The first edition of Psychobiology of Personality, published in 1991, provided a comprehensive treatment of a subject about which most psychologists have only a fragmentary knowledge, if any. Most behavior geneticists know a lot about the genetics of personality but little about its psychophysiology or psychopharmacology. Conversely, most psychopharmacologists know a lot about the psychopharmacology of personality and psychopathology but little about behavioral genetics. Most social and many personality psychologists know a lot about traits and their assessment but little about the psychobiology of traits. The first edition was based on a levels approach to personality (see Figure 7-1) that attempts to explore all levels of personality, from the genetic to the trait with stops at the neurological, biochemical, physiological, conditioning, and behavioral levels of explanation. The goal was not reductionism but a connectivism that respects the mode of explanation at each level and attempts to understand, as much as possible, the connections between phenomena at all levels. Although causation is usually assumed to work up from the more basic levels to the behavioral ones, it can work in the opposite direction. Only experimentation can decide the issues of causation; correlation cannot do this, as we patiently explain to students in introductory psychology (although clinicians sometimes forget this).

The psychobiology of personality has been explored by “top-down” and “bottom-up” approaches. The top-down approach is to identify basic personality traits in humans and then see what correlates of these traits can be found in physiology, biochemistry, neurology, and genetics.