This chapter discusses twin research in psychiatry from a critical perspective, using as little technical jargon as possible. It describes how reliance on twin research and the mistaken acceptance of its underlying assumptions has misled psychiatry and psychiatric genetics in much the same way as behavioral genetics and other behavioral and social science fields have been misled. Apart from a MISTRA study of substance abuse/dependence and antisocial personality, all psychiatric twin studies assessing genetic influences are based on twin method comparisons of MZT versus same-sex DZT pairs reared together in the same home. Psychiatric genetic researchers usually cite the body of EEA-test studies in support of the validity of the EEA, and conclude that the assumption, and genetic interpretations of psychiatric twin studies, is valid or are at least reasonable. The historical crimes committed by the 'racial hygienist' founders of psychiatric genetics such as Ernst Rudin in Nazi Germany have been documented.