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Biology of Personality and Individual Differences Editor: Turhan Canli (2006). New York: The Guilford Press, 462 pp, US$72.60, ISBN 1-59385-252-5.

Twin Research and Human Genetics Volume 9 Number 4 pp.615–619 As a social species, we are in the habit of continuously predicting each other’s behavior. How do we do that? How do we know what our family members, colleagues, friends (or adversaries) are probably going to do next? In ‘guesstimating’ behavior we use three crucial pieces of information. First, we take into account the environmental context in which we observe a person. We expect a different response to insulting behavior than to a friendly smile, but also expect that distinction to be less sharp at a party compared to a work setting or a traffic jam. Second, age and the sex of the person importantly shape our expectations regarding her or his behavior. We don’t expect bungeejumping in a 100-year-old, nor being shot by an 11-year-old (although both events have actually been recorded), and we rightfully consider recidivist female pedophiles a rare breed (although they do apparently exist). Finally, if we know the person reasonably well, we take into account her or his general tendency to respond with certain classes of behavior across a wide range of situations. This tendency is known as personality. Whereas the environment is in constant flux and causes changes in an individual’s behavior over time, personality is considered the constant factor that causes stability of behavior over time. This constancy is likely to be hardwired in our biology, and the brain seems the obvious place for such hardwiring. The key question, of course, is the exact shape of all this hardwiring of behavior and how it exactly comes about. The book Biology of Personality and Individual Differences addresses the biological processes underlying the major dimensions of personality as well as individual differences in behavioral tendencies coupled to age and sex (Canli, 2006). Integrating compelling lines of inquiry that until now have largely remained disparate, it brings together leading investigators from molecular and behavior genetics, neurobiology, neuroimaging, comparative psychology, personality psychology, clinical psychology and psychiatry. Coverage includes a wide range of topics, ranging from the structure of personality, its roots in genetics, its mapping onto brain processes, and the importance of personality in the vulnerability for psychopathology. This is a comprehensive book on the biology of individual differences that will be of interest to graduate students and researchers alike. To set the scene, Fowles (chapter 2) opens with a comprehensive review of Jeffrey Gray’s contribution to the theory of personality. In Gray’s view there are two basic mechanisms that control behavior. One mechanism, the behavioral inhibition system (BIS), is activated by novelty and stimuli associated with negative reinforcement, that is, punishment or omission of reward. The other mechanism, the behavioral approach system (BAS) is activated by stimuli associated with positive reinforcement, that is, reward or termination of punishment. Gray located these systems to the septohippocampal system (to which the amygdale was later added) and the ventral striatum (including the nucleus accumbens). Engagement of both systems is associated with arousal, as reflected in changes in autonomic nervous activity and hormonal secretion. Applied to personality, Gray proposed that the most salient individual differences reflect the variation in sensitivity to stimuli associated with negative or positive reinforcement and the coupled behavioral tendencies of avoidance and approach. Specifically, individual differences in the functioning of the reward system in response to appetitive stimuli are implicated in the personality traits extraversion and sensation seeking/ impulsivity. Individual differences in activity of the punishment system in response to aversive stimuli are implicated in the personality traits of neuroticism and harm avoidance. Extraversion and neuroticism, in Gray’s view, relate to harm avoidance and sensation seeking/ impulsivity by a 45° rotation such that sensation seeking is in the neurotic/extraverted quadrant and harm avoidance is in the neurotic/introverted quadrant. With these main dimensions of reward and punishment sensitivity as a guideline, the book proceeds with a number of chapters on individual differences in the reward system. Zuckerman (chapter 3) reviews the biological and social factors shaping sensation seeking. As an example of their complex interplay, a twin study is cited that showed high heritability for twins raised in nonreligious families (61% for females and 49% for males) but no genetic effects for Biology of Personality and Individual Differences