Heritability.
Studies carried out in behaviour genetics, with most research studying twins, since we know the proportion of genes they have in common.

It is a statistical concept and so does not give information about individuals, but assesses the relative contribution of genetics and the environment to individual differences, e.g. the heritability of neuroticism has been found to be or 30%. Criticised for it's methodological grounds assuming that twins grow in the same environment and using biased samples.

Psychometrics.
Uses tests to measure psychological characteristics e.g. intelligence and personality, a tradition started in the late 19th century with Galtons attempt to measure intelligence using sensory-motor criteria.

The problems with psychometrics.
Because tests involve questionnaires, there are possible problems with demand characteristics, and people giving what they perceive to socially desirable answers. This can be countered by the inclusion of questions designed to identify these kinds of influences. Another issue is that they often have forced-choice formats, where people must choose from among the answers provided, none of which may be entirely accurate for that person, so the results may present a distorted picture

Implicit Personality Theories.
Lay theories based on traits such as 'sociable' or 'kind'. They assume coherence within personality, someone assertive is more likely to be energetic than someone who is passive for example.They assume stability and consistency of behaviour over time and situations.

These in turn, at the lowest and most observable level of the heirarchy, express themselves in behaviour, and the beliefs, feelings and preferences which an individual expresses, e.g. 'I never miss deadlines'.

Extraversion.
Eysencks type theory of personality suggests that personality can be expressed on two dimension, one of which is extroversion-introversion. Everyone can be placed somewhere on a continuum from extreme extraversion to extreme introversion. Extraversion is associated with a range of traits;including sociable;risk-taking;sensation-seeking; lively and active.

Buss & Plomins research in to temperament lends some support to this theory thourhg there studies of infants, as they claim to have indetifed three factors in young infants; Emotionality; sociability; and activity the last two are traits of extroversion. There is also psychometric and behavioural evidence though no clear evidence to establish that extraversion has biological basis.ARAS (ascending reticulocortical activatin system)

The area Eysenck believed controlled levels of cortical arousal which effect extroversion and introversion. Extroverts are chronically under aroused and so engage in behaviour likely to increase arousal.