There are two major categories for how biological and behavioral scientists study the relation between genetics and personality. The first and most direct method for establishing a genetic basis for personality traits is known as the "quantitative trait loci" (QTL) approach (see chapter 2). The essence of this approach is "not to find the gene for a particular personality trait, but rather some of the many genes that make contributions of varying effect sizes to the variance of the trait."5
QTL research has begun to uncover some of the many genetic markers involved in basic nonpathological personality traits, such as novelty- or thrill-seeking, neuroticism/anxiety, and indirect genetic markers in potentially pathological levels of aggression, sexuality, impulsivity, or lack of constraint.6 To take just one example: thrill seeking appears to be associated with long repeating sequences of base pairs (rather than short sequences) in an allele for a dopamine receptor (DRD4) on chromosome 11. Having a short form of the allele appears to result in a more efficient method for binding dopamine, and hence having a long repeating sequence rather than short repeating sequence results in a dopamine deficiency. This deficiency in turn is associated with a greater need for thrill seeking, such as riding roller coasters or playing the stock market. Dopamine appears to be related to the experience of pleasure, and those with low baseline levels would then naturally seek experiences that give them a rush, that raise their low levels of dopamine as it were. It is important to point out, however, that QTL research is just a first step toward locating a particular region on the chromosome rather than the specific gene involved, and in that sense it is a beginning rather than an end in the process of uncovering the genetic basis for personality traits.
Yet there is a second, more traditional, method for examining the effect that genetics plays in behavior and personality, and that is through studying twins, both identical (monozygotic) and fraternal (dizygotic), who have been raised to gether and apart. As discussed in chapter 2, the logic of this twin-study approach is simple yet powerful. Identical twins are essentially 100 percent genetically alike, whereas fraternal twins, like all siblings, are on average 50 percent genetically alike. If genes play little role in personality, then identical twins (who are genetically identical) reared apart should be no more alike than any two people chosen at random from different environments. And yet, if environment plays little role, two identical twins separated at birth should be very similar—genes should influence personality regardless of environment. The emerging conclusion from twin-study research is that most basic personality traits have heritabil-ity estimates of between 40 and 50 percent. For instance, extroversion often correlates around .50 for monozygotic twins and around .20 to .25 for dizygotic twins, which leads to a heritability coefficient of between .50 and .60 (the simple model of heritability is calculated as twice the difference between mono- and dizygotic twin correlations). Neuroticism tends to have a heritability of about 50 percent, and conscientiousness, agreeableness, and openness slightly less, at between 40 and 50 percent. Such a figure leaves roughly 50 percent of the variance to be explained by three nongenetic sources: shared environment, unshared environment, and error. The surprising conclusion of recent researchers is that most of the environmental effects are of the unshared kind, such as birth order or different peer groups, and almost no variance is explained by shared environment. That is, environment does influence personality structure but not the environment that most people think, namely, growing up in a particular household. The environment that seems to matter most is the "unshared" environment between siblings, that is, their having different birth orders or different peer groups or even changes in parenting style and attitudes over time.7
These genetic and central nervous system (CNS) differences, in turn, have their most direct behavioral effect on temperament, that is, "a small number of traits that are present early in life, are biologically rooted, and relatively stable."8 Temperament is the foundation for personality development and such differences in basic personality dimensions as extroversion, neuroticism, and conscientiousness. As Mary Rothbart and her colleagues put it: "Temperament arises from our genetic endowment. It influences and is influenced by the experience of each individual, and one of its outcomes is the adult personality."
There are a number of different theoretical approaches to temperament and they do not all agree on the dimensions that constitute temperament. There is not yet a widely accepted taxonomy of temperament, as exists in the field of personality (with the "Big-Five" Model; see above). Some temperament researchers argue for three, and other researchers argue for nine distinct categories of temperament. On the low end, Arnold Buss and Robert Plomin argue for the dimensions of activity level, sociability, and emotionality. On the high end, and perhaps one of the most influential models of temperament, Alexander Thomas and Stella Chess propose nine dimensions: activity level, approach-withdrawal, adaptability, emotional intensity, mood, persistence, distractibil-ity, threshold, and rhythmicity.9 For instance, some infants are generally more active than inactive; are more approaching than withdrawing in novel social situations; are more often in positive than negative mood states; and some have lower thresholds for stimulation; finally some are more regular and predictable in their feeding, eating, and eliminating cycles (rhythmicity). These are the types of things parents are likely to notice about how their children differ from their very first day of life. It is my argument that some of these temperamental and personality dispositions function to lower thresholds for interest, talent, and achievement in science. That is, they make becoming a scientist more likely and influence both the kind and quality of scientist one becomes. The question now is which specific traits lower these thresholds.
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