught from another person, it may have affected Geoff's brain badly, leading to an abnormality and making Geoff act out.
If the neurochemicals in Geoff's brain are slightly out of balance, and he has more of one than the other, this could also cause behaviour to change. For example, if someone has lower levels of sertotonin in the brain, they are more likely to develop depression and/or anorexia nervosa and if someone has high levels of dopamine, they are more likely to develop schizophrenia. If Geoff has high levels of serotonin, he may feel depressed sometimes, however, because of the thrill he gets from fighting in the 'inner city crew', he believes it's helping him to not spiral into depression which makes him repeat the behaviour over and over. On the other hand, if he has higher levels of dopamine, he could have a case of schizophrenia which may be pushing him to fight and get into trouble.
The Behaviourist approach suggests that Geoff will have learnt the law-breaking actions from his friends and therefore think that it's an acceptable way of acting. Also, because he gets such a thrill from being in the 'inner city crew', this will make him want to stay a member of this crew. Geoff associates the crew with the fun he experiences during the rush of the fights. You could also look at it as though Geoff has trained his brain to think that this behaviour is acceptable.
For Operant conditioning, Geoff will have had positive and negative reinforcement for people. He would have received positive reinforcement from his friends within the crew and this would make him want to carry on with the behaviour. However, he would also receive negative reinforcement from the police and other legal people, which would be expected to make him want to stop the behaviour through discipline.
Social Learning Theory suggests that Geoff may have observed his friends in the ‘inner city crew’ and they were like role models to him and therefore, he acts in a similar way to them by watching and learning. These theories show that we learn our behaviour from others and that they do not come naturally.
The Psychodynamic approach could suggest that Geoff has a weak Superego because he cannot distinguish between what is right and wrong, otherwise he would not be involved with the ‘inner city crew’ because he would know that what he is doing is illegal and morally wrong. His Id is more dominant which makes him want to join in with the fights because it’s a basic desire for him to enjoy himself, and he gets a thrill from the fights, so therefore satisfies his Id. His Ego seems the most balanced because he knows that the fighting can be done realistically however because his Superego is weaker, it cannot say whether his behaviour is the right or wrong thing to do.
The Cognitive approach may justify Geoff's behaviour because he has seen how his friends from the 'inner city crew' perceive things and deal with situations, so he has picked up on this and does the same. This approach considers peoples' thinking processes.
The Evolutionary approach suggests that the relationships between Geoff and the other members of the 'inner city crew' is like a competition - survival of the fittest. From the work of Charles Darwin, we know that humans have evolved to adapt to the environment and situations around them so that they out-live others and can survive. This could be applied to the way Geoff behaves because he may think that the fighting will give him a stronger power over others and therefore he will survive best. His behaviour will have become normal to him after a while and may seem like an ordinary everyday thing for him, because he has adapted to the behaviour.