The Personality Hierarchy Reconsidered

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Typically, higher order domains emerge from factor analytic studies of a large number of lower order traits. Although these factors are essentially statistical entities, trait theories assume that they reflect basic psychological and, in some cases, biological entities. They also appear to assume that these entities are the primary dimensions of personality: Research effort has largely focused on the higher order domains with comparative neglect of lower order traits. This leaves the status of the lower order traits unclear. Are they merely facets or subcomponents of higher order traits or distinct entities with their own etiology?

Lower Order Traits

The designation of lower order traits as "facet traits" implies that they are conceptualized as components of the broader domains. Behavioral genetic perspectives have tended to adopt this assumption. Because the higher order traits are heritable, there has been a tendency until recently to assume that the genetic contribution to personality largely operates at the higher order level. Lower order traits are assumed to be heritable because of their association with higher order traits (Loehlin, 1992). That is, the subtraits defining a domain derive their genetic underpinning from the same genetic factor. Recent research showing that most subtraits have a unique heritable component questions this assumption (Jang et al., 1998; Livesley et al., 1998).

Jang and colleagues (1998) estimated the heritability of the 30 NEO-PI-R facet traits after all genetic influence due to the higher order traits was removed from each using a regression technique. Substantial residual heritability was found for each trait that accounted for between 25% (competence) and 65% (dutifulness) of the variance in each trait. Livesley and colleagues (1998) reported similar findings for personality disorder traits.

The occurrence of specific genetic influences on subtraits has important implications for conceptualizing trait structure. These specific sources of genetic variance suggest that subtraits are not derivative structures but etiologically distinct entities. The emphasis of trait theory on global dispositions, such as extraversion and neuroticism, may have been misplaced or, at least, needs to be supplemented with greater attention to the significance of subtraits and their contribution to trait structure. Thus far, little attention has been paid to conceptualizing their relationship to higher order structures or to developing methods to identify them. Most trait theories, including the five-factor model, have relied on rational analysis to identify subtraits presumably because they were considered merely subcomponents of a higher order domain and hence reflected largely arbitrary distinctions. Given consistent evidence of specific genetic influences, greater attention needs to be given to identifying and defining these fundamental building blocks of personality. An etiological approach suggests that just as domains may be defined as clusters of traits sharing a common genetic influence, subtraits may be defined as clusters of behaviors (or test items) sharing the same specific genetic influence. This definition offers a criterion to refine the item content of a scale to foster convergent and discriminant validity.

Having demonstrated that the lower order traits are not merely subcomponents of the higher order domains but have a distinct genetic component besides a common component shared with other traits forming the same domain, we are left with intriguing questions about how the common and specific genetic components are organized to form the coherent constellations of traits described by trait theories and about the nature or status of the higher order constructs. Do the common genetic factors give rise to phenotypic structures or mechanisms with a distinct biological and psychological basis, as assumed by most trait models? With this model, the common genetic component underlying a domain is assumed to have an indirect effect on subtraits via this higher order structure. Or, do the common genetic entities have a direct influence on the expression of specific components and the resulting trait? This is a critical issue for trait models of personality.

Evaluating Models of Genetic Influence

These different models of genetic influence on personality traits may be explored using a model-fitting approach (Neale & Cardon, 1992). The common pathway model is structurally similar to the model of exploratory factor analysis used to specify the phenotypic structure of traits. It postulates a single latent variable (higher order trait) that mediates the covariation among a set of variables (lower order traits) that also have their own genetic and environmental basis. As shown in Figure 6.1, the covariation in a set of variables is hypothesized to be mediated by a single superordinate latent phenotypic variable (P), such as a higher order trait, which is influenced by a single additive genetic (Ap h. k), one shared (Ct; c.k) and one nonshared environmental factor (Ep e. k). Genetic (A'k, a'k) and environmental effects (C'k, c'k and E'k, e'k) specific to each variable are also specified. As applied to each domain of the five-factor model, the model postulates a single latent factor that mediates the influence of genetic and environmental effects on each lower order trait. Thus, a latent variable of neuroticism is hypothesized that mediates the influence of genetic and environmental influences on each of the six facets of Anxiety, Hostility, Depression, Self-consciousness, Impulsivity, and Vulnerability.

In contrast to this model, the independentpathway model specifies direct links between one or more genetic and environmental influences to each lower order trait. In Figure 6.2, the subscript . identifies the common factor and k identifies the variable and direct links (h. k, c. k, e.k) are shown from one or more additive genetic, shared and nonshared environmental influences common to all the variables (denoted A., E., respectively). Like

Figure 6.1 Common Pathway Model. Note: The figure shows the model for only one twin and only some paths are marked in this figure for clarity; a = 1.0 for MZ/.50 for DZ twin; A, E = Common additive genetic and nonshared environmental factors; A', E' = Variable specific additive genetic and nonshared environmental factors; N1 = Anxiety; N2 = Hostility; N3 = Depression; N4 = Self-consciousness; N5 = Impulsivity; N6 = Vulnerability. (Source: From "Genetic and Environmental Influences on the Covariance of Facets Defining the Domains of the Five-Factor Model of Personality," by K. L. Jang, W. J. Livesley, A. Angleitner, R. Riemann, and P. A. Vernon, 2002, Personality and Individual Differences, 33, pp. 83-101. Reprinted with permission.

Figure 6.1 Common Pathway Model. Note: The figure shows the model for only one twin and only some paths are marked in this figure for clarity; a = 1.0 for MZ/.50 for DZ twin; A, E = Common additive genetic and nonshared environmental factors; A', E' = Variable specific additive genetic and nonshared environmental factors; N1 = Anxiety; N2 = Hostility; N3 = Depression; N4 = Self-consciousness; N5 = Impulsivity; N6 = Vulnerability. (Source: From "Genetic and Environmental Influences on the Covariance of Facets Defining the Domains of the Five-Factor Model of Personality," by K. L. Jang, W. J. Livesley, A. Angleitner, R. Riemann, and P. A. Vernon, 2002, Personality and Individual Differences, 33, pp. 83-101. Reprinted with permission.

the common pathways model, unique genetic and environmental influences are also specified for each lower order trait (A'k, a'k, and E'k, e'k, respectively).

It should be noted that both models are consistent with the findings discussed earlier that subtraits are influenced by common and specific genetic factors. The models are tested by examining the extent to which they fit the data derived from twin studies. If the common pathways model is found to provide the best fit, the implication is that the hierarchical structure of personality arises from the effects of higher order factors that have a genetic and environmental basis. The task is then to explain how this entity differs from lower order or facet traits and the role it plays in the formation of the hierarchy. If the independent pathways model provides the best fit, the implication is that the higher order constructs of phenotypic analyses reflect the pleiotropic action of genes shared by all subtraits forming that domain rather than the effects of a phenotypic entity. In this case, the task is to explicate the mechanisms that lead to trait clusters. Besides the value of the models in evaluating personality structure, the approach also provides information on the

Figure 6.2 Independent Pathways Model. Note: The figure shows the model for only one twin and only some are paths marked in this figure for clarity; a = 1.0 for MZ/.50 for DZ twin; A, E = Common additive genetic and nonshared environmental factors; A', E' = Variable specific additive genetic and nonshared environmental factors; N1 = Anxiety; N2 = Hostility; N3 = Depression; N4 = Self-consciousness; N5 = Impulsivity; N6 = Vulnerability. (Source: From "Genetic and Environmental Influences on the Covariance of Facets Defining the Domains of the Five-Factor Model of Personality," by K. L. Jang, W. J. Livesley, A. Angleitner, R. Riemann, and P. A. Vernon, 2002, Personality and Individual Differences, 33, pp. 83-101. Reprinted with permission.

Figure 6.2 Independent Pathways Model. Note: The figure shows the model for only one twin and only some are paths marked in this figure for clarity; a = 1.0 for MZ/.50 for DZ twin; A, E = Common additive genetic and nonshared environmental factors; A', E' = Variable specific additive genetic and nonshared environmental factors; N1 = Anxiety; N2 = Hostility; N3 = Depression; N4 = Self-consciousness; N5 = Impulsivity; N6 = Vulnerability. (Source: From "Genetic and Environmental Influences on the Covariance of Facets Defining the Domains of the Five-Factor Model of Personality," by K. L. Jang, W. J. Livesley, A. Angleitner, R. Riemann, and P. A. Vernon, 2002, Personality and Individual Differences, 33, pp. 83-101. Reprinted with permission.

magnitude of genetic and environmental influences unique to each facet. This provides the basis for determining which facets should be grouped together within the taxonomy.

Jang and colleagues (2002) used this approach to evaluate the structure of the five-factor model assessed with the NEO-PI-R (see Table 6.1). Common and independent pathways models were applied to two samples of twins: a sample of 253 identical and 207 fraternal twin pairs from Canada and 526 identical and 269 fraternal pairs from Germany. An independent pathways model specifying two additive genetic factors and two non-shared environmental factors provided the best fit in each domain in both sets of twins.

In the case of Neuroticism, the Angry Hostility facet marked the first genetic factor, and the second factor influenced all facets except Angry Hostility and Impulsivity. With Extraversion, the most salient component of the first genetic factor was Gregariousness in both samples. All facets except Gregariousness and Excitement Seeking loaded on the second factor. Although this factor appeared similar in the two samples, in the Canadian sample the factor emphasized Warmth and Positive Emotions, whereas in the German sample it emphasized Assertiveness and Activity. The first general genetic factor contributing to the Openness to Experience domain was defined by the Fantasy, Aesthetics,

Table 6.1 Multivariate Genetic Analysis of Two Cross-National Samples

Proportions of the Total Variance Accounted for by Each Genetic and Environmental Factor (Independent Pathways Model) of the NEO-PI-R Neuroticism Facets on a Sample of German and a Sample of Canadian Twins and Proportions of the Total Variance Accounted for by Each Genetic and Environmental Parameter

Variance Accounted by Each Parameter

Facet Scale

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