Personality Traits

The structure of personality traits shows consistency across different groups of people in different cultures. Furthermore, traits are stable across time, and there is evidence to indicate that some of them may have a tractable biological basis. Therefore, it seems reasonable to enquire to what extent individual differences in personality traits are caused by genetic and environmental factors.

There is the tendency to see this as a difficult area, because biometric behaviour geneticists and molecular genetics researchers both use advanced statistical techniques and specialised jargon. This chapter to introduces, in a non-technical way, the main study designs and findings in these areas. Studies of twins and adopted people can indicate the relative proportions of genetic and environmental influence on personality traits. Molecular genetic studies indicate which individual genes might influence personality. Genetics researchers make some surprising contributions. For example, genetic studies can make a contribution to the study of personality change, and even the genetic contribution to personality traits may change with age or over time. Genetic studies are just as informative about the environmental factors that influence personality traits. Plomin, Asbury and Dunn (2001) commented that 'behavioural-genetic research provides the best available evidence for the importance of environmental influences' (p. 225).

Once it has been established that traits are in part inherited, we might start to wonder how genetic variability in personality relates to the evolutionary processes that have influenced human nature (Buss, 1999). At present, there are no good answers to this question, although it is likely that future research will increasingly inter-relate the genetics and evolutionary psychology of traits. Box 6.1 describes some possible research strategies for making such connections.

Genetic and environmental research on personality traits–and on other psychological and physical traits–is based on three simple research designs: twin studies, adoption studies and molecular genetic studies (Bouchard and Loehlin, 2001; Plomin et al., 2001). Each may be elaborated upon to ask more complex questions. Twin and adoption studies are called 'genetically informative' and are carried

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