BIOLOGICAL AND GENETIC THEORIES – THE WAY WE ARE MADE
INHIBITION AND AROUSAL
Why do extraverts seek excitement?
In 1967 Eysenck developed inhibition theory. He argued that individual differences in extraversion–introversion are strongly determined by heredity and have their origins in the central nervous system. According to this theory, information from the environment is transmitted from the sense organs along neural pathways to the brain, where excitatory and inhibitory cortical processes result in either the facilitation or inhibition of behavioural and cognitive responses, in certain specific ways. Eysenck maintained that extraverts have relatively strong inhibitory processes and weak excitatory processes. Their ‘strong’ nervous system enables them to tolerate a high degree of stimulation. The brain’s slower and weaker reaction to stimuli creates a hunger or desire for strong sensory stimulation. So extraverts seek excitement from the environment.
Introverts, on the other hand, have strong excitory processes and weak inhibitory process. Their nervous systems are ‘weak’, but they have brains that react more quickly and strongly to stimuli. So they can tolerate only relatively small amounts of stimulation. Developing the theory Eysenck and Eysenck (1985) redeveloped inhibition theory to formulate arousal theory, which identifies the physiological systems underlying introversion–extraversion. The differences in the behaviour of extraverts and introverts are traced to various parts of the ascending reticular activating system (ARAS) – a network of fibres travelling upwards from the lower brain stem to the thalamus and cortex. Stimulation of the ARAS results in increases in alertness and arousal of the cortex. Other fibres descending from the lower brain stem influence bodily musculature and the autonomic nervous system. At the same time, fibres descending from the cortex can modulate the activity of the brain stem, increasing or inhibiting the excitability of the ARAS. So the relationship between the ARAS and the cortex is reciprocal.
The high cortical arousability of introverts is supposed to amplify incoming stimulation. According to this framework, very high and very low levels of stimulation are considered to produce negative hedonic tone, which is experienced as negative feelings and negative evaluation of the experience. Positive hedonic tone occurs only at intermediate levels of sensory stimulation. The levels at which negative and positive hedonic tone occur will be different for introverts and extraverts.
Testing the theory
Using this theoretical formulation psychologists have been able to make predictions about the behaviour of introverts and extraverts in experiments ranging from sensory deprivation to students’ study habits. For example, Campbell and Hawley (1982) predicted that introverts would prefer study locations that minimize intense external stimulation (such as study carrels) whereas extraverts would prefer large, open reading areas where socializing is permitted and both auditory and visual stimulation is high. These researchers gave students the EPQ, noted their preferred seating areas in a campus library, and asked them to fill out a study habits questionnaire. Their predictions turned out to be correct. They also found that extraverts took more study breaks, looking and walking around the room, going out for coffee etc.
Davies and Parasuraman (1982) found that extraverts also make more errors than introverts on long vigilance tasks. Eysenck explained this finding by suggesting that extraverts generate reactive inhibition (fatigue) more quickly and at greater levels than introverts when they are performing long tasks. Despite evidence that appears to support Eysenck’s theory, a comprehensive review by Stelmack (1990) showed that introverts and extraverts show no difference in brain-wave activity when at rest or asleep. It therefore seems likely that extraverts and introverts differ in terms of their sensitivity to stimulation, rather than in base rate levels of cortical activity.
Stress and performance
There has not been much direct investigation of how neuroticism affects performance, but many studies have examined the effect of anxiety – one of the component traits of neuroticism. According to Eysenck, the adverse effects of anxiety on performance are attributable to task-irrelevant processing activities, such as worry. Consistent with this, Eysenck and Eysenck (1985) found that students who report high levels of worry perform less well on tests. And when highly anxious people do perform well, it is at the expense of more effort and distress.
Sensation seeking
The differences between those who prefer bungee-jumping and those who world rather watch a good movie can also be addressed using a biological theory of personality.
Zuckerman (1994) conducted research into sensation seeking over a 30-year period, developing a questionnaire to measure the phenomenon and a biological theory to explain it. Zuckerman, Kolin, Price and Zoob (1964) identified four aspects of sensation seeking:
1. thrill and adventure seeking (risky sport)
2. experience seeking (desire for novelty)
3. disinhibition (stimulation through social activity)
4. boredom susceptibility (low tolerance for repetitive events)
Sensation seekers are more likely to have more sexual partners, use illegal drugs, take part in risky sport, be more complex, original and creative, and have more liberal and nonconforming attitudes. Zuckerman (1994) explained differences in sensation seeking in terms of level of arousal in the catecholamine system. (This system comprises neurons communicating via catecholamines, which include epinephrine, or adrenaline, norepinephrine and dopamine; see chapter 3.) According to Zuckerman, those with a low optimal level in this system work to reduce the stimulation in their environment, whereas those with a high optimal level seek to increase it.
GENETICS VS. ENVIRONMENT
Recent work in behavioural genetics has examined the contribution of genetic and environmental factors to human behaviour. A good example of this approach is represented by the twin studies of intelligence. Evidence for and against genetic influence In 1976 Loehlin and Nicholls examined the scores on self-report personality questionnaires of 800 pairs of twins. Nearly all traits showed moderate genetic influence, with monozygotic (identical) twins being much more similar than dizygotic (fraternal or non-identical) twins. A more extensive study (Loehlin, 1992) of 24,000 twin pairs in many different countries confirmed that monozygotic twins are much more similar than dizygotic twins on the Big Five personality dimensions. Riemann, Angleitner and Strelau (1997) found the same results when twins were rated by their friends on the same factors. Studies of genetically unrelated family members (parents and their adopted children) show no similarity in personality traits such as extraversion and neuroticism (Loehli n, 1992). This suggests that family environment itself does not contribute to similarities in personality between family members. Interestingly, recent studies have also shown only very slight similarities in personality between adopted children and their biological parents. A study by Plomin, Corley, Caspi, Fulker and DeFries (1998) found some evidence for a genetic basis for sociability, but almost no similarities in emotionality between biological parents and their adopted-away children, or between adoptive parents and their adopted children. Thus both adoption studies and twin studies are consistent with a genetic influence on personality. Only identical twins have exactly the same combinations of genes; dizygotic twins, just like ordinary siblings, will each inherit a different random sampling of half of each parent’s genes. It is also possible that research findings from twin studies are partly explained by the unique circumstances of being a twin. For example, twins who look similar may be encouraged t act in a similar way, whereas non-identical twins may be encouraged to behave differently.
INHIBITION AND AROUSAL
Why do extraverts seek excitement?
In 1967 Eysenck developed inhibition theory. He argued that individual differences in extraversion–introversion are strongly determined by heredity and have their origins in the central nervous system. According to this theory, information from the environment is transmitted from the sense organs along neural pathways to the brain, where excitatory and inhibitory cortical processes result in either the facilitation or inhibition of behavioural and cognitive responses, in certain specific ways. Eysenck maintained that extraverts have relatively strong inhibitory processes and weak excitatory processes. Their ‘strong’ nervous system enables them to tolerate a high degree of stimulation. The brain’s slower and weaker reaction to stimuli creates a hunger or desire for strong sensory stimulation. So extraverts seek excitement from the environment.
Introverts, on the other hand, have strong excitory processes and weak inhibitory process. Their nervous systems are ‘weak’, but they have brains that react more quickly and strongly to stimuli. So they can tolerate only relatively small amounts of stimulation. Developing the theory Eysenck and Eysenck (1985) redeveloped inhibition theory to formulate arousal theory, which identifies the physiological systems underlying introversion–extraversion. The differences in the behaviour of extraverts and introverts are traced to various parts of the ascending reticular activating system (ARAS) – a network of fibres travelling upwards from the lower brain stem to the thalamus and cortex. Stimulation of the ARAS results in increases in alertness and arousal of the cortex. Other fibres descending from the lower brain stem influence bodily musculature and the autonomic nervous system. At the same time, fibres descending from the cortex can modulate the activity of the brain stem, increasing or inhibiting the excitability of the ARAS. So the relationship between the ARAS and the cortex is reciprocal.
The high cortical arousability of introverts is supposed to amplify incoming stimulation. According to this framework, very high and very low levels of stimulation are considered to produce negative hedonic tone, which is experienced as negative feelings and negative evaluation of the experience. Positive hedonic tone occurs only at intermediate levels of sensory stimulation. The levels at which negative and positive hedonic tone occur will be different for introverts and extraverts.
Testing the theory
Using this theoretical formulation psychologists have been able to make predictions about the behaviour of introverts and extraverts in experiments ranging from sensory deprivation to students’ study habits. For example, Campbell and Hawley (1982) predicted that introverts would prefer study locations that minimize intense external stimulation (such as study carrels) whereas extraverts would prefer large, open reading areas where socializing is permitted and both auditory and visual stimulation is high. These researchers gave students the EPQ, noted their preferred seating areas in a campus library, and asked them to fill out a study habits questionnaire. Their predictions turned out to be correct. They also found that extraverts took more study breaks, looking and walking around the room, going out for coffee etc.
Davies and Parasuraman (1982) found that extraverts also make more errors than introverts on long vigilance tasks. Eysenck explained this finding by suggesting that extraverts generate reactive inhibition (fatigue) more quickly and at greater levels than introverts when they are performing long tasks. Despite evidence that appears to support Eysenck’s theory, a comprehensive review by Stelmack (1990) showed that introverts and extraverts show no difference in brain-wave activity when at rest or asleep. It therefore seems likely that extraverts and introverts differ in terms of their sensitivity to stimulation, rather than in base rate levels of cortical activity.
Stress and performance
There has not been much direct investigation of how neuroticism affects performance, but many studies have examined the effect of anxiety – one of the component traits of neuroticism. According to Eysenck, the adverse effects of anxiety on performance are attributable to task-irrelevant processing activities, such as worry. Consistent with this, Eysenck and Eysenck (1985) found that students who report high levels of worry perform less well on tests. And when highly anxious people do perform well, it is at the expense of more effort and distress.
Sensation seeking
The differences between those who prefer bungee-jumping and those who world rather watch a good movie can also be addressed using a biological theory of personality.
Zuckerman (1994) conducted research into sensation seeking over a 30-year period, developing a questionnaire to measure the phenomenon and a biological theory to explain it. Zuckerman, Kolin, Price and Zoob (1964) identified four aspects of sensation seeking:
1. thrill and adventure seeking (risky sport)
2. experience seeking (desire for novelty)
3. disinhibition (stimulation through social activity)
4. boredom susceptibility (low tolerance for repetitive events)
Sensation seekers are more likely to have more sexual partners, use illegal drugs, take part in risky sport, be more complex, original and creative, and have more liberal and nonconforming attitudes. Zuckerman (1994) explained differences in sensation seeking in terms of level of arousal in the catecholamine system. (This system comprises neurons communicating via catecholamines, which include epinephrine, or adrenaline, norepinephrine and dopamine; see chapter 3.) According to Zuckerman, those with a low optimal level in this system work to reduce the stimulation in their environment, whereas those with a high optimal level seek to increase it.
GENETICS VS. ENVIRONMENT
Recent work in behavioural genetics has examined the contribution of genetic and environmental factors to human behaviour. A good example of this approach is represented by the twin studies of intelligence. Evidence for and against genetic influence In 1976 Loehlin and Nicholls examined the scores on self-report personality questionnaires of 800 pairs of twins. Nearly all traits showed moderate genetic influence, with monozygotic (identical) twins being much more similar than dizygotic (fraternal or non-identical) twins. A more extensive study (Loehlin, 1992) of 24,000 twin pairs in many different countries confirmed that monozygotic twins are much more similar than dizygotic twins on the Big Five personality dimensions. Riemann, Angleitner and Strelau (1997) found the same results when twins were rated by their friends on the same factors. Studies of genetically unrelated family members (parents and their adopted children) show no similarity in personality traits such as extraversion and neuroticism (Loehli n, 1992). This suggests that family environment itself does not contribute to similarities in personality between family members. Interestingly, recent studies have also shown only very slight similarities in personality between adopted children and their biological parents. A study by Plomin, Corley, Caspi, Fulker and DeFries (1998) found some evidence for a genetic basis for sociability, but almost no similarities in emotionality between biological parents and their adopted-away children, or between adoptive parents and their adopted children. Thus both adoption studies and twin studies are consistent with a genetic influence on personality. Only identical twins have exactly the same combinations of genes; dizygotic twins, just like ordinary siblings, will each inherit a different random sampling of half of each parent’s genes. It is also possible that research findings from twin studies are partly explained by the unique circumstances of being a twin. For example, twins who look similar may be encouraged t act in a similar way, whereas non-identical twins may be encouraged to behave differently.
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