The Behavioural Genetics of Personality Development in Adulthood—Classic, Contemporary, and Future Trends WIEBKE BLEIDORN 1 * , CHRISTIAN KANDLER 2 and AVSHALOM CASPI 3,4 1 Tilburg University, The Netherlands 2 Bielefeld University, Germany 3 Duke University, USA 4 King’s College London, UK Abstract: Behavioural genetic research has led to important advances in the field of personality psychology. When carried out on longitudinal data, behavioural genetic studies also offer promising ways to examine the genetic and environmental origins of personality stability and change. Here, we review the findings of longitudinal twin studies, discuss their implications for our understanding of adult personality development, and point out open questions that need to be addressed by future research. Three general conclusions stand out. First, there is a strong and relatively stable genetic foundation of individual differences in personality throughout the adult life span; second, environmental influences become more important and contribute to an increasing rank-order stability of personality traits from early to middle adulthood; and third, both genetic and nonshared environmental influences contribute to both stability and change in personality traits. Equipped with this knowledge, the most urgent tasks for the next generation of behavioural genetic studies on personality development will be to (i) identify measurable environmental factors that matter and (ii) to capture the interplay between genetic and environmental influences on personality stability and change throughout adulthood. Copyright © 2014 European Association of Personality Psychology Key words: behavioural genetics; personality development; gene–environment interplay; adulthood Behavioural genetic research has led to important advances in the field of personality psychology (Krueger & Johnson, 2008). Since the early days of behavioural genetics, numer- ous twin and adoption studies have examined the genetic and environmental origins of individual differences in personality. Hence, it is now considered a well-established fact that about 50% of the variance in personality characteristics is genetic in origin, whereas the remaining variance can be traced back to nonshared environmental influences—that is, those environmental influences that make individuals within the same family more different from each other. In contrast, shared environmental influences that make individuals within the same family more similar to each other seem to play only a negligible role (for reviews, see Bouchard & Loehlin, 2001; Johnson, Vernon, & Feiler, 2008). From a developmental perspective, however, the relative proportion of genetic and environmental influences on indi- vidual differences might change with age and time and so might the degree to which genetic and environmental forces influence stability and change in personality during different stages across the life span (Krueger, Johnson, & Kling, 2006). Thus, the typical cross-sectional quantitative genetic design is not ideally suited to address questions about the genetic and environmental origins of personality development. When carried out on longitudinal data, behavioural genetic de- signs offer promising ways to examine the genetic and envi- ronmental origins of personality stability and change across the life span (Johnson, 2008). In the present article, we first outline the advantages and specifics of the quantitative genetic design—especially longitudinal twin studies—for the study of individual differences in personality development over the adult life span. In doing so, we aim to show that quantitative genetics can go beyond the rudimentary ‘how much’ question about nature versus nurture (Haworth & Plomin, 2010). In the following, we review the findings from the growing number of longitudinal behavioural genetic studies on adult personality trait development in view of three major research questions. Thereby, we aim to carve out well-replicated findings, to discuss their implications for two contemporary theoretical accounts of adult personality development, and to point out challenges and opportunities for future research. QUANTITATIVE GENETIC STUDIES OF PERSONALITY DEVELOPMENT IN THE ERA OF MOLECULAR GENETICS Although recent advances in molecular genetics provide exciting additions to the behavioural geneticist’s toolkit (Plomin, 2013), traditional quantitative genetic designs— especially twin studies –remain a favourite means to study *Correspondence to: Wiebke Bleidorn, Department of Developmental Psychology, Tilburg University, PO Box 90153, 5000 LE Tilburg, The Netherlands. E-mail: wiebkebleidorn@gmail.com European Journal of Personality, Eur. J. Pers. 28: 244–255 (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/per.1957 Received 14 June 2013 Revised 12 March 2014, Accepted 12 March 2014 Copyright © 2014 European Association of Personality Psychology