Originally published in 1969, this book deals extensively with the description and measurement of personality.

Beginning with a statement of the principles of typological research in psychology, set against the background of general taxonomic principles in biology, the study discusses in detail results and generalisations from the Eysencks' previous work. The second part of the book describes several large-scale studies using personality questionnaires prepared by the authors, as well as the standard ones of Cattell and Guilford. There is a comparative study of the Eysenck, Cattell and Guilford inventories, which analyses the degree to which similar factors can be found in these three instruments and discusses areas of agreement and disagreement between the three authors. The third part deals with personality studies in children, and includes a chapter on personality structure in subnormal subjects. These studies are concerned with discovering the extent to which personality structure changes with increasing age, and to what extent it is possible to measure personality in younger children. They also examine sex differences in personality structure, and show quite marked differences between the sexes on a number of primary personality traits.

The results of the Eysencks' work in this field directed new light on the structure of personality and cast doubt on many widely accepted findings of the time.