Describe one individual cause of criminal behaviour and explain how one study investigates it

An individual cause of criminal behaviour could be biological influences with an example being Brunner - who investigated how genes and serotonin can lead to criminal behaviour. He investigated 5 males from the same family diagnosed with “borderline mental retardation” who had low IQ levels. They were investigated for their criminal past which involved arson, attempted rape and general aggressive behaviour. Brunner took urine samples for 24 hours where he noticed that their urine had low levels of the enzyme MAOA. This enzyme is important as it breaks up neurotransmitters such as serotonin, which is responsible for mood, where it has been found that too much or too little serotonin can cause aggressive behaviour. This lack of the enzyme is due to a genetic mutation. Therefore, the genetic makeup of a person, individual to them, can be responsible for their criminal behaviour.