CHAPTER 1 Genetics of Personality

In this current chapter, we review findings from an important approach to understanding the etiology of personality—behavior genetics. Behavior genetics encompasses a series of methods for disentangling the relative influence of genes and environment on the variation in a phenotype (observed variable). Personality, or the characteristic ways that people think, feel, and behave, varies considerably within and between different populations. There are people who are more or less aggressive, more or less modest, more or less sociable, more or less humorous, and so on. It is the variation in personality that defines us and makes us distinct from every other individual on the planet. There are certainly those who have questioned the existence of consistent personality dispositions (Mischel, 1968), or theorized as to why any variation in personality exists (Penke, Denissen, & Miller, 2007). Behavior genetics is vitally important in showing that personality does exist, that it does vary in meaningful ways between people, and that this variation is due, in part, to genetic influences. Indeed, the field of behavior genetics, particularly the use of genetically informative family data, was a driving force in establishing the importance of both nature and nurture in the development of personality. Quantitative behavior genetic methods, which parse out the relative influence of genes, family environment, and unique environmental experiences, have consistently demonstrated the importance of genetic influences. Biometric modeling with twin data reveal one of the most reliable findings in behavior genetics and in the personality literature—heritability, or proportion of variance due to genetic influences, for most major personality traits is approximately 40 to 50%. A robustly replicated finding, this has caused consternation in the molecular genetic field of personality research, where scientists have been largely stymied in their attempts to find measured genes that explain a significant portion of the variance in personality traits. Why, they have asked, is it so difficult to replicate measured gene-measured personality findings, if we know that genetic influences explain 50% of personality variation? In the current chapter, we review what has been found, and how improving technology may partly solve the problem. We also note that part of the difficulty for molecular genetics is the complex interactions that might be occurring, not only between different genes but also between genes and the environment. Geneenvironment interaction, or the impact the environment has on the expression of genetic influences, may be important in our search for the etiology of personality. It might be a return to quantitative genetic methods, which can provide estimates of gene-environment interplay, which best informs our understanding of personality development. Thus, the field of behavior genetics comes full circle, from an early reliance on quantitative methods that set the stage for molecular work, which again must now rely on statistical modeling of family data to move forward. In the current chapter, our goal is to present a general conceptual overview of behavior genetics methodology, including both biometric models and molecular genetics techniques, as well as a snapshot of seminal work from the past, current issues and controversies, and recommendations for future research.