Animal Models of Personality
A.R. Cools and B.A. Ellenbroek
In practice, we need to be able to assess personality in order to predict how patients will respond to illness and to treatment. We also need to be able to manage people with personalities that cause serious problems to themselves or other people. As elaborated in the next paragraph on 'basic dimensions of personality', the term 'personality' refers to the balance between various traits. If one or more dominant traits are strong enough to cause difficulties to the person or to other people, the person is said to have a personality disorder. However, the borderline between normal personality and personality disorders is hard to define. In fact, there is no reliable and valid measure that allows a cut-off for distinguishing between normality and disorder. In clinical practice, therefore, a personality is said to be disordered when it causes suffering to the person or to other people. It is evident that such a criterion is impossible to model in animals. Indeed, animal models of personality disorders are not available, unless one adheres to the traditional classification system for personality disorders: in that case, we are dealing with animal models for anxiety disorders, mood disorders, etc., as discussed in several other chapters of this textbook.
Recently, however, Cloninger et al. (1994) have introduced a new classification system that allows differential diagnosis of personality disorders with the help of temperament and character descriptors. As outlined below, it is possible to model a number of these temperament descriptors in animals. In this chapter, we will focus on these and related studies.
In the first place we should clarify what we want to model. Thus, we should delineate the basic units of personality. As elaborated by Zuckerman (1991), the basic unit in personality psychology is the trait, defined as a group of related habits that, in turn, are defined as consistent behaviours in specific situations. Using this definition, Zuckerman suggests that the term 'basic dimensions of personality' should refer to
supertraits that have internal reliability and can be identified in terms
of their constituent traits across methods, genders, and cultures; these
supertraits should be identifiable in childhood but not necessarily in
infancy; the basic personality traits should show consistency over
time (but because the phenotype and environment interact throughout
life, one would not expect perfect or even near perfect consistency
over long periods of time); they should be identifiable in species
other than the human on the assumption that they have been shaped
by natural selection; they should show at least moderate heritability
and be related to significant biological markers; and, eventually,
their biological basis in the structure and physiology of the nervous
system should be identifiable, though classification is a necessary
first step in biological bases of traits (pp. 40–41).
Unfortunately, there is no general agreement about the kind of classification system to be used for classifying fundamental traits. In psychiatry, there are at least two types of classification systems: the traditional system that distinguishes 5–7 dimensions resulting from factor analyses of the phenotypic structure of personality, and a more recently introduced functional classification system that is based on largely hypothetical characteristics of central nervous system functioning. The traditional classification system has resulted, among others, in the three-factor model of Eysenck (1947), encompassing the factors introversion-extraversion (E), neuroticism (N) (or emotional instability), and psychoticism (P); the five-factor model of Fiske (1949) and, later, Norman (1963), encompassing the factors extraversion, agreeableness, consciousness, emotional stability, and culture; and the seven personality dimensions of Zuckerman, which are activity, sociability, impulsivity, socialization, sensation seeking, emotionality (subdivided into general, anxiety, and hostility), and social desirability (Zuckerman et al., 1988). As elaborated elsewhere, this traditional classification system suffers from several shortcomings (Cloninger et al., 1994; van Praag, 1986; Tuinier and Verhoeven, 1995), the most important one being the fact that the entities classified are still complex behaviours that are not yet sufficiently broken down into specific and elementary items that are mutually exclusive and, in addition, theoretically tractable in the brain. Anyhow, animal models for some of these dimensions, especially anxiety in particular and emotionality in general, are discussed elsewhere in this book.
More recently, Cloninger has developed a neurobiologically based model to guide the rational development of descriptors for temperament and character (Cloninger, 1994). Temperament refers to those components of personality that are heritable, developmentally stable, emotion-based, or uninfluenced by sociocultural learning (Goldsmith et al., 1987), whereas character refers to those components of personality that are weakly heritable, and are moderately influenced by sociocultural learning (Loehlin, 1982). His model is based on the synthesis of information from twin and family studies, studies on longitudinal development, and neuropharmacological and neurobehavioural studies of learning in human and other animals, as well as psychometric studies of personality in individuals and twin pairs. Though the neurobiological processes are not yet identified, he proposes four temperament dimensions on the basis of individual differences in associative learning that are postulated to be genetically homogeneous and independent of one another: 'novelty seeking', which is associated with the behavioural activation system; 'harm avoidance', which is associated with the behavioural inhibition system; 'reward dependence', which is associated with the behavioural dependence system; and 'persistence', which is associated with the behavioural persistence system. The dimension 'novelty seeking' nicely corresponds with Zuckerman's dimension 'sensation seeking' (McCourt et al., 1983). Cloninger complements his model of personality with three character dimensions, self-directed behaviour, cooperativeness, and self-transcendence.