303 0001-8244/00/0700-0303$18.00/0 © 2000 Plenum Publishing Corporation Behavior Genetics, Vol. 30, No. 4, 2000 ies of psychiatric illness, especially in biometrical models which fit genetic, shared environment, and un- shared environmental influences to risk for a particular disorder. The EEA presupposes that the environmen- tal experiences which may contribute to the risk of INTRODUCTION The equal-environments assumption (EEA) is critical to establishing the validity of results from twin stud- Self-Reported Zygosity and the Equal-Environments Assumption for Psychiatric Disorders in the Vietnam Era Twin Registry Hong Xian, 1a, 2, 13 Jeffrey F. Scherrer, 1a, 3 Seth A. Eisen, 1b, 2, 3a William R. True, 1a, 3, 3a Andrew C. Heath, 3a, 4, 5 Jack Goldberg, 11, 12 Michael J. Lyons, 6–8 and Ming T. Tsuang 7–10 Received 8 Dec. 1999—Final 23 Apr. 2000 The equal-environments assumption (EEA) in twin studies of psychiatric disorders assumes that the family environment which contributes to risk for a disorder is equally correlated between monozygotic (MZ) and dizygotic (DZ) twin pairs. In a study of psychiatric disor- ders in female twins, Kendler and colleagues (1993) have demonstrated the utility of a test of the EEA which includes a specified family environmental factor defined by using meas- ures of perceived zygosity. We tested the EEA assumption among 3155 male–male twin pair members of the Vietnam Era Twin Registry for the following DSM-III-R lifetime disorders: alcohol dependence, marijuana dependence, any illicit drug dependence, nicotine depend- ence, major depression, and posttraumatic stress disorder. The majority of MZ (81.6%; n = 1593) and DZ (90.2%; n = 1086) twin pairs agreed with the investigator’s assigned zygos- ity. The best-fitting model for each of these disorders did not allow for a specified family environmental influence. These results support the usefulness of perceived zygosity in tests of the EEA. In male twin pairs, perceived zygosity has little impact on twin similarity for common psychiatric disorders. KEY WORDS: Equal-environments assumption; twins; psychiatric disorders; veterans; males. 1a Research Service, St. Louis Veterans Affairs Medical Center, St. Louis, Missouri. 1b Research and Medical Service, St. Louis Veterans Affairs Med- ical Center, St. Louis, Missouri. 2 Division of General Medical Sciences, Department of Medicine, Washington University School of Medicine, St. Louis, Missouri. 3 School of Public Health, St. Louis University Health Sciences Cen- ter, St. Louis, Missouri. 3a Missouri Alcoholism Research Center. 4 Department of Psychiatry, Washington University School of Med- icine, St. Louis, Missouri. 5 Department of Psychology and Genetics, Washington University School of Medicine, St. Louis, Missouri. 6 Department of Psychology, Boston University, Boston, Massa- chusetts. 7 Harvard Medical School Department of Psychiatry at the Brockton / West Roxbury VA, Brockton, Massachusetts. 8 Harvard Institute of Psychiatric Epidemiology and Genetics, Boston, Massachusetts. 9 Harvard Medical School Department of Psychiatry at Massachu- setts Mental Health Center, Boston, Massachusetts. 10 Harvard School of Public Health Department of Epidemiology, Boston, Massachusetts. 11 Department of Veterans Affairs, Health Services Research and De- velopment Service and Cooperative Studies Program Coordinat- ing Center, Hines, Illinois. 12 Epidemiology Program, School of Public Health, University of Illinois, Chicago, Illinois. 13 To whom correspondence should be addressed at St. Louis VAMC (151JC), 915 North Grand, St. Louis, Missouri 63106. Fax: 314- 289-7604. e-mail: hxian@im.wustl.edu.