As in the 1991 first edition of this book, I begin with a discussion of temperament and personality and their basic dimensions. This is a necessary first step in a top-down approach because one cannot begin a levels analysis of psychobiology without a classification of traits at the top level. It would be like a science of astronomy without distinctions between planetary bodies such as asteroids, planets, stars, and galaxies; geology as a science of “rocks” sorted by size; or biology that makes distinctions only between two-legged and four-legged creatures – putting humans and chickens in the same category. Classification of phenomena is basic to any science. Without it, all is chaos.

There is a difference, however, between the classification of behavioral traits and other types of scientific classification. We are not defining “types” in the sense of clear-cut assortment of individuals. The concept of continuous, normally distributed trait dimensions is not widely understood outside of psychology. I purposely use the labels “high” and “low” sensation seekers, rather than type terms, such as “Big-T” and “Little-t,” to define persons falling near the extreme ends of the continuum on this trait. Types, however, may be defined from a combination of independent dimensions as particular combinations of high or low scores on these dimensions in the same way that syndromes of psychopathology are defined by particular combinations of symptoms.

In the earlier volume, I discussed certain issues about traits that preoccupied personality researchers in the 1970s.