The study of personality lies within the broader context of the interplay between human nature and individual differences. It identifies human nature as a set of qualities that are characterized to all humans, in other words what humans have in common and are genetically equipped with from birth (Hergenhahn, 2011). In contrast, individual differences is concerned with how and why there are discrepancies between human beings, in terms of behaviour and more specifically, personality. In this essay I aim to portray a clear understanding of what is meant by personality, focusing on the genetic influences as well as the antithetic approach, environmental influences. This will reflect the nature versus nurture debate, and as this debate reaches common ground, I intend to gain a holistic perspective of personality by disentangling the two, whilst simultaneously displaying the interaction between genetics (nature) and the environment (nurture) in shaping ones personality.
Personality is a universal phenomena, it exists everywhere. Its broad nature makes it extremely difficult for theorists to generate a universally acceptable definition of personality. There is subjectivity in defining personality, as theorists have different perceptions of what it is, resulting in many definitions which vary according to the assumptions of a specific personality theory. The word personality comes from the Latin word ‘persona’, which translates to ‘mask’. The mask represents the aspects of ourselves that we choose to display, however it also implies that there are parts of our personality that remain concealed, something that theorist are keen to uncover. A generic definition that allows us to focus on many aspects of the person has been created; ‘Personality represents those characteristics of the person that account for consistent patterns of feeling, thinking and behaving’ (John & Pervin, pp. 4, 1996), summing up what the many theories of personality strive to explore.
Studying personality poses as a challenge for researchers because it is a psychological construct; we cannot physically see the characteristics, except when they are expressed through behaviour. The primary starting point is to look at the biological influences on behaviour, as behavioural geneticists infer that the behaviour of an individual, is directed by biological processes, determined by the information found in the genes (Pervin, 2003). Francis Galton (1876; cited in Pervin, 2002) the founder of individual differences created a movement within and beyond the field of psychology, after finding a strong correlation between the genetic closeness of two men and the probability of them being eminent (intelligent), he concluded that eminence ran in families. These findings have led subsequent researchers to question whether personality is also hereditary (inherited through genes). Behavioural geneticists have to disentangle the interaction between genes and the environment, in order to clearly identify their effects on behaviour separately and twin studies enable them to do so. There are two types of twins, the first are called monozygotic (MZ), they are from the same fertilised egg and therefore genetically identical. Dizygotic (DZ) twins are fraternal, they are two eggs fertilised separately. Twin studies are particularly helpful because it is assumed that MZ twins share exactly the same genes, so any differences in their personality are due to the environment, in the same way, DZ twins only share half of their genes, therefore if they inhabit the same environment, any difference in their personality are due to their genetic dissimilarity. These assumptions are the basis of the heritability estimate (h²), which is a statistical proportion of how much an observed variance in a population can be attributed to genetic factors (Pervin, 2003). The formula is h² = 2(rMZ – rDZ), where ‘r’ is the correlation between MZ twins and DZ twins respectively.
The extensive nature of personality makes it difficult to reduce it to a quantitative level for this test statistic to be used, fortunately, Costa and McCrae (1992; cited in Hampson, 1999) developed the five factor model which reduces personality to five traits: Extraversion, agreeableness, conscientiousness, emotional stability and openness, allowing the heritability estimate to show how much each trait can be attributed to genetic factors. However, this reductionism oversimplifies the concept of personality and it is contrary to the holistic stance that should be taken. In addition, Goldberg (1993; cited in Pervin, 1996) questioned the cultural relativity of each trait. He investigated whether these traits were semantically the same across all cultures, and found that there was a cross-cultural agreement on the meaning of these factors in most languages of the world. His findings can be used to support the genetic approach on personality, as it exposes the universality of personality, suggesting that it is not culturally or environmentally bound, rather it is a product of human nature. Moreover, the longitudinal study by Conley (1985; cited in Pervin, 1996) showed that after the age of thirty, personality loses its plasticity; the five traits are consistent over time across different situations. These findings led him to characterize personality as unchanging, fixed and ‘set like plaster’, (Pervin, pp. 53, 1996) and just as genetics typically remain constant, so does personality, strengthening the genetic influences approach. This statement must be considered with a degree of caution, firstly even if personality does not usually change, it does not mean that it cannot change, because genes which are predominantly constant are also affected by mutations from the environment, therefore concluding that the same traits are displayed across all situation lacks mundane realism, as in everyday life the characteristics we show vary according to the situation at hand. Secondly, it is deterministic to say that personality is solely a product of genetics, as the implications of doing so means that responsibility is taken away from the individual, for example, if one is classified as having an antisocial personality from their genes, it justifies their antisocial behaviour.
The results of h² showed evidence of a genetic contribution to all the five traits, with a mean heritability estimate of h²=.40, suggesting that 40% of variance in personality can be attributed to genetic factors. These results do not necessarily mean that 40% of an individual’s personality is inherited, as correlation does not imply causation. The heritability estimate varies with the trait being measured and the population type, so caution must be taken when using it to make inferences about personality in the broader population.
Though twin studies have shown to be beneficial in the field of personality, further research has highlighted its fallible nature. Some MZ twins are more similar than others. Dichorionic MZ twins do not share the same placenta and have separate amniotic sacs, whereas, Monochorionic MZ twins share the same placenta, and amniotic sac, effectively sharing the same environment prenatally (Wright, 1997, cited in; Pervin, 2003). The h² estimate does not currently account for these prenatal differences and may lead to an inaccurate estimate of heritability in regards to personality. Perhaps future research would look at the correlational difference between monochorionic MZ and dichorionic MZ twins to see whether there is a manifested difference in the heritability of traits between these twins.
Behavioural geneticists have concluded that 40% of variance in personality can be attributed to genes, so what about the remaining 60%? The environmental approach has been considered, but first a distinction must be made between shared and non-shared environments. Shared environments are common across all the siblings in a family e.g. family values and child rearing practices. Non-shared environments consist of unique experiences outside the family setting. Family and adoption studies aid in distinguishing between the two, and identifying what type of environment has a greater influence on personality. If shared environments are more important, then biological siblings raised together ought to be more similar beyond the degree accounted for by genes, than those raised apart. This assumption was falsified by Dunn & Plomnin, (1990), using families, they devised the sibling inventory of differential experiences (SIDE) questionnaire to study the perceptions of siblings in their family environment. The results showed that siblings from the same shared environment are very different in personality, implying that even if the environment is absolutely the same, researchers cannot control for how the individual perceives, interprets and interacts with the environment. Plomnin (1990) concluded that the attributed variance in personality due to genetics, shared, and non-shared environments are 40%, 35% and 5% respectively, suggesting that unique experiences in non-shared environments are vital in shaping personality. While the influence of shared environments appears to be weak, it should not be ignored. Borkenau et al (2001, cited in; Pervin, 2003) found that when video rating was used as a source of information instead of questionnaires, more shared environment effects were displayed. This highlights the limitations of using questionnaires that require the individual to be retrospective because reconstructive memory is prone to inaccuracy. Furthermore, the social desirability bias is present (Pervin, 2002), causing individuals taking the questionnaire to present themselves in a favourable light. These methodological flaws affect the reliability of the data and in turn the confidence in generalising it to the broader population is reduced. On the other hand, Piedmont, et al (2000, cited in Pervin, 2002) found that the accumulated concurrent validity (when different tools measuring the same thing yield the same results) of major personality questionnaires are enough evidence to support its use. Nevertheless, Plomnin (1990) responded to these criticisms by apportioning 20% of the total variance as an error margin, as penance for the possible flaws in personality research.
The siblings used in family studies have different genetic endowments, and genes do not just affect the individual, but they affect the environment around them. Theorists are confronted by the cause and effect dilemma, as by a certain point it becomes hard to distinguish whether the individual is the recipient of the environment or the creator of it due to their genetic makeup. So, they have looked at both perspectives, appreciating the fact that genes affect our environment and similarly the environment has an effect on our genes. Genetic effects will not manifest themselves unless they have the adequate environment to do so; Scarr & McCartney (1983, cited in Pervin, 2003) called this environment-seeking. When looking at infants, there appeared to be an innate drive in them to express their genes so they would actively seek environments that enabled them to. An example of this is an adoption study by Bouchard, et al (1990); he looked at twin sisters raised apart, who both enjoyed reading despite the non-shared environment. The sister that was in the poorer environment did not have books readily available to her, but she still sought out the library environment which allowed her to express her innate drive for reading. This example of environment-seeking can be applied to personality; an extrovert individual would seek stimulating environments, whereas an introvert individual would seek calm environments (Pervin, 2003). Kagan’s (1994; cited in Pervin 2002) temperament hypothesis suggests that infants are born with innate characteristics (temperament), which determines their future interactions, especially with their primary caregiver. Longitudinal adoption studies have shown that adopted children who were classified with a temperament that predisposes them to antisocial behaviour, were more likely to receive negative parenting from their adoptive parents (Deates, et al, 1998; cited in Pervin, 2003). This is an example of an evoked environment effect, the child’s temperament elicits the parenting style (environment), however, one could argue whether it is the child that elicits the parenting style, or whether the parent is living up to the self-fulfilling prophesy, feeling that they have to behave a certain way, and again the cause and effect dilemma appears, what comes first, the temperament or the parenting style?
Genetic and environmental influences are not mutually exclusive approaches, they are interdependent. Historically, the consensus as to whether personality is due more to genetic influences of environmental influences changes over time. In today’s era, where there are advances in science technology it is inevitable that researchers will sway towards the nature approach, especially as it is more empirically sound, which will be useful for future research looking at whether the prenatal shared environment of MZ twins affect their personality. Environmental effects cannot be ruled out, the results from Pervin (2003) show that the attributed percentage variance in personality is equally split between genetics (40%) and the environment (35% non-shared and 5% shared=40%), this ties in well with this quote, ‘there is never gene without environment, or environment without gene’ (Pervin, pp. 149, 2003)
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