Every individual is unique and different from one another. Although all human beings share the same nature and common humanity, each individual possesses their own thoughts and feelings, and between our individuality and humanity, personality is what lies within. Genetics play a significant role to the disposition of an individual’s development of personality traits, and that the environment influence the actual development within the genetically predisposed limit. In this essay,various theories to defining personality will first be described, following the extent to which genetics exert substantial effects to individual differences on personality traits, predominantly using twin studies, family studies and adoptions studies, as these studies are valuable in order to separate environmental influences from genetic influences for personality traits. The impact of environment on personality traits will then be discussed and how a gene-environment covariation is present in personality.
Personality can be referred to as a set of characteristics possessed by an individual within the psychobiological system, which leads to a unique pattern towards their own behaviour, thoughts and emotions. These traits contributes to a person’s individuality and thus influences how an individual responds differently to the environment around them and causes the individual to behave in certain ways. There have been numerous theories approached by different perspectives attempting to explain how personality stems and develops, which are all equally important to the fundamental understanding of personality. Carl Jung’s (1921) key theory to personality is the development of two attitude types, extraversion and introversion, which Jung described as an individual’s energy moves towards the outer world and their inner thoughts, and that the individual’s view of the world acts as a fundamental bases to personality. Sigmund Freud proposed one of the earliest theory on personality development from a psychoanalytical perspective, which emphasises on childhood and the unconscious mind. Freud claimed that the human mind consists of three components, the id, ego and superego. Freud stresses that the interaction and constant conflicts between these elements throughout an individual’s childhood are crucial in shaping an individual’s personality in adulthood, and believes that a healthy personality is resulted from a solid ego, which is able to balance the instinctive drives of the id and incorporating the superego’s moral principles. Contrasting from Freud’s psychoanalytic perspective and Jung’s trait perspective, the learning theory of personality proposed by B.F Skinner, suggests that the individual differences in personality occur depending on the individual’s learning experiences. Skinner proposed that depending on what the individual’s wants to experience, the individual would then develop certain personality traits in order to receive various reinforcements. Hans Eysenck, a personality psychologist, attempted to explain personality from a trait perspective, and believed that personality is genetically inherited. Eysenck proposed the three dimensions of personality, extroversion vs. introversion, neuroticism vs. emotional stability, and psychoticism vs socialisation. These traits all contribute to an individual’s hierarchy of personality where higher-order traits determine the lower-order traits, hence developing the individual’s behavioural patterns.
Throughout the years, many behaviour-genetic studies have been showing a consistency of heritability influences on personality traits between 20%-50% (Saudino, 1997). The role of genetic influence on personality traits gained considerable attention when Loehlin and Nichols (1976) conducted the first authoritative study using a large sample of identical twin pairs (n=514) and fraternal twin pairs (n=336). The participants were assessed with a series of personality and interest questionnaires and the statistical result showed a 50% heritability influence on all personality traits. Since then, numerous studies have been conducted to demonstrate how our genotypes influence individual’s behaviours, thoughts and emotions. Loehlin (1992, as cited in Holden, 1987) compiled a series of twin studies, adoption studies and family studies that have been conducted over the years on the genetic effects on personality traits, Loehlin’s meta-analysis concluded that genes are approximately 28%-59% contributable to one’s personality, whereas shared environments, which are factors that contribute to twin and sibling similarity, only accounted for 0%-11% the individual differences in personality. Both studies therefore strongly suggests that, in comparison to the strictly limited influence of environment, the genetic effects plays an unneglectable role to an individual’s personality. However, even though shared environments contributed little to individual differences of personality, non-shared environments on the other hand, was found to have accounted for 44%-55% in the variances of personality (Loehlin, 1992, as cited in Borkenau, 2001), therefore suggesting that one’s personality is not sole due to genetic influences.
The genetic effects of heritability on personality strengthens with studies of monozygotic twins (MZ) and dizygotic twins (DZ) being reared apart (Bouchard, 1994). Bouchard studied 44 pairs of MZ twins who were reared apart, who were assesed with self-report personality questionnaires. The questionnaires were measured using Eysenk’s proposed personality dimensions, and were further divided into 11 primary personality dimensions, in order to include as much characteristical triats within an individual as possible. It was found that genetic influences yielded an approxiate of 40% and that shared environments only yielded 7%. A similar study conducted by the Minnesota Twin Ministry, using the same method as Bouchard’s study but a larger sample, has found that the heritability of personality traits yielded 50% approximately. Moreover, when assessing the 11 primary personality dimensions, it was shown that the correlations between MZ twins reared apart and MZ twins reared together were closely similar, at which their correlations were 0.49 and 0.52 respectively (Holden, 1987). This therefore suggests that shared environment contributes very limited influence on personality. Furthermore, attitudes towards traditionalism such as disciplinary were found to have an estimated heritability of 0.45, which subsequently implies that social attitudes are not only affected by the environment itself, but are also the result of the predisposition of innate personality traits. As a result, the statistical evidences shown within these two studies, espeically the latter one, strongly implies that genetic influences exceed environmental influences on personality. Yet, an important point to consider here is that, almost all the populations being studied could be criticised as being too euro-centric (Saudino, 1997). The degree of impact of genes on personality might differ when it comes to assessing other countries, therefore it could be suggested that cultural bias is present when attempting to claim that genetics exert the same influence on personality to the general population. Moreover, participants were generally reared in similar homogenous environments, therefore generalisation becomes another issue if heritabilities influences were to be implemented on individuals who are not reared in well-conditioned environments. Furthermore, the studies could be criticised as being subjective, as the findings were based on self-reports and parent-rated reports, which therefore suggests the possibility of a certain degree of rating bias. This could ultimately mean that genetic influences in twin studies might be overestimated.
Another aspect suggesting a degree of genetic influence on personality that needs to be considered is that, genes have the possibility to develop and alter its genotypes and thereby exerts an influence over an individual’s development, rather than the environment implementing an effect on the development of personality. This explanation has been addressed in the by Ronald Wilson, who conducted the Louisville Twin Study, a longitudinal study assessing various measures related to temperament, sampling nearly 500 pairs of twins starting from infancy to adolescence (Wilson, 1983, as cited in Holden, 1987). The results found that there is a correlation between genetics acting as an intrinsic bases towards stimulating a further growth to the termperamental development. This finding could therefore be implying that, personality traits, and in this case, an individual’s teperament, is syncronised with the changing patterns of the genes. However, the correlational findings in this study could not precisely indicate the cause and effect between the genetics and temperament. It is vital to note that, gene-environment effects could be present, meaning that the contribution of an individual’s genotype could alter and is dependent of the environment. This therefore suggests that it could be the surrounding environment causing an alteration of an individual’s genetics, and in turn causing a change in temperament.
Although it is widely established that individual differences of personality is due to genetic influences, it is inaccurate to disregard the influences environment could exert on personality. A study was conducted to examine the effect of family environments, in terms of child-rearing style, on personality traits (Kazuhisa et al, 2000). Children participants were directly observed and interviewed, where 13 behavioural characteristics were identified, which were then factorised into three personality traits – extraversion, maturity and intellect. Participants’ parents were also interviewed in order to assess parent participation in child rearing. It was found that family environment and child-rearing style had a degree of influence on the participants’ personality traits. The effect of family environment on variances in extraversion, maturity and intellect were 8%, 14% and 10% respectively. As a result, this implies that the environment a child is reared in do exert a certain magnitude of influence on their personality traits. Yet, it should be considered that the impact of family environment on personality traits vary from trait to trait, as suggested by the different percentage of variance on the three personality traits being examined. This therefore implies that some personality traits are more susceptible to genetic influences than environmental influences, in this case, extraversion is more determined by the individual’s genetic effects, and that maturity is more affected by family environment. Furthermore, formal reliability test was not performed, this study could therefore be criticised of its reliability, as consistency of such results have not been proven.
In fact, a more plausible approach to determining the magnitude of genetic influences on personality traits, is to look at the interaction between genes and the environment in shaping an individual’s personality trait. It is suggested that, an individual’s environment has been genetically mediated, which means genes predispose an individual’s preferences to create their own environment and experiences which is a reflection of their personality (Saudino, 1997). Studies using multivariate genetic analyses have found that there’s a certain extent of genetic effects on environmental measures being correlationed to genetic effects on personality (Saudino, 1997). One study has found that personality contributed an approximate 30-40% of genetic influence to the risk factors relating to divorce (Jockin, Mcgue, & Lykken, 1996, as cited in Saudino, 1997). Another study assessing the life events of females in their adulthood, has found that the five factor personality traits contributed entirely to the genetic influences on life events (Saudino, Pedersen, Lichtenstein, McClearn, & Plomin, 1997, as cited in Saudino, 1997). These studies therefore suggests that, genetic effects predispose a bases, or boundary, to an individual personality, and that the environment exerts an influence on the development of personality within the limit. However, these findings raises uncertainty in terms of its accuracy. This is due to the fact that the environment is based on the participant’s self-report, implicating that the individual’s own perception of environment, and that this perception is affected by the individual’s genetically influenced personality. Still, objective events such as divorce, would not necessarily be affected by the individual’s perceptual process. Therefore, a gene-environment correlation is nevertheless existent.
In conclusion, solid evidences from twin studies, family studies and adoption studies have sufficiently proven that genetic influence exerts an unneglectable role to the individual differences of personality traits, and that the evidence becomes more substantial when comparing MZ twins reared apart and MZ twins reared together, showing the similar correlation of personality traits and suggesting the minor to none influence of shared environments. However, it is unreasonable to deny the influence of environments on one’s personality, as it is shown that nonshared environments do have a certain impact on an individual’s personality trait. Nonetheless, it is more plausible to suggest that there is a covariation between genes and environment on the development of personality.
