ABAJO EN ESPAÑOL
by Rich Deem
Born Gay?
There is a common belief among liberals that people are born either gay or straight. Conservatives tend to believe that sexual orientation is actually sexual preference, which is chosen by the individual. This page represents a review of the scientific literature on the basis for homosexual orientation.
Rich Deem
Are people born gay or straight? Much of the current media sources assume the question is a solved scientific problem with all the evidence pointing toward a biological (probably genetic) basis for a homosexual orientation. Contrary to this perception, the question has been poorly studied (or studied poorly), although there is some evidence on both sides of question. In addition, many of the initial studies, which were highly flouted by the media as “proof” for a biological basis for homosexuality, have been contradicted by later, more thorough studies. This evidence falls into four basic categories:
1. Brain structure
2. Possible hormonal influences
3. Concordance of homosexuality in twins
4. Concordance of genetic markers in siblings
Until a few years ago, sexual orientation used to be called sexual preference. Obviously, the two terms denote significant differences in the the manner by which sexuality develops. A preference is something that is chosen, whereas orientation is merely something that defines us. The differences are potentially important regarding how the law applies to those who are gay. If homosexuality is not chosen, but actually is a biologically-determined characteristic over which we have no choice, then laws should not treat gays and straights differently, since homosexuality would be equivalent to one’s race, over which we have no control.
Sexual orientation – brain studies
Since sexual attraction begins in the brain, researchers first examined the question of sexual orientation by comparing the anatomy of brains from males and females. These studies showed that male and female brains showed sexual dimorphism in the pre-optic area of the hypothalamus, where males demonstrated a greater than two-fold difference in cell number and size compared to females.1 A second study found that two of four Interstitial Nuclei of the Anterior Hypothalamus (INAH) were at least twice as large in males as females.2 Since the INAH was involved in sexual dimorphism, it was hypothesized by Simon LeVay that there might be differences in this region in heterosexual vs. homosexual men. Postmortem examination of the brains of AIDS patients vs. control male subjects (presumed to be heterosexual) showed that the presumably heterosexual men exhibited INAH3 that were twice the size of both females and presumably homosexual men who had died of AIDS.3 The study has been criticized for its uncertainty of sexual orientation of the subjects, and potential complications caused by the AIDS virus (which does infect the human brain), and also by lowered testosterone levels found in AIDS patients. A popularized Newsweek cover story, “Is This Child Gay?”4 characterized LeVay as a “champion for the genetic side,” even though the study involved no genetic data at all.
A subsequent study by Byne, et al. examined the question of INAH3 size on the basis of sex, sexual orientation, and HIV status.5 The study found large differences in INAH3 volume on the basis of sex (with the male INAH3 being larger than the female INAH3). However, the volume of IHAH3 was decreased in male heterosexual men who had contracted AIDS (0.108 mm3 compared with 0.123 mm3 in male controls). There was no statistically significant difference between IHAH3 sizes of male heterosexuals vs. male homosexuals who had contracted AIDS (0.108 mm3 and 0.096 mm3, respectively). The study also found that there were no differences in the number of neurons in the INAH3 based upon sexual orientation, although researchers found significant differences between males and females, as in other studies.5 It was obvious from this study that LeVay’s study was fatally flawed due to the AIDS complication, and that there were no differences in the INAH3 based upon sexual orientation.
The role of the hypothalamus in sexual orientation was further studied by Swaab, et al. Other researchers had hypothesized that differentiation of the hypothalamus occurred before birth. However, Swaab’s study showed that the sexually dimorphic nucleus (SDN) of more than 100 subjects decreased in volume and cell number in the females only 2-4 years postnatal. This finding complicated the findings of the brain studies, since not only chemical and hormonal factors, but also social factors, might have influenced this process.6
A study by Allen and Gorski examined the anterior commissure of the brain, finding that females females and homosexual males exhibited a larger size than heterosexual males.7 However, later studies using larger sample sizes found no such differences.8
Complicating the issue of brain differences between homosexuals and heterosexuals is the problem that sexual experiences themselves can affect brain structure.9 So, the question will always be whether homosexual practice changes the brain or whether the brain results in homosexual practice.
Since sexual differentiation occurs within the womb, as a result of hormonal influences, it has been hypothesized that homosexuality may result from a differential hormone balance in the wombs of those who eventually exhibit a homosexual orientation. Since hormonal levels within the womb are not available, proxies for hormonal influences have been used to examine the question of how hormonal influences might impact sexual orientation. These proxies include differences in skeletal size and shape, including the ratio of the long bones of the arms and legs relative to arm span or stature and the hand bones of adults (the ratio of the length of the various phalanges).
Digit ratio vs. orientation
Studies have shown that ratios of digit length are predictors of several hormones, including testosterone, luteinizing hormone and estrogen.10 In women, the index finger (2D, second digit) is almost the same length as the fourth digit (4D). However, in men, the index finger is usually shorter than the fourth. It has been shown that this greater 2D:4D ratio in females is established in two-year-olds. It has been hypothesized that the sex difference in the 2D:4D ratio reflects the prenatal influence of androgen on males. A study by Williams, et al. showed that the 2D:4D ratio of homosexual men was not significantly different from that of heterosexual men for either hand.11 However, homosexual women displayed significantly smaller 2D:4D ratios compared with heterosexual women (see figure to right). It has been hypothesized that women exposed to more androgens in the womb tend to express a homosexual orientation. However, since these hormone levels were never measured, one is left with the proxy of finger lengths as a substitute. Studies have found that the more older brothers a boy has, the more likely he is to develop a homosexual orientation.12 This study also found that homosexual men had a greater than expected proportion of brothers among their older siblings (229 brothers: 163 sisters) compared with the general population (106 males: 100 females). Males who had two or more older brothers were found to have lower 2D:4D ratios,11 suggesting that they had experienced increased androgens in the womb. Why increased androgens would predispose both males and females to be homosexual was not explained in the study.
Another study examined the length of long bones in the arms, legs and hands. Both homosexual males and heterosexual females had less long bone growth in the arms, legs and hands, than heterosexual males or homosexual females.13 Accordingly, the researchers hypothesized that male homosexuals had less androgen exposure during development than male heterosexuals, while female homosexuals had greater steroid exposure during development than their heterosexual counterparts. Of course, with regard to male homosexuality, this study directly contradicted the presumed results of the Williams study above, which “showed” that males with multiple older brothers (who tended to be homosexual) experienced increased androgen exposure.
A study of one homosexual vs. two heterosexual male triplets found that the homosexual triplets scored more on the female side of the Masculinity-Femininity scale of the Minnesota Multiphasic Personality Inventory,14 suggesting a possible hormonal influence (decreased androgens) involved in male homosexual orientation.
All of the studies reporting possible hormonal influence on homosexuality suffer from the lack of any real evidence that hormones actually play any role in sexual orientation. The fact that contradictory studies report increased11,15 vs. decreased13-14 androgens as a basis for homosexuality doesn’t provoke confidence that the proxies are really true. Obviously, a study that documented real hormone levels, as opposed to proxies, would probably provide more definitive data.
The observation that familial factors influence the prevalence of homosexuality led to a the initiation of number of twin studies, which are a proxy for the presence of possible genetic factors. Most of these early studies suffered from methodological flaws. Kallmann sampled subjects from correctional and psychiatric institutions – not exactly representative “normal” populations.16 Bailey et al. published a number of studies in the early 1990′s, examining familial factors involved in both male and female homosexuality. These studies suffered from the manner in which subjects were recruited, since the investigators advertised in openly gay publications, resulting in skewed populations.17 Later studies by the same group did not suffer from this selection bias, and found the heritability of homosexuality in Australia was up to 50 and 60% in females but only 30% in males.18
A study by Kendler et al. in 2000 examined 1,588 twins selected by a random survey of 50,000 households in the United States.19 The study found 3% of the population consisted of non-heterosexuals (homosexuals and bisexuals) and a genetic concordance rate of 32%, somewhat lower than than found in the Australian studies. The study lost statistical significance when twins were broken down into male and female pairs, because of the low rate (3%) of non-heterosexuals in the general U.S. population.
A Finnish twin study reported the “potential for homosexual response,” not just overt homosexual behavior, as having a genetic component.20
On a twist on homosexual twin studies, an Australian research group examined the question of whether homophobia was the result of nature or nurture.21 Surprisingly, both familial/environmental and genetic factors seemed to play a role as to whether or not a person was homophobic. Even more surprising, a separate research group in the U.S. confirmed these results (also adding that attitudes towards abortion were also partly genetic).22 Now, even homophobes can claim that they were born that way!
Twin studies suffer from the problem of trying to distinguish between environmental and genetic factors, since twins tend to live within the same family unit. A study examining the effect of birth order on homosexual preference concluded, “The lack of relationship between the strength of the effect and degree of homosexual feelings in the men and women suggests the influence of birth order on homosexual feelings was not due to a biological, but a social process in the subjects studied.”12 So, although the twin studies suggest a possible genetic component for homosexual orientation, the results are certainly not definitive.
Genetic studies – the “gay gene”
An examination of family pedigrees revealed that gay men had more homosexual male relatives through maternal than through paternal lineages, suggesting a linkage to the X chromosome. Dean Hamer23 found such an association at region Xq28. If male sexual orientation was influenced by a gene on Xq28, then gay brothers should share more than 50% of their alleles at this region, whereas their heterosexual brothers should share less than 50% of their alleles. In the absence of such an association, then both types of brothers should display 50% allele sharing. An analysis of 40 pairs of gay brothers and found that they shared 82% of their alleles in the Xq28 region, which was much greater than the 50% allele sharing that would be expected by chance.24 However, a follow-up study by the same research group, using 32 pairs of gay brothers and found only 67% allele sharing, which was much closer to the 50% expected by chance.25 Attempts by Rice et al. to repeat the Hamer study resulted in only 46% allele sharing, insignificantly different from chance, contradicting the Hamer results.26 At the same time, an unpublished study by Alan Sanders (University of Chicago) corroborated the Rice results.27 Ultimately, no gene or gene product from the Xq28 region was ever identified that affected sexual orientation. When Jonathan Marks (an evolutionary biologist) asked Hamer what percentage of homosexuality he thought his results explained, his answer was that he thought it explained 5% of male homosexuality. Marks’ response was, “There is no science other than behavioral genetics in which you can leave 97.5% of a phenomenon unexplained and get headlines.”28
Sexual preference or orientation?
If homosexual orientation were completely genetic, one would expect that it would not change over the course of one’s life. For females, sexual preference does seem to change over time. A 5-year study of lesbians found that over a quarter of these women relinquished their lesbian/bisexual identities during this period: half reclaimed heterosexual identities and half gave up all identity labels.29 In a survey of young minority women (16-23 years of age), half of the participants changed their sexual identities more than once during the two-year survey period.30 In another study of subjects who were recruited from organizations that serve lesbian/gay/bisexual youths (ages 14 to 21 years) in New York City, the percentage that changed from a lesbian/gay/bisexual orientation to a heterosexual orientation was 5% over the period of just 12 months (the length of the survey).31 Other studies have confirmed that sexual orientation is not fixed in all individuals, but can change over time, especially in women.32
It always amazes me when people say that they were born gay. Looking back on my own experience, I would never say that I was born straight. I really didn’t have any interest in females until about the seventh grade. Before that time, they weren’t really interesting, since they weren’t interested in sports or riding bikes or anything else I liked to do.
Homosexuality and Darwinism
I am not a huge fan of Neo Darwinian evolution. Nevertheless, there is some clear evidence that natural selection (and sexual selection) does act upon populations and has acted on our own species to produce racial differences.33 Natural selection postulates that those genetic mutations that favor survival and reproduction will be selected, whereas those that compromise survival and reproduction will be eliminated. Obviously, a gene or series of genes that produce non-reproducing individuals (i.e., those who express pure homosexual behavior) will be rapidly eliminated from any population. So, it would be expected that any “gay gene” would be efficiently removed from a population. However, it is possible that a gene favoring male homosexuality could “hide” within the human genome if it were located on the X-chromosome, where it could be carried by reproducing females, and not be subject to negative selection by non-reproducing males. In order to survive, the gene(s) would be expected to be associated with higher reproductive capacity in women who carry it (compensating for the generation of non-reproducing males). I can’t imagine a genetic scenario in which female homosexuality would ever persist within a population.
Within the last decade, genetic analysis of heritable traits has taken a huge step forward with the advent of DNA microarray technology. Using this technology, it is possible to scan large lengths of the human genome (even an entire genome wide scan – GWAS) for numerous individuals, at quite reasonable costs. This DNA microarray technology has led to the discovery of genes that are associated with complex diseases, such as Crohn’s Disease, which is the topic of my research. If homosexuality truly has a genetic component, DNA microarray studies (probably beginning with the X-chromosome) would not only definitively prove the point, but would identify specific gene(s) or loci that might be associated with those who express a homosexual orientation. Why haven’t these studies been done?
The question of how homosexual orientation originates has been the subject of much press, with the general impression being promoted that homosexuality is largely a matter of genes, rather than environmental factors. However, if one examines the scientific literature, one finds that it’s not quite as clear as the news bytes would suggest. The early studies that reported differences in the brains of homosexuals were complicated by HIV infection and were not substantiated by larger, better controlled studies. Numerous studies reported that possible hormonal differences affected homosexual orientation. However, these studies were often directly contradictory, and never actually measured any hormone levels, but just used proxies for hormonal influences, without direct evidence that the proxies were actually indicative of true hormone levels or imbalances. Twin studies showed that there likely are genetic influences for homosexuality, although similar studies have shown some genetic influences for homophobia and even opposition to abortion. The fact that sexual orientation is not constant for many individuals, but can change over time suggests that at least part of sexual orientation is actually sexual preference. Attempts to find a “gay gene” have never identified any gene or gene product that is actually associated with homosexual orientation, with studies failing to confirm early suggestions of linkage of homosexuality to region Xq28 on the X chromosome. The question of genetic influences on sexual orientation could be definitively answered by using DNA microarray technology, although, to my knowledge, no such studies are planned or in progress.
· Smoking and Alcohol: It’s Just Another Lifestyle: What Health Risks?
· Rebuttal to Why the Christian Right is Wrong About Homosexuality
· The Biblical Design for Human Sexuality
1. Swaab, D.F. and E. Fliers. 1985. A sexually dimorphic nucleus in the human brain. Science 228:1112-1115.
2. Allen, L.S., M. Hines, J.E. Shryne and R.A. Gorski. 1989. Two sexually dimorphic cell groups in the human brain. J. Neurosci. 9:497-506.
3. LeVay, S. 1991. A difference in hypothalamic structure between heterosexual and homosexual men. Science 253:1034-1037.
4. Gelman, D. Is This Child Gay? Born or Bred: The Origins of Homosexuality. Newsweek September 9, 1991, p. 52.
5. Byne W., S. Tobet, L. A. Mattiace, M. S. Lasco, E. Kemether, M. A. Edgar, S. Morgello, M. S. Buchsbaum, and L. B. Jones. 2001. The interstitial nuclei of the human anterior hypothalamus: an investigation of variation with sex, sexual orientation, and HIV status. Horm. Behav. 40:86-92.
6. Swaab, D.F., L.J.G. Gooren, and M.A. Hofman. 1992. Gender and Sexual Orientation in Relation to Hypothalamic Structures. Horm. Res. 38 (Suppl. 2):51-61.
7. Allen, L. S. and R. A. Gorski. 1992. Sexual orientation and the size of the anterior commissure in the human brain. PNAS 89: 7199-7202.
Allen, L. S. and R. A. Gorski. 1991. Sexual dimorphism of the anterior commissure and massa intermedia of the human brain. J. Comp. Neurol. 312: 97-104.
8. K. M. Bishop, K. M. and D. Wahlsten. 1997. Sex Differences in the Human Corpus Callosum: Myth or Reality? Neuroscience & Biobehavioral Reviews 21: 581-601.
Lasco, M. S., T. J. Jordan, M. A. Edgar, C. K. Petito and W. Byne. 2002. A lack of dimorphism of sex or sexual orientation in the human anterior commissure. Brain Research 936: 95-98.
9. Breedlove, M.S. 1997. Sex on the brain. Nature 389: 801.
10. Manning, J.T., D. Scutt, J. Wilson and D. I. Lewis-Jones. 1998. The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen. Human Reproduction 13: 3000-3004..
11. Williams, T. J., M. E. Pepitone, S. E. Christensen, B. M. Cooke, A. D. Huberman, N. J. Breedlove, T. J. Breedlove, C. L. Jordan, and S. M. Breedlove. 2000. Finger-length ratios and sexual orientation. Nature 404: 455-456.
12. McConaghy, N., D. Hadzi-Pavlovic, C. Stevens, V. Manicavasagar, N. Buhrich, and U. Vollmer-Conna. 2006. Fraternal birth order and ratio of heterosexual/homosexual feelings in women and men. J. Homosex. 51:161-74.
13. Martin, J. T. and D. H. Nguyen. 2004. Anthropometric analysis of homosexuals and heterosexuals: implications for early hormone exposure Hormones and Behavior 45: 31-39.
14. Hershberger, S.L., and N.L. Segal. 2004. The cognitive, behavioral, and personality profiles of a male monozygotic triplet set discordant for sexual orientation. Arch. Sex. Behav. 33:497-514.
15. McFadden, D. 2002. Masculinization effects in the auditory system. Arch. Sex. Behav. 31:99-111.
16. Kallmann, F.J. 1952. Twin and sibship study of overt male homo-sexuality. Am. J. Hum. Genet. 4:136-146.
17. Bailey, J.M., and R.C. Pillard. 1991. A genetic study of male sexual orientation. Arch. Gen. Psychiatry 48:1089-1096.
Bailey, J.M., R.C. Pillard, M.C. Neale, and Y. Agyei. 1993. Heritable factors influence sexual orientation in women. Arch. Gen. Psychiatry 50:217-223.
Bailey, J.M., and A.P. Bell. 1993. Familiality of female and male homosexuality. Behav. Genet. 23:313-322.
Bailey, J.M., and D.S. Benishay. 1993. Familial aggregation of female sexual orientation. Am. J. Psychiatry 1993; 150:272-277.
18. Bailey, J.M., M.P. Dunne, and N.G. Martin. 2000. Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. J. Person Soc. Psychol. 78:524-536.
Kirk, K., J. Bailey, and N. Martin. 2000. Etiology of male sexual orientation in an Australian twin sample, Psychology, Evolution & Gender 2: 301–311.
Kirk, K., J. Bailey, M. Dunne, and N. Martin. 2000. Measurement models for sexual orientation in a community twin sample. Behavior. Genetics 30: 345–356.
19. Kendler, K. S., L. M. Thornton, S. E. Gilman, and R. C. Kessler. 2000. Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs. Am. J. Psychiatry 157:1843-1846.
20. Santtila, P., N. K. Sandnabba, N. Harlaar, M. Varjonen, K. Alanko, and B. von der Pahlen. 2008. Potential for homosexual response is prevalent and genetic. Biological Psychology 77: 102–105.
21. Verweij, K. J. H., S. N. Shekar, B. P. Zietsch, L. J. Eaves, J. M. Bailey, D. I. Boomsma, and N. G. Martin. 2008 Genetic and Environmental Influences on Individual Differences in Attitudes Toward Homosexuality: An Australian Twin Study Behav. Genet. 2008 May; 38(3): 257-265.
22. Eaves, L. J., and P. K. Hatemi. 2008. Transmission of Attitudes Toward Abortion and Gay Rights: Effects of Genes, Social Learning and Mate Selection. Behav. Genet. 38:247–256.
23. Dean Hamer gained even more notoriety by publishing a book entitled The God Gene: How Faith Is Hardwired into Our Genes, which a Scientific American Review of The God Gene said should have been titled, “A Gene That Accounts for Less than One Percent of the Variance Found in Scores on Psychological Questionnaires Designed to Measure a Factor Called Self-Transcendence, Which Can Signify Everything from Belonging to the Green Party to Believing in ESP, According to One Unpublished, Unreplicated Study.”
24. Hamer, D. H., S. Hu, V. L. Magnuson, N. Hu, and A. M. Pattatucci. 1993. A linkage between DNA markers on the X chromosome and male sexual orientation. Science 261: 321.
25. Hu S., A.M. Pattatucci, C. Patterson, L. Li, D.W. Fulker, S.S. Cherny, L. Kruglyak, and D.H. Hamer. 1995. Linkage between sexual orientation and chromosome Xq28 in males but not in females. Nat. Genet. 11:248-56.
26. Rice, G., C. Anderson, N. Risch, and G. Ebers. 1999. Male Homosexuality: Absence of Linkage to Microsatellite Markers at Xq28. Science 284: 665-667.
27. Wickelgren, I. 1999. Discovery of ‘Gay Gene’ Questioned. Science 284: 571.
28. Marks, J. 2002. What It Means to Be 98% Chimpanzee: Apes, People, and Their Genes .
29. Diamond, L. M. 2003. Was it a phase? Young women’s relinquishment of lesbian/bisexual identities over a 5-year period. J. Pers. Soc. Psychol. 84: 352-64.
30. Diamond, L. M. 2000. Sexual identity, attractions, and behavior among young sexual-minority women over a 2-year period. Dev. Psychol. 36: 241-50.
31. Rosario M., E.W. Schrimshaw, J. Hunter, and L. Braun. 2006. Sexual identity development among gay, lesbian, and bisexual youths: consistency and change over time. J Sex Res. 43: 46-58.
32. Kinnish, K.K., Strassberg, D.S., Turner, C.W., 2005. Sex differences in the flexibility of sexual orientation: a multidimensional retrospective assessment. Archives of Sexual Behavior 34, 173–183.
33. See online slideshow beginning with Origin of the Races: Race Facts.
http://www.godandscience.org/evolution/genetics_of_homos exuality.html
La Genética y la Homosexualidad: ¿Nace la gente, homosexual?
La Base Biológica para la Orientación Sexual
Por Rich Deem
Nacido Gay?
Hay una creencia común entre los liberales que las personas nacen, ya sea homosexuales o heterosexuales. Los conservadores tienden a creer que la orientación sexual es de hecho una preferencia sexual, la cual es optada por el individuo. Esta página representa una revisión de la literatura científica acerca de la base para la orientación homosexual.
Rich Deem
¿Nacen algunas personas como homosexuales o más bien como heterosexuales? Mucho de las fuentes de los medios de comunicación actuales asumen que la pregunta es un problema científico solucionado con toda una evidencia que señala hacia una base biológica (probablemente genética) para la orientación homosexual. En contra de esta percepción, la cuestión ha sido pobremente estudiada (o se estudia pobremente), aunque hay algo de evidencia en ambos lados de la pregunta. Además, muchos de los estudios iniciales que fueron presentados por los medios noticiosos como “prueba” para una base biológica para la homosexualidad, han sido contradichos más tarde, por más estudios cabales. Esta prueba se subdivide en cuatro categorías básicas:
La estructura del cerebro
Las posibles influencias hormonales
La concordancia de homosexualidad en gemelos
La concordancia de marcadores genéticos en hermanos
¿Por qué tiene importancia?
Hasta hace unos cuantos años atrás, la orientación sexual solía llamarse preferencia sexual. Obviamente, los dos términos denotan diferencias significativas en la manera por la cual la sexualidad se desarrolla. Una preferencia es algo que se escoge, mientras que la orientación es meramente algo que nos define. Las diferencias son potencialmente importantes referente a cómo la ley se aplica a aquellos que son gays. Si la homosexualidad no se escoge, sino que realmente es una característica biológicamente determinada sobre la cual no tenemos alternativa, entonces las leyes no deberían tratar a los homosexuales varones y a los heterosexuales diferentemente, puesto que la homosexualidad sería equivalente a la raza de uno, sobre la cual no tenemos control.
La orientación sexual – el cerebro estudia
Puesto que la atracción sexual comienza en el cerebro, los investigadores primero examinaron la cuestión de la orientación sexual comparando la anatomía de los cerebros de varones y mujeres. Ehttp://www.godandscience.org/evolution/genetics_of_homosexuality.htmlstos estudios demostraron que los cerebros de los varones y las mujeres mostraron dimorfismo sexual en la área preóptica del hipotálamo, donde los varones demostraron más de dos veces en el número de células que las mujeres1 Un segundo estudio encontró que dos de cuatro núcleos intersticiales del Hipotálamo Anterior (INAH) eran al menos el doble de grande en los varones como en las mujeres.2 Puesto que el INAH estaba involucrado en el dimorfismo sexual, fue conjeturado por Simón LeVay que podrían haber diferencias en esta región en hombres heterosexuales vs. homosexuales. La autopsia de la materia gris de pacientes de SIDA vs. sujetos masculinos de control (presuntos de ser heterosexuales) demostró que los hombres probablemente heterosexuales exhibieron INAH3 que eran dos veces al tamaño de ambos mujeres y hombres presumiblemente homosexuales que habían muerto de SIDA.3http://www.godandscience.org/evolution/genetics_of_homosexuality.html El estudio ha sido criticado por la incertidumbre de la orientación sexual de los sujetos, y las complicaciones potenciales causadas por el virus del SIDA (que infecta el cerebro humano), y también niveles de testosterona aminorados encontrados en pacientes de SIDA. Un artículo destacado en la portada Newsweek, “¿Es Este Niño Gay”?4 caracterizó a LeVay como un “campeón para el lado genético,” si bien el estudio no requirió datos genéticos en absoluto
Un subsiguiente estudio por Byne, et al. examinó la cuestión del tamaño del INAH3 en base al sexo, orientación sexual, y estatus HIV.5http://www.godandscience.org/evolution/genetics_of_homosexuality.html El estudio encontró grandes diferencias en el volumen del INAH3 sobre la base del sexo (con el INAH3 de los hombres siendo mayor que el INAH3 de las mujeres). Sin embargo, el volumen de IHAH3 era menor en hombres heterosexuales masculinos que habían contraído SIDA (0.108 mm3 comparado con 0.123 mm3 en los controles de los hombres). No hubo estadísticamente diferencia significativa entre las dimensiones del IHAH3 de hombres heterosexuales vs. hombres homosexuales que habían contraído SIDA (0.108 mm3 y 0.096 mm3, respectivamente). El estudio también encontró que no hubo diferencias en el número de neuronas en el INAH3 basado en la orientación sexual, aunque los investigadores encontraron diferencias significativas entre varones y hembras, como en otros estudios.5http://www.godandscience.org/evolution/genetics_of_homosexuality.html Era obvio de este estudio que el estudio de LeVay fue fatalmente defectuoso debido a la complicación del SIDA, y que no hubo diferencias en el INAH3 basado en la orientación sexual.
El papel del hipotálamo en la orientación sexual fue además estudiado por Swaab, et al. Otros investigadores habían conjeturado que la diferenciación del hipotálamo ocurrió antes del nacimiento. Sin embargo, el estudio de Swaab demostró que el núcleo sexualmente dimórfico (NSD) de más que 100 sujetos disminuyó en volumen y el número de células en las mujeres sólo 2-4 años postnatales. Este descubrimiento complicó las conclusiones de los estudios del cerebro, puesto que no sólo factores químicos y hormonales, sino que también los factores sociales, podrían haber influenciado este proceso.6http://www.godandscience.org/evolution/genetics_of_homosexuality.html
Uhttp://www.godandscience.org/evolution/genetics_of_homosexuality.htmln estudio por Allen y Gorski examinó la anterior comisura del cerebro, encontrando que las mujeres y los hombres homosexuales exhibieron mayores tamaños que hombres heterosexuales7. Sin embargo, estudios posteriores usando muestras más grandes no encontraron diferencias8
Complicando el asunto de las diferencias del cerebro entre homosexuales y heterosexuales es el problema que las experiencias sexuales mismas pueden afectar la estructura del cerebro.9http://www.godandscience.org/evolution/genetics_of_homosexuality.html Así que, la pregunta siempre será si la práctica homosexual cambia el cerebro o si el cerebro da como resultado la práctica homosexual.
Las influencias hormonales
Puesto que la diferenciación sexual ocurre dentro del vientre, como resultado de las influencias hormonales, ha sido conjeturado que la homosexualidad puede resultar de un balance diferencial de hormonas en los vientres de aquellos que eventualmente exhiben una orientación homosexual. Puesto que los niveles hormonales dentro del vientre no están disponibles, los defensores para las influencias hormonales han estado acostumbrados a examinar la cuestión de cómo las influencias hormonales podrían afectar la orientación sexual. Estos defensores incluyen diferencias en tamaño esquelético y forma, incluso la proporción de los huesos largos de los brazos y piernas con relación a la envergadura del brazo o la estatura y los huesos de la mano de adultos (la proporción de la longitud de varias falanges).
Los estudios han mostrado que ratios de longitud digital son pronosticadoras de varias hormonas, incluyendo la hormona de la testosterona, luteínica y estrógeno.10http://www.godandscience.org/evolution/genetics_of_homosexuality.html En mujeres, el dedo índice (2D, segundo dígito) es casi la misma longitud como el cuarto dígito (4D). Sin embargo, en hombres, el dedo índice es usualmente más corto que el cuarto. Ha sido mostrado que este ratio mayor 2D: 4D en mujeres es establecida en los dos años de edad. Ha sido conjeturado que la diferencia sexual en el ratio 2D:4D refleja la influencia prenatal de andrógeno en varones. Un estudio por Williams, et al. mostró que el ratio 2D:4D de hombres homosexuales no fue significativamente diferente a aquellos de hombres heterosexuales para una o la otra mano.11http://www.godandscience.org/evolution/genetics_of_homosexuality.html Sin Embargo, las mujeres homosexuales exhibieron significativamente un ratio 2D:4D más pequeño comparado con las mujeres heterosexuales (vea figura para verificar). Ha sido conjeturado que las mujeres expuestas a más andrógenos en el vientre tienden a expresar una orientación homosexual. Sin embargo, puesto que estos niveles de hormona nunca fueron medidos, uno queda con el apoderado de las longitudes del dedo como un substituto. Los estudios han encontrado que más hermanos mayores un niño tiene, lo más probable es que él deba desarrollar un orientación homosexual.12http://www.godandscience.org/evolution/genetics_of_homosexuality.html Este estudio también encontrado que los hombres homosexuales tuvieron una más grande proporción de la esperada de hermanos entre sus hermanos mayores (229 hermanos: 163 hermanas) comparadas con la población general (106 varones: 100 hembras). Los varones que tuvieron dos o más hermanos mayores se encontró que tenían ratios inferiores 2D:4D, 11 sugiriendo que habían experimentado aumento de andrógenos en el vientre. Porqué andrógenos aumentados predisponían a ambos varones y mujeres para ser homosexuales, no fue explicado en el estudio.
Otro estudio examinó la longitud de los huesos largos en los brazos, piernas y manos. Ambos varones homosexuales y mujeres heterosexuales tuvieron menos crecimiento de los huesos largos en los brazos, piernas y manos, que los varones heterosexuales o mujeres homosexuales.13http://www.godandscience.org/evolution/genetics_of_homosexuality.html Consecuentemente, los investigadores conjeturaron que los homosexuales varones tuvieron menos exposición de andrógeno durante el desarrollo que los hombres heterosexuales, mientras que las mujeres homosexuales tuvieron más grande exposición del esteroide durante el desarrollo que sus contrapartes heterosexuales. Por supuesto, acerca de la homosexualidad de los hombres, este estudio directamente contradijo los presuntos resultados del estudio de Williams arriba, que “mostró” que los varones con hermanos mayores múltiples (quien se inclinaban a ser homosexuales) experimentaron aumentada exposición de andrógeno.
Un estudio de un homosexual versus 2 varones heterosexuales trillizos se encontró que los trillizos homosexuales marcaron más en el lado femenino de la escala de Feminidad/masculinidad del Inventario de la Personalidad Multifásica de Minesota, 14 sugiriendo una posible influencia hormonal (disminución de andrógenos) implicado en la orientación homosexual masculina.
Todos los estudios que reportan una posible influencia hormonal en la homosexualidad padecen de falta de cualquier evidencia verdadera de que las hormonas realmente juegan algún papel en la orientación sexual. Ehttp://www.godandscience.org/evolution/genetics_of_homosexuality.htmlhttp://www.godandscience.org/evolution/genetics_of_homosexuality.htmlhttp://www.godandscience.org/evolution/genetics_of_homosexuality.htmll hecho de que los estudios contradictorios reportan incremento11,15 versus decrecimiento de andrógenos como una base para la homosexualidad no provocan confianza de que los alegatos son realmente verdaderos.
La observación de que factores familiares influencian el predominio de la homosexualidad condujo uno la iniciación de un número de estudios de gemelos, para analizar la posible presencia de factores genéticos. La mayor parte de estos estudios tempranos padecieron de desperfectos metodológicos. Kallmann probó a sujetos de instituciones correccionales y psiquiátricas – no exactamente poblaciones representativas “normales” .16 Bailey et al,http://www.godandscience.org/evolution/genetics_of_homosexuality.html publicaron un número de estudios en los inicios de 1990′s, examinando factores domésticos involucrados en la homosexualidad de hombres y de mujeres. Estos estudios adolecieron de la forma en la cual los sujetos de estudio fueron reclutados, puesto que los investigadores anunciaron abiertamente en publicaciones Gay, resultando en poblaciones sesgadas. Estudios posteriores por el mismo grupo no padeció de este prejuicio de selección, y encontró que la heredabilidad de la homosexualidad en Australia fue arriba de 50 y 60% en mujeres pero sólo 30% en hombres.18
Un estudio por Kendler Et Al. en el 2000 examinó 1,588 gemelos seleccionados por una encuesta aleatoria de 50,000 grupos familiares en el Estados Unidos.19 El estudio encontró que 3 % de la población constó de no heterosexuales (homosexuales y bisexuales) y una tasa genética de concordancia de 32 %, algo más bajo que el encontrado en los estudios australianos. El estudio perdió significado estadístico cuando los gemelos fueron descompuestos en pares de hombres y mujeres, debido a la tasa baja (3 %) de no heterosexuales en la población general de Estados Unidos.
Un estudio terminado de gemelos reportó el “potencial para la respuesta homosexual,” no sólo sobre comportamiento homosexual abierto, como que tiene un componente genético.20 http://www.godandscience.org/evolution/genetics_of_homosexuality.
En un viraje en los estudios homosexuales de gemelos, un grupo de investigación australiano examinó la cuestión de si la homofobia era el resultado de la naturaleza o de la educación.21http://www.godandscience.org/evolution/genetics_of_homosexuality.html Sorprendentemente, ambos factores domésticos / medioambientales y genéticos parecieron jugar un papel en lo que se refiere a si una persona era homofóbica o no. Más sorprendente aún, un grupo de investigación separada en EEUU confirmó que estos resultados (también añadiendo las actitudes hacia el aborto fueron también en parte genéticas) .22http://www.godandscience.org/evolution/genetics_of_homosexuality.html Ahora, aun los homofóbicos pueden afirmar que nacieron de ese modo.
Los estudios de gemelos padecen del problema de intentar distinguir entre factores medioambientales y genéticos, puesto que los gemelos tienden a vivir dentro de la misma unidad familiar. Un estudio que examina el efecto del orden de nacimiento en la preferencia homosexual concluyó, “la falta de relación entre la fuerza del efecto y el grado de sentimientos homosexuales en los hombres y mujeres sugiere que la influencia del orden de nacimiento en los sentimientos homosexuales no fue debido a un proceso biológico sino social, en los sujetos estudiados”.12 Entonces, aunque el estudio de gemelos sugiere un componente genético posible para la orientación homosexual, los resultados son ciertamente no definitivos
Continuará la traducción…no dejen ver ver el cuadro en el link del autor que aparece arriba…es importante para entender los ratios explicados.
www.apologista.blogdiario.com