06 Apr 2018
To do what extent do genes influence one’s personality?
Personality is, traditionally, one of the most important objects of study for psychologists due to its major contribution to individual differences. Previous research explored the mystery of human personality to some extent, people gradually realize what determines their behaviours, why somebody feel excited in a certain situation while others do not. However, there is still huge debate with respect to various perspectives of personality. First debate is about the definition of personality which is much more complex than everyday uses of this term. It can depend on what psychologists believe about the causes and formation of personality. A generally accepted definition was proposed by Robins who defined personality as the distinctive and relatively enduring ways of thinking, feeling and acting that characterizes a person’s response to life situations (Robins et al, 2007). Furthermore, psychologists also ask “what causes personality differences?” Why some twins raised in the same family could have different personalities while others could have more similar personalities (Bouchard et al. 2001)? It is generally believed that personality is formed under the combing effect of genes and environment (Riemann et al., 1997; Bouchard et al.,2001; Caspi et al., 2002); however, there is no final conclusion about to what extent genes and environment influence personality. Some psychologists suggest that genes could play a more important role in shaping one’s personality than environment (Schuett, 2013), and some believe that the plasticity of personality is lost after the age of thirty, that is, people’s personality remains constant as a result of their unchanged genes (Conley, 1985). In this essay, the influence of genes on personality will be discussed and evaluated in relation to two perspectives: Eysenck’s biological basis personality theory and twin studies carried out by various psychologists (Eave et al., 1989; Bouchard, 1997; Waller, 1999; Jang et al., 1996).
One important theory which is still regularly cited today is Eysenck’s biological basis personality theory (Eysenck 19xx, 19xx; Eysenck and Eysenck, 19xx). This theory explains individual differences in personality in terms of genetic factors based on Eysenck’s “Gigantic three theory”, which is one of the major theories and instrument for assessing personality traits. According to Eysenck’s account, People’s personality traits can be classified according to three dimensions: Neutroticism - the extent to which people experience negative emotion; Extraversion - the extent to which people prefer to be alone or with others; and Psychoticism - the extent to which people are tough-minded. After the description of “gigantic three theory”, Eysenck states (1985) that there are two major systems responsible for physiological and psychological variations between individuals: the reticulo-cortical which is located in the brain-stem reticular formation as well as the reticulo-limbic which is located in the visceral area and which consists of the amygdala, hippocampus, septum, cingulum and hypothalamus (Eysenck, 1985, cited in Matthews & Gilliland, 1999). Although mutation can occur, these differences in the cortex are genetically determined. According to Eyseneck (1985), differences in the Reticulo-cortical could determine whether someone is extroverted or introverted, as the reticulo-cortical controls the cortical arousal produced by nerve impulses which determines levels of motivation, emotion and condition depending on either inhibitions or excitations of the cerebral cortex. To illustrate, Eysenck claims that introverts are more likely to be cortically aroused than extraverts, as a consequence, they are likely to generate larger arousal by themselves than extraverts when they are in exactly the same situations such as in a party (Eysenck&Eysenck, 1985, cited in Matthews & Gilliland, 1999). Discrepancy between cortical arousal of extroverts and introverts, which have a genetic origin, means that the former feel comfortable to talk to people in the party because they are seeking external stimulation compensating for their low level of arousal, while the later remain quiet as they have higher level of cortical arousal in their brain and do not need the external stimulation (ibis).
Eysenck also accounts for people’s emotional stability in terms of reticulo-limbic activities (Eysenck & Eysenck, 1985, cited in Matthews & Gilliland, 1999). Reticulo-limbic in the brain controls people’s response to emotional stimuli. Emotional stimuli induces arousability of limbic system. Eysenck states that the brains of neurotic people are more easily aroused than emotionally stable people (ibid). The arousing activities can then be translated into a predisposition to experience intense emotions such as anxiety or sadness. Therefore, when the same event happens to neurotic and stable person, strong arousability could be generated by the brain of the former, as a result, he or she may response intensely either psychologically and physiologically, on the contrast, little arousability is generated in the brain of the later, leading to very few response in return (ibid).
A major limitation of Eysenck’s particular biological account of personality is concerned with its testability. The difficulty to test Eysenck’s hypotheses is considerable, especially due to the lack of appropriate technological instruments (Matthews&Gilliland,1999). For example, a measure of cardiovascular activity is sometimes used to measure arousal differences between introverts and extraverts. However, the cardiovascular system has a considerable interaction with the respiratory system which could be a confounding variable in this experiment (ibid). It is very difficult for scientists to exclude all confounding variables and reach a valid result, hence it is difficult to verify the relationship between arousability of brain and personality traits. Nevertheless, further experiments carried out by other psychologists offer evidence that Eysenck’s biological theory was not as well substantiated as predicted. To illustrate, associations between extraversion and tonic measures of central nervous system and automatic nervous system arousal are actually much weaker than that were expected from Eysenck’s prediction (ibid.). It is worth considering, therefore, whether there is any other more important factor giving rise to the differences between extraverts and introverts apart from arousability of cortex as proposed by Eysenck.
While the genetic account of personality theory proposed by Eysenck (1985) has remained purely theoretical and is difficult to substantiate, more recent research on twin studies also reveals that personality could largely be influenced by genetic factors (Eave et al., 1989; Bouchard, 1997; Waller, 1999; Jang et al., 1996). Twins can be divided into two types: monozygotic (MZ) and dizygotic (DZ) twins. Monozygotic twins come from the same fertilized egg and are, therefore genetically identical. Dizygotic twins refer to twins who come from two separate fertilized eggs and only share half of their genetics. In the study of influence of genetics and environments on personality, twin studies prove to be an important strand of research (Bouchard, 2001). Because MZ twins shared exactly the same genes, it is reasonable to assume that any variation in their personality could be attributed to environmental differences in their upbringing, which indicates, to a certain extent, the possible environmental influence on personality. Meanwhile, DZ twins shared only half of identical genes and most of them are raised in the same environment. Their personality differences could largely be a result of genetic variations instead of environmental influence. In 1989, Eaves et al. carried out twin studies using the “Gigantic Three”(Eysenck, 1985) as three major personality dimensions and differences between twins’ personality traits( Neuroticism, Extraversion and Psychoticism) were thus investigated. Twin meta-analysis suggested a nearly zero correlation between shared environments and twins’ personality traits for all three traits, while correlation for MZ was twice that for DZ, indicating that genes play an important role in this difference. Eaves et al. then used falconer heritability to describe the heritability of a personality trait based on the difference between twin studies. The Falconer heritability for Extraversion, Neuroticism and Psychoticism was 0.58, 0.44 and 0.46 respectively, which means around half of one’s personality traits could be inherited from parents (Bouchard&Loehlin, 2001).
While Eysenck (1985) described personality traits according to his “Gigant three ” and proposed a genetic account for them. Costa and McCare (1992) proposed an alternative personality trait theory: “the Big Five” which classifies personality trait into Extroversion, Neuroticism, openness to experiences, agreeableness and conscientiousness.
Neuroticism can be described as the tendency to experience negative emotions, notably anxiety, depression and anger. Extraversion refers to high activity, the experience of positive emotions, impulsiveness, assertiveness and a tendency towards social behavior. Openness to experience represents the tendency to engage in intellectual activities and experience new sensations and ideas. Agreeableness refers to friendly, considerate and modest behavior. Conscientiousness is associated with proactivity, responsibility and self-discipline (Costa & McCare, 1992, p.xx).
There are psychologists, however, who suggested that the “Big five model” is a more robust and superior interpretation of personality traits compared to “Gigantic three”, as it could include wider personality scales in the input data(Aziz&Jackson,2000) and more scientifically acceptable nowadays. Similar twin study of “the Big five” rather than “the Gigantic Three” was conducted. Those studies yielded similar results as those for the Giant three (Waller,1999 ; Jang et al., 1996a ; Riemann et al ., 1997), stating that around 40 per cent to 60 per cent personality traits was influenced by genes and the rest of them most influenced by non-shared environment(also 40-60 per cent).
Although twin studies(Eave et al., 1989; Bouchard, 1997; Waller, 1999; Jang et al., 1996) has provided a relatively convincing answer for the question to what extent do genes influence one’s personality, studies from different researchers have not always provided consistent results (Bouchard&Loehlin, 2001). Furthermore, a main limitation of all twin studies is that MZ twins can not be treated as a homogenous group. MZ twins can be further divided into two types: monochorionic MZ twins who share the same placenta and amniotic sac and dichorionic MZ twins who do not. Sharing the same placenta means the twins have the same prenatal environment, which could lead to their higher correlation in personality traits. However, this high correlation was not resulted from similarity of genes but the same prenatal environment. This could be a large confounding variable of twin studies, giving rise to larger proportion of genetic accounts than reality. REF??
In conclusion, both Eysenck’s biological basis of personality theory and twin studies have revealed that genetics can play an essential role in the formation of one’s personality. However, limitations of these theories and experiments exist and cannot be excluded due to difficulties of current technology. On the other hand, it is worth pointing out that the extent of influence of genes and environment have on personality could be different depending on individuals. For example, people in adverse situations are more likely to change their personalities sharply, while those without big change happened in their lives could keep relative constant personalities. Nevertheless, it is still very difficult to draw a valid conclusion; more research needs to be conducted in this field in the future in order to draw a more valid conclusion.
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