<P> In general, patients with brain damage have been difficult to find and study . In the 1990s, researchers began to use electroencephalography (EEG), positron emission tomography (PET), and more recently functional magnetic resonance imaging (fMRI), which is now the most widely used imaging technique to help localize personality traits in the brain . </P> <P> Ever since the Human Genome Project allowed for a much more in depth understanding of genetics, there has been an ongoing controversy involving heritability, personality traits, and environmental vs. genetic influence on personality . The human genome is known to play a role in the development of personality . Previously, genetic personality studies focused on specific genes correlating to specific personality traits . Today's view of the gene - personality relationship focuses primarily on the activation and expression of genes related to personality and forms part of what is referred to as behavioural genetics . Genes provide numerous options for varying cells to be expressed; however, the environment determines which of these are activated . Many studies have noted this relationship in varying ways in which our bodies can develop, but the interaction between genes and the shaping of our minds and personality is also relevant to this biological relationship . DNA - environment interactions are important in the development of personality because this relationship determines what part of the DNA code is actually made into proteins that will become part of an individual . It has been noted that while different choices are made available by the genome, in the end, the environment is the ultimate determinant of what becomes activated . Small changes in DNA in individuals are what lead to the uniqueness of every person as well as differences in looks, abilities, brain functioning, and all the factors that culminate to develop a cohesive personality . </P> <P> Cattell and Eysenck have proposed that genetics have a strong influence on personality . A large part of the evidence collected linking genetics and the environment to personality have come from twin studies . This "twin method" compares levels of similarity in personality using genetically identical twins . One of the first of these twin studies measured 800 pairs of twins, studied numerous personality traits, and determined that identical twins are most similar in their general abilities . Personality similarities were found to be less related for self - concepts, goals, and interests . </P> <P> Twin studies have also been important in the creation of the five factor personality model: neuroticism, extraversion, openness, agreeableness, and conscientiousness . Neuroticism and extraversion are the two most widely studied traits . A person that may fall into the extravert category can display characteristics such as impulsiveness, sociability, and activeness . A person falling into the neuroticism category may be more likely to be moody, anxious, or irritable . Identical twins, however, have higher correlations in personality traits than fraternal twins . One study measuring genetic influence on twins in five different countries found that the correlations for identical twins were . 50, while for fraternal they were about . 20 . It is suggested that heredity and environment interact to determine one's personality . </P>

One of the first attempts at researching what is meant by the term personality