Xref: utzoo sci.bio:3355 sci.med:18873 sci.psychology:3081 Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!usc!apple!portal!cup.portal.com!geoffp From: geoffp@cup.portal.com (Geoffrey Scott Puterbaugh) Newsgroups: sci.bio,sci.med,sci.psychology Subject: Re: The persistance of homosexuality in a gene pool Message-ID: <32214@cup.portal.com> Date: 29 Jul 90 17:35:43 GMT References: <1990Jul23.022511.28161@mtcchi.uucp> <11095@netcom.UUCP> <10615@cs.utexas.edu> <1990Jul29.050038.24791@wolves.uucp> Organization: The Portal System (TM) Lines: 152 In response to Daniel Levy's request for reputable research into a possible link between heredity and sexual desires for members of the same sex... Mr. Levy:>> Note please that I am not saying that this would make acting on that desire _right_ ... I believe it is contrary to God's will, and I don't care who flames... Such irrelevant twaddle has no place on sci.bio, and makes me wonder how sincere you are in asking for "reputable research." But you get a reply anyway. :-) For better or worse, I have a book coming out next month on this subject ("Twins and Homosexuality: A Casebook," Garland Publications, NY, should be August 1990). You asked for references to articles. I have put them at the end. What follows is a summary of what appears in the periodical literature. The concordance rate for homosexuality among male MZ (identical) twins is the interesting number addressed by these studies. If the theory of environmental causation were true, then one would expect no concordance for homosexuality at all; that is, homosexuals would appear at the basic Kinsey rate of 5 to 10 per cent of the male population. But that is not the number which medical research gives us. If we add up all the pairs reported in these eight articles (including those summarized in Zuger and in Heston and Shields), we obtain the following totals: 65 pairs of male MZ twins, 50 of whom are concordant and 15 discordant. This amounts to 77 per cent concordance among identical male twins. (Out of caution, this count excludes all females and all cases where schizophrenia was present.) Furthermore, if we eliminate the recent spate of "freak" articles -- articles published to study the puzzling case of discordant MZ twins, and which have a sample size of one discordant pair -- we are left with 37 from Kallmann, 2 from Lange, 6 from Sanders, 5 from Habel, 4 from Heston and Shields, 1 from Farber, and 2 from Eckert and Bouchard. This makes 57 pairs, 50 of whom are concordant, and 7 of whom are discordant. Analyzed in this way, the data would seem to suggest a concordance rate of about 88 per cent for homosexuality among male identical twins. The true concordance rate is not yet known. If it should prove to be as high as 80 or 90 per cent, then it would probably be enough to settle the nature/nurture debate. This conclusion may seem startling, since many people believe that even one discordant MZ pair must disprove the entire genetic case. Identical twins have identical genes: if they are discordant in regard to homosexuality, then clearly something else is at work. The error in such reasoning lies in a faulty assumption, which is pointed out by two authors, Money and Klintworth: we already know that MZ twins are not necessarily identical in their gene complements. This has appeared in cases of MZ twins discordant for Down's syndrome and Turner's syndrome. Slight differences in the genotype can and do occur in ways which we are trying to determine (even Gregor Mendel has been rightly faulted for doctoring his pea studies to obtain exact ratios, where nature was not quite so exact). And this very slight difference may well be the mechanism which makes the concordance rate for male homosexuality less than 100%. We must, given these figures for MZ twin pairs, compare them to the figures for DZ (fraternal) twins. The DZ twin pairs show no concordance at all for homosexuality, or a very slight concordance. In every case so far known to medical research, the DZ concordance rate has been so much lower than the MZ concordance rate that the DZ rate has generally been compared to the rate for siblings. And the DZ rate is consistent, whether the DZ twins were raised together or apart. Since both DZ and MZ twins are usually raised together in the same environment, the champions of an environmental etiology are, rightly, puzzled. In any case, the numbers from these twin studies are obviously much too high to allow any further credence to be given to theories of exclusively environmental causation. -------------- It is very hard to write about this subject, because news is coming in all the time. A Canadian group just now claims to have discovered that homosexual men *and* women are disproportionately left-handed. Vast anecdotal evidence claims that the great majority of woodwind players and organists are gay men. What is going on? This much seems fairly certain right now: there is no "period of choice" -- for choosing homo, hetero, or bi. Everyone seems to be looking at genetic factors or at events within the womb before birth. The "events within the womb" group needs to look at the numbers from the twin studies, because both MZ and DZ twins share the same womb -- but somehow the MZ twins emerge from the womb much more concordant in their sexuality than the DZ twins. I hope this is helpful in enabling you to make up your own mind on the question, or in beginning to research the question on your own. Right now, my own impression is that human sexuality is built upon a very strong genetic foundation which is probably modifiable by hormonal events within the womb. But the question is decided, for most people, at or before birth. So finger-pointing is not a reasonable activity. Partial bibliography (significant articles in the literature, send me e-mail if you want a more complete bibliography of around 200 items) Kallmann, F.: "Comparative Twin Study on the Genetic Aspects of Male Homosexuality," The Journal of Nervous and Mental Disease, v. 115, pp. 283-98 (1952). Rainer, J.D., Mesnikoff, A., Kolb, L.C. and Carr, A.: "Homosexuality and Heterosexuality in Identical Twins," Psychosomatic Medicine, v. 22, pp. 251-59 (1960). Klintworth, G.K.: "A Pair of Male Monozygotic Twins Discordant for Homosexuality," The Journal of Nervous and Mental Disease, v. 135, pp. 113-25 (1962). Heston, L.L. and Shields, J.: "Homosexuality in Twins: A Family Study and a Registry Study," Archives of General Psychiatry, v. 18, pp. 149-60 (1968). Zuger, B.: "Monozygotic Twins Discordant for Homosexuality: Report of a Pair and Significance of the Phenomenon," Comprehensive Psychiatry, v. 17, pp. 661-69 (1976). McConaghy, N. and Blaszczynski. M.A.: "A Pair of Monozygotic Twins Discordant for Homosexuality: Sex- Dimorphic Behavior and Penile Volume Responses," Archives of Sexual Behavior, v. 9, pp. 123-131 (1980). Ruse, M.: "Are There Gay Genes? Sociobiology and Homosexuality," The Journal of Homosexuality, v. 6, pp. 5-34 (1981). Eckert, E.D., Bouchard, T.J., Bohlen, J. and Heston, L.L.: "Homosexuality in Monozygotic Twins Reared Apart," The British Journal of Psychiatry, v. 148, pp. 421-425 (1985).