The Four Levels of Explanation:
Biological Level (focuses on the biological and chemical processes underlying behaviour): Interpersonal aggression may be focused on the effect or role of hormones or activity in specific brain areas.
Alcohol can contribute to aggressive behaviour
Low blood sugar levels can boost aggressiveness
Testosterone: males with high testosterone levels are more prone to delinquency, hard drug use and aggressive responses when provoked.
High temperature: heat
The amygdala in humans is the brain structure which has been liked to aggressive behaviour.
Basic Process (focuses on the psychological processes that are widespread among humans): Aggression may focus on the perceptions and emotions that commonly precede it.
Anger: an emotional readiness to perform an aggressive behaviour (anger arises when a person who frustrates an individual could have chosen to act otherwise).
When experiencing pain
Berkowitz revised the frustration-aggression theory: the theory states frustration leads to anger and anger can potentially lead to aggressive behaviour. Creates a motive for aggression. Fear of punishment or disapproval may cause the aggressive behaviour to be displaced against a target or oneself.
Person Level (focuses on the individual differences in behaviour): Focuses on different levels of aggression displayed by people with different types of personalities.

Sociocultural Level (focuses on the influence that other people exert on behaviour by studying behaviour in social and cultural contexts): focuses on the role of onlookers or on different levels or types of interpersonal aggression displayed in different cultures.
Crowding: places where there's tons of peoples such as festivals.
Social influences on aggression: environmental cues can increase the likelihood or amplify aggressive behaviour.
Aggression can be learnt.