Hans Eysenck was a German psychologist who developed personality theories that were based on genetics and temperament. He theorized that personality traits develop from innate genetic influences that are biological and inherited.
Although he recognized the importance of the environment on behavior he believed that personality traits developed from mostly biological origins. Eysenck's main focus was on temperament which he saw as long term patterns of behavior.
He originally identified two areas of temperament: neuroticism and extroversion/introversion.
Neuroticism is a temperament level that ranges from calm to nervous. Eysenck theorized that this aspect of personality was influenced by the sympathetic nervous system which is responsible for the 'fight or flight' response. People who were high in neuroticism levels have issues with anxiety and panic attacks.
The extroversion and introversion area of temperament is a range between shyness and an outgoing nature. Individuals vary widely on this spectrum.
Later on Eysenck added a third area of psychoticism to incorporate people with mental illness. Ensynck's theories were influential because they propose a cause of personality traits which most personality theories do not account for.