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 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 proportion of genetic and environmental influence on personality traits. Molecular genetic studies try to discover 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 ask how genetic variability in personality relates to the evolutionary processes that have influenced human nature (Buss, 1999).