By studying consistencies in behaviour and emotional reactions from birth through early adulthood, the study of temperament aims to demonstrate that genes contribute to personality. Temperament, as defined by Buss and Plomin (1984, reported in Thomas, 2002) is a range of personality traits genetically inherited. They believe that temperaments observed in childhood will predict personality later in adulthood. Temperament has to be established as early in life as possible in order to minimize environmental influences. An example from the longitudinal studies of Kegan and colleagues illustrates that there is substantial consistency in temperament throughout childhood from an early age. They observed cautiousness (inhibited behaviour) and boldness (uninhibited behaviour) in 600 children at four months old, and later at two years old (Kegan, 1994, reported in Thomas, 2002). They found that at four months, 40 per cent of children were ‘uninhibited’ and that at the age of two years two-thirds of these still demonstrated a relaxed reaction to stimulation. This study shows that boldness is, to some extent, genetically determined.
With increasing advances in technology there are more ways to explore the influence of genetics on our personality. The relatively new scientific discipline of molecular genetics provides new insights with biological data. With our increasing understanding of DNA, it is now possible to explore the link between certain aspects of behaviour and specific genes. In terms of personality, there has been a tentative finding that neuroticism is linked to the neurotransmitter serotonin which is coded for by one gene (Goldman, 1996; Lesch et. al., 1996, reported in Thomas, 2002).
As shown above, there is clear evidence that genes influence our personality. Genes cannot, on their own, account for all aspects of personality. But how much do nature and nurture contribute to a range of individual differences, including temperament, personality traits and intelligence?
The study of heritability offers to answer this question. Heritability is concerned with establishing how much our genes are responsible for our behaviour and personality. It is important to note that heritability is a statistical concept – it only helps to estimate the relative contribution of genetics and environments.
Twin studies are the most commonly used experimental design in estimating the extent of genetic effects. If genes influence personality, then the closer the genetic relationship between two people, the greater should be the correspondence between their personalities. This means that monozygotic (MZ, or identical) twins should have very similar scores on personality tests, considering that they share all their genetic material. In comparison, dizygotic (DZ, or fraternal) twins should have less similar scores. Empirical studies have shown that the observed heritability for neuroticism between MZ twins is about 30 per cent. It is inferred that the remaining 70 per cent is due to the influence of the environment (Thomas, 2002).
In order to estimate how much are the genes responsible for personality, twin studies have to be carefully designed to eliminate environmental influences. This is not always easy as it is known that as the genetic similarity between people increases, so does the similarity of their environments. Parents and their children usually live in the same households, whereas unrelated people do not. To overcome this problem, researchers attempted to study MZ twins reared together (in the same family) and apart (in different families). It has been found that MZ twins reared together do not, in fact, share the same environment. The shared environments are non-standard since each child experiences the environment differently. Also, parents do not always treat their children the same. In Loehlin’s review of twin studies (1992, reported in Thomas, 2002) the contrast in childhood temperaments like shyness, inhibition and different levels of extraversion were present in MZ twins that were reared together, in the same family. In contrast, MZ twins reared apart, in different families were more likely to have similar scores.
As illustrated above there is evidence that the complexity of environments has significant and complex effect on the development of individual differences in personality. However, evidence provided by studies of temperament and molecular genetics also strongly suggest that genes influence our personality. Environments and genetics are simultaneously present. It is therefore clear that nature and nurture, together and in interaction, have effect on our personality. Heritability studies have attempted to quantify their relative contribution. With increasing evidence from rapidly expanding new scientific disciplines (such as molecular genetics), it is now possible to understand the interaction between nature and nurture on a very fundamental, biochemical bases. There is no need to argue whether it is nature or nurture that is the more important. There is, however, a need to gain more insight into their mutual interaction.
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References
EPoCH (Exploring psychology’s content and history) CD-ROM for DSE212 (2002), in DSE212 Course Team The nature-nurture debate (1955 – present day), The Open University.
Thomas, K. (2002) The individual differences approach to personality (Chapter 5), in Miell, D., Phoenix, A. and Thomas K. (eds) Mapping Psychology 1, Milton Keynes, The Open University.
