Hans Eysenck’s theory of criminal personality suggests that personality is biologically based and that personality traits include dimensions of extraversion and neuroticism that can be measured using a personality questionnaire. Extraversion refers to a biological need individuals have for high or low levels of environmental stimulation, determined by the level of arousal in a person’s central and autonomic nervous system. This means that people with high levels of extraversion have a low level of arousal, meaning they require more environmental stimulation to fuel their excitement. In the context of forensic psychology, this environmental stimulation may include criminal behaviour. Neuroticism refers to the stability of personality and a high neuroticism score would represent someone who is more reactive and volatile and perhaps more likely to engage in offending behaviour. Later, Eysenck added a third personality dimension, psychoticism, which relates to the degree to which somebody is anti-social, aggressive and uncaring. Eysenck (1964) argued for there being a criminal personality, which would characterise people who score highly on these three dimensions.