Biosocial approaches to personality have traditionally been more metaphorical than empirical. Freud (1954) began a project to describe the complex functioning of the mind in terms of a brain model, but recognizing that neurophysiological knowledge was not ready to support such a theory, he abandoned this project for a psychological model that used mechanistic but strictly hypothetical constructs.Pavlov (1927/1960) and his followers (e.g., Teplov, 1964) also approached temperament using a neurophysiological model, but in their case it was developed empirically from studies of individual dogs and was based on the dogs' reactions to human experimenters and to conditioning situations. Two types of dogs described by Pavlov could be regarded as extreme types on a dimension that will be called sensation seeking. One type of dog was vivacious, exploratory, demonstrative, and emotionally adient toward humans, whereas the other was inhibited and cautious and avoided humans and other novel stimuli and situations. Although these types were sometimes used to describe human personalities, the extension was largely by metaphor rather than from systematic study of common behavioral and physiological correlates.However the work of Pavlov and his students showed the feasibility of using animal models for human individual differences in temperament.In modern times, Eysenck (1967) has been the outstanding pioneer in the effort to build a personality theory on a biological basis. The trait of extraversion, in particular, has been related to Pavlov's idea of a "strong nervous system" type that is characterized by insensitivity to low levels of stimulation but by reactivity at high levels of stimulation. Conversely, the introvert is said to be more sensitive to weaker stimuli but shows "transmarginal inhibition" (a Pav-lovian construct) in response to intense stimuli.The "optimal level of stimulation" is a construct described by Wundt (1893) and by modern translators such as Hebb (1949).The discovery of the role of the reticular activating system in regulating central nervous system (CNS) arousal led a number of theorists, including Berlyne (1960) and Hebb (19SS), to reformulate the optimal level of stimulation in terms of an optimal level of arousal. Eysenck's (1967) later theory also incorporated this idea, making the sensitivity of the reticulo-cortical system the basis of the introversion-extraversion trait. Unlike prior theorists, Eysenck has marshalled a vast amount of data on behavioral and psychophysiological correlates of extraversion (usually denned by a questionnaire) to support his theory.