A layman would define personality as the characteristics of an individual in terms of how he thinks and behaves. Many theorists, however, interpret personality differently resulting in various theories of personality. Personality is determined by traits that are behaviors displayed by a person in most given situations. How a person reacts to common circumstances can also be used to predict future behavior. Traits are then classified into types that allow easier comparisons between each individual's attributes. The focus of this article would be on the biological aspects of personality whereby traits are believed to be heritable through genetics and associated with the central nervous system. Behavioral genetics is research that uses the results of studies conducted on families, twins and adoptions. The results of studies on how both genes and environment influence personality will be discussed. Some researchers have found possible problems with the representativeness of such studies. The outline of Eysenck's biological model of personality and arousal, Gray's BAS/BIS theory, and Cloninger's biological model of personality will further explain the biological effect on personality. An individual's behavior may differ depending on the circumstances they find themselves in, although there are definite signs of repetitiveness. behavior when placed in most situations. These characteristics are known as traits that make up each person's personality (Engler, 2014). Personality theorists do not have mutual agreement on how the term personality should be used. They each have their own definition of personality thus providing a large number of different personality theories (Engler, 2014). For example, Eysenck (1970) defines personality as a relatively permanent and coherent composition of an individual's dispositions which in turn establishes how the person adapts to his or her needs.