In Unit #2 the twin study method will be introduced and general findings from twin studies in psychology and psychiatry will be reviewed. The two types of twins, monozygotic (MZ) and dizygotic (DZ), will be described and methods for assessing their similarity will be given. We will also look critically at the limitations of the twin study method and discuss alternative research designs, including adoption studies and the study of reared-apart twins. The importance of convergent evidence from multiple research designs will be emphasized.