From the beginnings of research on personality, the nature±nurture debate has been a central issue in theorizing about the causes of individual dierences. The metaphor of a pendulum, swinging back and forth between nature and nurture, is frequently used to characterize the opinion of the majority or the `main stream' of social scientists and psychologists (Plomin and Petrill, 1997) in this debate. This metaphor, however, does not provide a fully accurate description of behaviour genetic research. Like few other ®elds of psychological research, behaviour genetics has accumulated an impressive body of knowledge over more than one century. Acknowledging that a swinging pendulum has detrimental eects on research eorts, funding and careers, the controversial nature of and the continued interest in the debate about nature versus nurture seems to have contributed to the advancement of theories, study designs and methods.
Today, the basic debate of whether nature prevails over nurture or vice versa seems to be settled: both are important for personality development. If behaviour genetic research had nothing more to oer than an answer to the question of what percentage nurture or nature contributes to the phenotypic expression of psychological outcomes, this special issue would not have been produced. Modern behaviour genetic research, however, goes far beyond this question and oers insights into numerous classic ®elds of personality research.
Behaviour genetic research on personality has largely been dominated by studies of twins reared together using self-report personality questionnaires. These studies share the predominant method of personality measurement with personality research in general and have provided important data on the etiology of personality. However, many eorts in personality research have been directed at disentangling substantive eects from method eects and these multi-method research strategies are increasingly adopted in behaviour genetic research (Rose, 1995). The major conclusions derived from self-report studies are: about 40 per cent to 50 per cent of the variance of almost any personality trait can be attributed to genetic variance (i.e., heritability); environmental in¯uences shared by members of a family have almost no eect on personality development, and there is no substantial dierence between heritability coecients for dierent traits. These conclusions contradict theories predicting a strong eect of between±family in¯uences (e.g., psychodynamic or social learning CCC 0890±2070/98/050303±03$17.50