Whereas Zuckerman's theory concerns levels of MAO, which breaks down neuro-transmitters, other researchers hypothesize that levels of neurotransmitters themselves are responsible for specific individual di ferences. Neurotransmitters are receiving a great deal of attention as possible sources of personality dif ferences. One neurotransmitter, dopamine, appears to be associated with pleasure. For example, animals will work to obtain doses of dopamine, much as they would work to obtain food. As such, dopamine appears to function like a reward system and has even been called the feeling good chemical (Hamer, 1997). Drugs of abuse, such as cocaine, mimic dopamine in the nervous system, which accounts for the pleasure associated with taking them. However, such drugs deplete a person' s natural levels of dopamine, leading to unpleasant feelings after the drug leaves the nervous system, creating a drive or ur ge to obtain more of the drug.
A second important neurotransmitter is serotonin. Researchers have documented the role of serotonin in depression and other mood disorders, such as anxiety . Specifically, drugs such as Prozac, Zoloft, and Paxil block the reuptake of serotonin, leaving it in the synapse longer , leading depressed persons to feel less depressed. In one study, Prozac was given to nondepressed subjects. Over several weeks of observation, they reported less negative af fect and engaged in more outgoing and social behavior than did those in a control group (Knutson et al., 1998). In studies of monkeys, the monkeys that were higher in dominance and that engaged in more grooming had higher levels of serotonin. The monkeys low in serotonin were frequently fearful and aggressive (Rogness & McClure, 1996). In summarizing animal studies, Depue (1996) notes that low serotonin is associated with irritable behavior .
A third important neurotransmitter , norepinephrine, is involved in activating the sympathetic nervous system for fight-o -flight. Not surprisingl , personality theories have been proposed based on the neurotransmitters dopamine, serotonin, and norepinephrine. Probably the most comprehensive is Cloninger' s Tridimensional Personality model (Cloninger, 1986, 1987; Cloninger, Svrakic, & Przybeck, 1993), in which three personality traits are tied to levels of the three neurotransmitters. The first trait novelty seeking, is based on low levels of dopamine. Recall that low levels of dopamine create a drive state to obtain substances or experiences that increase dopamine. Novelty, thrills, and excitement can make up for low levels of dopamine, so novelty-seeking behavior is thought to result from low levels of this neurotransmitter .
The second personality trait identified in Cloninger s model is harm avoidance, which he associates with abnormalities in serotonin metabolism. Although various descriptions of the theory indicate increased or decreased serotonin levels are associated with increased harm avoidance, Cloninger himself (personal communication, October 2003) states that it is unwise to suggest a simple linear correlation between harm avoidance and absolute levels of serotonin. Very low levels of the principal serotonin metabolite 5-HIAA in cerebrospinal fluid are associated with risk of sever depression, but serotonin levels can also be elevated in states of anxiety or stress. The selective serotonin uptake inhibitors (like the antidepressants Prozac, Zoloft, or Paxil) result in increased levels of serotonin at synapses, which may increase anxiety initially, but then lead to decreased vulnerability to overreact to stress, probably by down-regulating sensitivity to serotonin when it is released in response to stress. So we have to distinguish the acute role of serotonin, which is increased in states of acute stress, and the role of serotonin down-regulation over the life span, which is associated with lower levels of harm avoidance. People low in harm avoidance are described as energetic, outgoing, and optimistic, whereas people high in harm avoidance are described as cautious, inhibited, shy, and apprehensive. They seem to expect that harmful and unpleasant events will happen to them, so they are constantly on the lookout for signs of such threatening events. And, like a dog that bites out of fear rather than anger, such a person can be irritable, snappy , and hostile.
The third trait in Cloninger' s model is reward dependence, which Cloninger sees as related to low levels of norepinephrine. People high on this trait are persistent; they continue to act in ways that produce reward. They work long hours, put a lot of ef fort into their work, and often continue striving after others have given up.
Genes Work through Neurotransmitter Systems to Influence Personality Although we discussed behavior genetics in more detail in Chapter 6, it is worth mentioning here that many researchers interested in personality and genetics are focusing on the genes involved in regulating our neurotransmitter systems. For example, if low levels of dopamine are related to novelty seeking, then perhaps the genes involved in dopamine transmission would be a good place to start in the search for the genetic basis of this personality trait. Keltikangas-Jarvinen and her colleagues in Finland (2003) have found that the type 4 dopamine receptor gene (D4DR) is associated with heightened levels of novelty seeking. However , other studies have not found these particular genetic dif ferences associated with novelty seeking (Azar , 2002). A metaanalysis of genetic studies of novelty seeking has suggested that very specific type of repeated genetic codes on the D4DR gene (Schinka, Letsch, and Crawford, 2002) are reliably associated with novelty seeking. These findings imply that many gene will be involved in the creation of any single personality trait. So, while looking for one gene as the basis of a personality trait is like looking for the proverbial needle in the haystack, now the researchers are looking for many dif ferent needles in the same big haystack. That is, they are looking for multiple genes that interact in complex ways to influence neurotransmitter systems. A prominent researcher in this area, Dean Hamer, recently commented, " After 10 years, it is quite clear to me that at least for most traits there are a very lar ge number of genes involved" (quoted in Azar, 2002). As new technology for analyzing gene sequences is developed, the search will likely become more tractable. Nevertheless, any answers that are found in the future are likely to reveal complicated and multiple interacting genetic contributions, possibly requiring environmental triggers, for the expression of any biologically based personality trait.
Cloninger's theory has had some impact in psychiatry , where it has been used to help explain various types of addictions. For example, alcoholics do not all become addicted for the same reasons. Cloninger ar gues that some alcoholics began drinking due to high novelty seeking, that they drink to make up for low levels of dopamine, and that they drink primarily for the pleasure af forded by boosting dopamine. Other alcoholics began drinking because they are high in harm avoidance, and they drink to relieve the stress and anxiety they chronically feel. These drinkers are motivated primarily for the relief from anxiety that alcohol provides (Cloninger, Sigvardsson, & Bohman, 1988). Understanding people's motivations for abusing substances may play a large role in helping them overcome their addictions. For example, some people may enjoy smoking because it relieves stress, whereas others enjoy smoking because it enhances pleasurable activities, such as drinking coffee and socializing.
It is probably clear that Cloninger' s model has much in common with Gray' s, Eysenck's, and Zuckerman's. For example, novelty seeking seems a lot like the reward sensitivity associated with the BAS of Gray' s theory. All of these theories have different explanatory bases for the traits (Depue & Collins, 1999). For example, Gray suggests that brain systems involved in learning through reward and punishment are important in determining these traits. Eysenck also implicates the brain and nervous system. Zuckerman focuses on the synapse and the neurochemicals found there. And Cloninger specifies particular neurotransmitters. All are perhaps describing the same behavioral traits but focusing on dif ferent levels of explanation within the body, ranging from the synapse to the brain.
Let's turn now to a consideration of two other personality dimensions, which appear to have a biological base that is not related to physiological reactivity— morningness-eveningness and brain asymmetry .
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