Twin Methodology in Psychological Studies

Abstract

Psychological traits, including temperament, cognitive functions, and emotions, vary between people. The key aim of twin studies is to investigate the role of genetic factors (nature), environmental factors (nurture), and their interplay, in individual differences of various traits. In the last few decades, the classical twin design (i.e., univariate twin studies and multivariate twin studies), as well as other types of twin studies (e.g., children of twins studies) have been widely used in investigations of psychological traits. Importantly, large-scale twin studies have been established in different countries around the globe, and they have facilitated nature and nurture investigations across different geographical, social and cultural settings. However, when interpreting findings of twin studies, specific methodological assumptions and limitations need to be considered. For example, the equal environments assumption (EEA) suggests that similarities for both monozygotic and dizygotic twin pairs reared in the same family are roughly the same, but this may not always be the case. Among the most important findings of twin studies of psychological traits is the establishment of relative contributions of genetic and environmental influences on health and behavior. Substantial contributions of genetic factors have been demonstrated for many psychological traits, but none of the studies psychological traits is 100% heritable. Also, twin studies have demonstrated that genetic effects can be modified by sex, age, as well as by environmental influences. Moreover, our genetic makeup can shape our environment. Taken together, studies using the twin design have made the significant contribution to our understanding of etiology of individual differences of many psychological traits.