ls Lesbianism Hereditary? Reprinted from the Harvard Mental Health Letter, November 1993 Evidence from a recent twin study sug- gests that female sexual orientation has a genetic basis. Through advertisements in lesbian publications requesting twin volunteers, investigators recruited 115 women, 86% of them (by their own ac- count) lesbian and 14% bisexual. Two- thirds had been raised with a mono- zygotic (MZ, or genetically identical) twin sister, and one-third had been raised with a dizygotic (DZ, or fraternal) twin sister. Some also had adoptive sisters who were unrelated genetically. Inter- viewers asked the women about their sis- ters’ sexual orientation and, with their permission, mailed the co-twins and adoptive sisters a questionnaire that cov- ered the topic. The sexual orientation of 108 co-twins and 35 adoptive sisters could be con- fidently determined by their own or their sisters‘ reports. Thirty—four (48%) of 71 MZ co-twins were homosexual or bisexual, as compared with six (16%) of 37 DZ co-twins and two (6%) of 35 adoptive sisters. MZ twins also had a much higher rate of homosexuality than DZ twins and adoptive sisters when two other definitions of homosexuality were used, and even when bisexuals were not counted as homosexual. The original volunteers were also con- fident about the sexual orientation of 73 genetically related sisters and 104 genet- ically related brothers who were not their twins. Fourteen percent of the sisters were homosexual or bisexual about the same as the rate among DZ co-twins. Only 5% of the brothers were homo- sexual, although male homosexuality is thought be more common than female homosexuality in the general popula- tion. According to the authors, this re- sult suggests that male and female ho- mosexuality do not run in the same families and probably have different causes. To judge the heritability of lesbianism — the proportion of the variance (in- dividual differences) in sexual orienta- tion associated with genetic difference — the authors used several models in- corporating different estimates of both ‘ the rate of female homosexuality in the general population and the likelihood that an unusually high proportion of the volunteer subjects would have homo- sexual relatives. On any plausible com- bination of assumptions about these mat- ters, the heritability of female homosexuality was at least .40 and at most .76. Shared environment - the com- mon upbringing of the twims and their adoptive sisters — had no significant ef- fect in any of the statistical models. Al- though environment counted for almost as much as heredity in determining women's sexual orientation, the crucial environmental influences were not the same even for twins brought up together. J.Michael Bailey, Richard C. Pillard, Mi- chael C. Neale, and Yvonne Agyei. Her- itable factors influence sexual orienta- tion in women. Archives of General Psychiatry, 50:217-223 (March 1993). V Send your questions to: Auntie Pearl PO Box 4156 Burlington, VT 05406 AIDS HOTLINES: Vermont New Hampshire New York State 1-800-882-2437 1-800-752-2437 1-800-541-2437 Massachusetts 1-617-522-4090 Maine 1-800-851-2437 National 1-800-342-2437 Canadian 1-613-563-2437 AIDS Community Awareness Project (ACAP) PO Box 608 St. Johnsbury VT 05819 (802) 748-1149 AIDS Community Resource Network (ACoRN) PO Box 2057 Lebanon NH 03766 (603) 448-2220 Bennington Area AIDS Project PO Box 1066 Bennington VT 05201 (802)442-4481 or 1-800-845-2437 Health Resource Organizations Brattleboro AIDS Project PO Box 1486 Brattleboro VT 05302 (802) 254-4444 (Helpline) (802) 254-8263 (Office) Franklin-Grand Isle AIDS Task Force P. O. Box 241, St. Albans VT 05478 (800) 524-7742 - (802) 638-7834 Friends Vermont ‘ Emotional, Spiritual, & Psychological Support PO Box 402 Bristol, _VT 05443 1-800-639-3316 Gay/Lesbian Alcoholics Anonymous GLAA, PO. Box 5653 Burlington, VT 05402 Friends Vermont Emotional, Spiritual & Psychological Support PO Box 402 - Bristol, VT 05433 1-800-639-3316 Lesbian Cancer Support Group (802) 660-8386 North Star (Holistic Health Care) RR2 Box 3255 Morrisville VT 05661 - (802) 888-2858 Vermont AIDS Council PO Box 275 Montpelier VT 05601 - (802) 229-2557 Vermont C.A.R.E.S. (Committee for AIDS Resources, Education, and Services) PO Box 5248 30 Elmwood Avenue Burlington VT 05401 (802) 863-2437 (Burlington) 1-800-649-2437 (Rutland and Washington County support groups) Vermont Department of Health 1-800-882-2437 (AIDS Hotline) or (802) 863-7245 (AIDS Program office) VT PWA Coalition PO Box 1055 ° Brattleboro, VT 05302 1-800-698-8792 or (802) 222-5123 Vermont Women’s Health Center 336 North Avenue Burlington VT 05401 - (802) 863-1386 January 1994