This chapter discusses some major issues and concepts that relate to behavioral genetic and psychiatric genetic research, which of course include twins reared-apart (TRA) studies and studies of twins reared together. The psychometric/behavioral genetic perspective accepts the validly of concepts such as stable personality traits, personality tests, heritability, model fitting, the biological reality of bell-shaped distributions, and the ranking of people on the basis of numerical test scores with an assumed genetic basis to such ranking. In TRA studies, the environments in which twins grow up are far from random, and many pairs are reared nearby to each other in different branches of the same family. Behavioral geneticist Lindon Eaves helped pioneer model fitting along with his Birmingham School colleagues. Genetic studies of IQ based on the assumption that standardized IQ tests accurately measure general intelligence. Psychologists such as Frederick Morgeson and some personnel psychology journal editors rejected self-report personality tests as a predictor of personnel selection and job performance.