Twins as a Tool of Behavioral Genetics Edited by T. J. Bouchard, Jr. P. Propping Every human being is genetically unique and consequently genetically different from every other human being. The one exception is identical (monozygotic) twins, who share exactly the same genome. Fraternal (dizygotic) twins share half of their genes in common by descent. Twins of both types constitute "an experiment of nature". Because it is unethical to carry out powerful experiments on human beings in order to explore the causes of variation in human traits, this natural experiment with all of its vicissitudes is one of the few windows we have with which to view the genetic and environmental determinants of complex human behavioral traits. Many scientists believe that twins can only be used to estimate "heritability" and that they reveal nothing about how genes influence behavior. In addition, they argue that modern molecular genetics will quickly make twin research obsolete. These widely held views are largely incorrect. Twins are a unique and very powerful tool for exploring a wide variety of hypotheses about both the distal (mostly genetic) and proximal (mostly environmental) origins of human individual differences. Scientific knowledge accumulates most rapidly when scientists ask the right questions and utilize the right tools—the right tools for the job. This book attempts to highlight the questions that might be most productively addressed through the use of twin designs. Every tool, however, has its limitations. This book carefully examines the limitations and assumptions associated with the application of the method to each of the domains discussed. Goal of this Dahlem Workshop: to evaluate the environmental and genetic mechanisms underlying the structure and development of behavior in twins studies: the achievements, limitations, and potentials.