Taxonomies of Individual Differences

Taxonomic work has focused on categorizing the infinite ways in which individuals differ in terms of a limited number of latent or unobservable constructs. This is a multi-step, cyclical process of intuition, observation, deduction, induction, and verification that has gradually converged on a consensual descriptive organization of broad classes of variables as well as on methods for analyzing them. Most of the measurement and taxonomic techniques used throughout the field have been developed in response to the demand for selection for schooling, training, and business applications.

Personality and Ability

Although to some the term personality refers to all aspects of a person's individuality, typical usage divides the field into studies of ability and personality. Tests of ability are viewed as maximal performance measures. Ability is construed as the best one can do on a particular measure in a limited time (speed test) or with unlimited time (power test). Personality measures are estimates of average performance and typically include reports of preferences and estimates of what one normally does and how one perceives oneself and is perceived by others.

The same procedures used to clarify the structure of cognitive abilities have been applied to the question of identifying the domains of personality. Many of the early and current personality inventories use self-descriptive questions (e.g., do you like to go to lively parties; are you sometimes nervous) that are rationally or theoretically relevant to some domain of interest for a particular investigator. Although there is substantial consistency across inventories developed this way, some of this agreement could be due to conceptually overlapping item pools. Other researchers have advocated a lexical approach to the taxonomic problem, following the basic assumption that words in the natural language describe all important individual differences. This shifts the taxonomic question from how are individuals similar and different from each other to how are the words used to describe individuals (e.g., lively, talkative, nervous, anxious) similar and different from each other.

Dimensional analyses of tests developed based on lexical, rational, or theoretical bases suggest that a limited number (between three and seven) of higher order trait domains adequately organize the thousands of words that describe individual differences and the logically infinite way that these words can be combined into self or peer report items. The broadest domains are those of introversion-extraversion and emotional stability-neuroticism, with the domains of agreeableness, conscientiousness and intellectual openness or culture close behind. These domains can be seen as asking the questions that one wants to know about a stranger or a potential mate: are they energetic and dominant (extraverted), emotionally stable (low neurotic), trustworthy (conscientious), lovable (agreeable), and interesting (intelligent and open).

Measures of ability and personality reflect observations aggregated across time and occasion and require inferences about stable latent traits thought to account for the variety of observed behaviors. However, there are other individual differences that are readily apparent to outside observers and require little or no inference about latent traits. The most obvious of such variables include sex, age, height, and weight. Differences that require some knowledge and inference are differences in ethnicity and social economic status. These obvious group differences are sometimes analyzed in terms of the more subtle measures of personality and ability or of real life outcomes (e.g, sex differences in neuroticism, mathematics ability, or income).

Personality Taxonomies

Some of the earliest work on personality taxonomies were descriptions of different "Characters" and may be found in the work of Plato, Theophrastus, and Galen. Plato's descriptions of the personalities characteristics needed for leadership: " ... quick intelligence, memory, sagacity, cleverness, and similar qualities, do not often grow together, and ... persons who possess them and are at the same time high-spirited and magnanimous are not so constituted by nature as to live in an orderly and peaceful and settled manner; they are driven any way by their impulses, and all solid principle goes out of them. ... On the other hand, those stable and steadfast and, it seems, more trustworthy natures, which in a battle are impregnable to fear and immovable, are equally immovable when there is anything to be learned; they are always in a torpid state, and are apt to yawn and go to sleep over any intellectual toil." (Plato, The Republic, Book 6 503c from Benjamin Jowett Fourth Edition). can be interepreted as a discussion of the dimensions of impulsivity and anxiety.

More well known are the four temperaments discussed by the early physician and philospher, Galen, and associated with Hippocrates. Although quaint by today's understanding, this was an attempt to provide a biological explanation for individual differences. The nature of man was seen as a function of "Blood, phlegm, yellow bile, and black bile are the particular elements of the nature of man." The four temperaments were seen as representing too much or too little phlegm, black bile, white bile, and blood. Peter Heineman's dissertation provides an excellent review of this early work.

Dimensional representations of personality dimensions have emphasized between three to seven fundamental dimensions. These dimensions are, in turn, used to organize research across different laboratories. Although there is strong agreement about the importance of Introversion/Extroversion and Emotional Stability, Psychoticism, the third factor in the "Giant 3", is seen as being composed of Agreeableness and Conscientiousness in the "Five Factor Model".

  • A great deal of work in the past 50 years has been devoted to developing an adequate description and explanation of the individual differences. The descriptive taxonomic work is associated with Lew Goldberg and his colleagues and is called the "Big 5" dimensions of personality, or Paul Costa and R.R. McCrae and their colleagues who talk about the "Five Factor Model". Both of these approaches are descriptive.

  • Much of the work on causal theories of personality can be organized around the biologically driven models associated with Eysenck, Gray, Depue, Fowles, and Cloninger. See an abbreviated reading list describing the Even Bigger 3 (The Giant 3) fundamental" dimensions and how they have been described.
    • For those with a biological orientation, the work of Hans Eysenck and Jeffrey Gray has focused on biological models of personality. Gray's 2000 text on the neuropsychology of anxiety is a must read.
    • see also Smillie, L.D., Pickering, A.D. and Jackson, C.J. (2006) The new reinforcement sensitivity theory: implications for personality measurement. Personality and Social Psychology Review, 10, 320-335. (abstract)
    • Revelle, W. (in press) The contribution of reinforcement sensitivity theory to personality theory. To appear in P. Corr (Ed.) Reinforcement Sensitivity Theory of Personality. Cambridge University Press. (pdf of the draft)
  • More recently, much of the personality taxonomic literature has been devoted to the descriptive taxonomies of the "Big 5" or the "Five Factor Model." An abbreviated reading list describing the "Big 5" or "Five Factor Model" Oliver John's chapter in the Pervin Handbook is an excellent resource, but is not available on the web.

  • Other taxonometric theories of temperament are reviewed by Peter Heineman.

  • Affective ratings as well as interpersonal traits including dominance and love may be organized into a special two dimensional representation known as a circumplex. Circumplex models are two dimensional factor models with items or scales tending to have equal communalities but with a factor structure such that many items have non-zero loadings on both factors.
    • Michael Gurtman has developed circumplex models of personality and interpersonal behavior
    • Tests for circumplex structure were developed by G. Scott Acton for his disseration and described in
      • Acton, G. S. , & Revelle, W. (2002). Interpersonal personality measures show circumplex structure based on new psychometric criteria. Journal of Personality Assessment, 79, 456-481. (pdf)
      • Acton, G. S., & Revelle, W. (2004). Evaluation of ten psychometric criteria for circumplex structure. Methods of Psychological Research. 9, 1-27.
      • Unfortunately, the tests described in these papers were written in Pascal for Mac Classics and are no longer available, but the more useful tests have converted to R and are available as part of the psych package.
    • Hofstee, W. K. B., de Raad, B., & Goldberg, L. R. (1992). Integration of the big five and circumplex approaches to trait structure. Journal of Personality and Social Psychology, 63, 146-163. This paper takes the "Big 5" trait dimensions and considers items in 10 dimensional planes (two dimensional slices of the 5 dimensions). Their basic observation is that many items are factorially complex, with a complexity of two, but few items have a complexity greater than two.
    • Another example of circumplex structure in the personality-affect domain, is a two dimensional representation of the structure of affect. See for instance Rafaeli, E. & Revelle, W. (2006). A premature consensus: Are happiness and sadness truly opposite affects? Motivation and Emotion. (pdf from Springer). Appendices to this article are available from Eshkol Rafaeli or from the PMC lab.
  • For items that are used to measure each of the Big 5 and for the facets of these higher order dimensions, see Lew Goldberg's International Personality Item Pool.

  • To take a short personality test based upon Big5 constructs and to get some feedback about your level of these 5 dimensions, visit the personality project's online personality inventory

  • Chapters in the John, Robins and Pervin (2008) Handbook of Personality devoted to the Big 5 and the Bigger (Giant) 3 include those of John and Srivastava, McCrae and Costa, and Clark and Watson.