Genetics and Human Behaviour: The Ethical Context
Trait definition and measurement
8.2 Different aspects of personality can be described at different levels. One can either choose the highest level, at which all the traits are independent of one another, or a variety of lower levels, at which the traits are to varying degrees correlated with each other. Genetic research into personality has largely concentrated on the first, highest level; what might be termed ‘global’ traits.
Table 8.1: The 'Big Five' Personality Traits
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8.3 The dominant view at present puts the number of independent personality traits at five; this is called the ‘Big Five’ model of personality. The ‘Big Five’ traits are: Neuroticism, Extraversion, Agreeableness, Conscientiousness and Openness to Experience (see Table 8.1).(2) Each trait has a normal distribution of scores. These five traits, or factors, are commonly referred to as ‘dimensions of personality’. There is disagreement among psychologists about the number of core personality traits; alternative views range from three to seven. The British psychologist Hans Eysenck originally suggested three – Neuroticism, Introversion and Psychoticism. However, in the Big Five Model, Psychoticism is broken down into three separate factors (Agreeableness, Conscientiousness and Openness to Experience). Impulsivity (sensation-seeking) is also sometimes separated out. It is important to note that these traits are used for descriptive convenience, rather than because there is evidence that they have distinct biological causes or pathways that affect personality.
8.4 Most research indicates that the two most robust traits are those of Neuroticism and Introversion–Extraversion. These are highly replicable, they account for a considerable portion of the variance across a very wide range of measures, and they can each be reliably measured by relatively short self-report scales ideally suited to large-scale genetic studies.
8.5 Using factor analysis, it is possible to determine how many independent factors or traits exist within a given body of data, but not how these factors are related. Thus, from the results of descriptive studies alone, we are unable to tell whether a given dimension of personality merely provides us with a convenient set of coordinates within which to locate an individual’s personality (as, say, East–West and North–South are used as convenient sets of coordinates within which to fix spatial location), or whether it has a basis in underlying causal reality (as, say, up–down has a basis in the force of gravity). Because of this ambiguity, many of the trait terms used in personality research (even when they all operate at the highest, dimensional, level) do not reflect entirely different traits, but rather rotations of one another. For example, the trait of novelty-seeking blends some lower-order traits that, in alternative descriptive systems, make up Extraversion with others that make up Psychoticism.
8.6 A further problem is that different investigators may refer to what is essentially the same dimension by different names (often reflecting different theories into which the dimension has been embedded). There are no clear pre-existing signposts to suggest for which personality traits researchers will most likely find genetic influences. However, if there is a substantial heritable component to a given personality trait, then this can itself provide both an external criterion by which to validate the factor purporting to represent the trait as well as a theoretical framework for prediction and experiment to establish its causal reality. For example, if research in quantitative genetics shows a substantial genetic influence on variation in Neuroticism, this simultaneously provides support for the reality of the underlying trait of Neuroticism, for the tests by which it is measured, and for the factor-analytic solution that has yielded the factor of Neuroticism.
Footnotes2 Since terms such as Neuroticism are also in common usage, we adopt the practice of psychologists of capitalising the first letter of each trait in order to indicate that we are referring to these traits as they are defined and studied by psychologists.
3 Quantitative genetics research has shown that the genetic influences on Neuroticism affect almost exclusively its comorbidity with other traits as distinct from the liability to any one particular disorder. The studies show also that scores on self-report scales of Neuroticism provide a good measure of the heritable component of the comorbidity of neurotic disorders. Thus, this trait is best regarded as one of susceptibility to the entire gamut of neurotic disorders, with the actual nature and occurrence of such a disorder depending upon life events.
4 This is the best established of all personality traits. Like Neuroticism, Extraversion has been embedded in several, experimentally testable, neurobiological theories and progress is being made towards identifying the underlying brain mechanisms.