personality is a particular pattern of behaviour an thinking that prevails across time and situations and differentiates one person from another. research on human personality requires two kinds of effort: Identifying personality characteristics: determining the variables that produce and control them. The earliest known explanation for individual differences in personality was proposed by the greek physician hippocrates in the fourth century b. c. e and refined by his successor galen in the second century c. e. personality types: different categories into which personality characteristics can be assigned based on factors such as developmental experiences. Individual differences in personality are of different degree, not the kind of difference. The degree to which an individual expresses a particular personality trait. personality trait is an enduring personal characteristic that reveals itself in a particular pattern of behaviour in different situations. gordon allport was one of the first psychologists to search systematically for a basic core of personality traits.