Twin studies are probably the most important form of study to researchers in their efforts to use biology to explain personality due to the identical genetic make-up of monozygotic twins researchers hypothesise that their personality would also score very similarly. Floderus-Myrhed, Pedersen, & Rasmuson, (1980) were such researchers who conducted a study on 12,898 same sexed Swedish twins, who were born between 1926 and 1958, to assess heritability of the personality variables of neuroticism and extraversion based on the Eysenck Personality Inventory. They found that dizygotic twins had a higher level of shared variance than the monozygotic twins for both neuroticism and extraversion, which could indicate genes have a distinct role to play in personality. However, there are two confound variables that this research fails to address. The first is that the researchers used a questionnaire to ask the twins how similar they were to assess zygosity, but this method does not guarantee the similarity of the genetic makeup. Kasriel & Eaves, (1976) found that up to 10% of twins who scored as monozygotic on a questionnaire of sameness were not actually monozygotic when a blood test was done. The second confound variable is that of environment and how the twins were treated when they were children and as adults. When in childhood twins are very often treated by family members similarly and they have similar friends therefore when assessing the similarities in personality environmental considerations cannot be discounted (Maltby et al., 2009). When in adulthood environmental differences must also be considered when scoring personality differences in monozygotic twins due to differences in lifestyle. Although McCrae & Costa, (2003) suggest that an individual’s personality becomes stable as they reach adulthood, changes can still happen after adulthood is reached due to environmental changes and society expectations e.g. each one of the twins living in a different location (Hopwood et al., 2011).
A twin study that did consider environment was that of Torgersen & Janson, (2002) when they examined the personality traits of monozygotic twins at the age of 29 who were raised together. They examined the life stressors that the twins experienced between the ages of 6 and 15 and found there was a positive significant correlation between childhood stressors and intrapair personality differences in Agreeableness, Openness, Conscientiousness, and five-factor profiles. Therefore this suggests that perhaps environmental issues also play a part in shaping personality.
So does the environment and interactions with others have an effect on personality differences? When a baby is in its first few years of life there is a strong belief by many e.g. Bowlby, (1988), that the interaction the parent has with the baby is crucial to the baby’s development of many of the traits that are used in defining personality e.g. self esteem, affection and positive belief systems (Levy & Orlans, 1998). So whilst the parent is interacting with their baby they are shaping their child’s affective responses and creating an important part of the baby’s personality. A number of studies have investigated the relationship between attachment styles and personality disorders and one such study was conducted by West, Keller, Links, & Patrick, (1993). In their study they used participants who had been diagnosed with borderline personality disorders and investigated their attachment style. West et al., (1993) found that there was a significant correlation between borderline personality disorder and extreme anxious attachment. However, this study does not address whether the participant’s attachment style affected their personality or whether their personality affected their attachment style. Shaver & Brennan, (1992) went some way to addressing this issue by looking at the personality traits of participants who have an insecure attachment style, which was suggested to have been caused by early childhood trauma, and found an association between insecure attachment and anxiety and depression.
Another, slow to emerge, competing theory to that of biology alone defines personality, is that of evolutionary theory. Evolutionary theory suggests that personality can be explained by the concept of adaption to achieve the major goals that humans must achieve to ensure survival of the species (Nichols, K. M. Sheldon, & M. S. Sheldon, 2008). So if we were to assume that one of the major goals for the human species is survival and reproduction then living and working in a group would assist this goal as the group would provide protection and more resources from collective hunting. However, working in a group can have its difficulties of conflict and the possibilities of social loafing, and this is where Buss, (2009) suggests that personality traits would evolve to allow group cooperation.
Though, if evolutionary theory suggests that personality is derived from goal directed strategies then everyone would have similar personality without any noticeable individual differences. Rands, Cowlishaw, Pettifor, Rowcliffe, & Johnstone, (2003) go some way to explaining evolutionary individual differences by their example of foraging animals. They suggest that leadership qualities would evolve in the animal that started smaller when they joined a pair as they would need to encourage the larger animal to go out to forage at the time the smaller animal needs to eat and follower qualities would emerge in the larger of the pairing animals.
Although Maltby et al., (2009) suggests the difficulties with the evolutionary theory of personality are achieving empirical measurement, the group are not aware of their goals and reductionism, there are researchers that believe evolutionary personality theory is the framework that brings the disparate aspects that make-up individuals and attempts to understand the complicated social processes spread over generations (Nichols et al., 2008).
This paper has briefly discussed some of the research that has been carried out to suggest biology shapes personality and also briefly touched on a few of the competing theories on personality, and the issues that arise from each of them. At this stage in research it is not possible to say which theory is the more convincing. However, in 2003 the Human Genome Project was completed, which was a 13 year project to identify the 20,000-25,000 genes in human DNA. Although 50% of the functions of these genes are still unknown, scientists are still striving to map these out and may be able to map personality traits to specific genes (Maltby et al., 2009). Though if genes are found to be associated with personality then this will have far reaching consequences for psychologists and society on the whole when it comes to behavioural disorders, and particularly crime. There is already some support that there is a genetic association to crime (DiLalla & Gottesman, 1991) but if projects such as the Human Genome project find there is a strong link between genes and crime then society will have to think about how to treat criminals as there would be no avenue for rehabilitation.
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