udies of cortical arousal have suggested that the brains of introverts and extroverts do in fact respond differently to various kinds of stimuli. Nevertheless, research has not yet shown biological correlations among other personality traits, indicating that, while genes may play an important role in some aspects of personality distinctiveness, they do not provide a complete explanation. John M. Oldham and Lois B. Morris (1990) note, "Genes themselves don't cause a baby to respond to a new face with a rush of tears . . . What genes determine is the way an organism's brain develops and the range of its normal neurobiological and biochemical reactions that characterize its personal style of responding to the environment" (p. 361).
Warren Norman expanded on Eysenck's model, developing a five-factor taxonomy that provides a more detailed set of scales by which to analyze personality. He accepts Eysenck's extroversion and emotional stability factors, but he adds measures of agreeableness, conscientiousness, and culture (later theorists prefer to call this
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