King, S.M. & Iacono, W.G. (2006) Twin Studies. In N.J. Salkind, K. DeRuyck, & K.Rasmussen (Eds.) Encyclopedia of Human Development Volume 3. (pp. 1280-1283). Thousand Oaks: Sage Publications.
Twin studies represent one of the oldest scientific methods of evaluating the influence of heredity on human development and behavior. During the course of the twentieth century, twin studies became more versatile and have encompassed a wider range of behavioral constructs. As the field advanced, debates about whether nurture or nature is more influential have been replaced with the notion of nature via nurture. This ideological shift has been driven by an increasing number of twin studies demonstrating that certain environments in combination with genetic influences produce a particular developmental outcome. What follows is a brief overview of the twin study method and applications of the method that have led to discoveries in human development.
Iacono, W.G., & McGue, M. (2002). Minnesota Twin Family Study. Twin Research, 5, 482-487.
The Minnesota Twin Family Study is a longitudinal study of 11-year-old and 17-year-old twins and their parents designed to examine factors related to the etiology of substance abuse and related problems. At study intake, the twins and their parents participate in a day-long assessment in our laboratory that includes measures of endophenotypes (e.g., event-related potentials, EEG, autonomic nervous system reactivity, startle eye-blink), psychopathology, personality, cognitive ability, anthropometry, and environmental risk/protective factors. DNA derived from blood is also collected. A parallel longitudinal study of adolescent adoptive siblings, biologically related siblings, and their parents is also underway. Over 1500 twin families and 350 adoptive and biological sibling families have already entered the longitudinal phase of the study. This article provides an overview of study methods, highlights published findings, and describes procedures in place to foster collaboration with other investigators.