Introduction

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.

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 touted 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:

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.

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 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).

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.

Studies involving a rare hormonal imbalance, congenital adrenal hyperplasia (CAH), caused by defective 21-hydroxylase enzyme, suggest that hormonal abnormalities can influence sexual orientation. CAH results in increased production of male hormones during development. In males, increased androgens has little effect. However, female fetuses that develop in this environment develop ambiguous external genitalia, which complicates subsequent development. In utero treatment with dexamethasone reduces the androgen imbalance, resulting in an individual who is genetically and phenotypically female. However, dexamethasone treatment also results in reduced homosexual orientation among treated females,16 suggesting that some homosexuality may result from hormonal influences during development. Homosexual rights groups have suggested that dexamethasone treatment not be given, because it reduces homosexual orientation in females affected by CAH.

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 institutionsnot exactly representative "normal" populations.17 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.18 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.19

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.20 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.21

On a twist on homosexual twin studies, an Australian research group examined the question of whether homophobia was the result of nature or nurture.22 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).23 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.

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 Hamer24 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.25 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.26 Attempts by Rice et al. to repeat the Hamer study resulted in only 46% allele sharing, insignificantly different from chance, contradicting the Hamer results.27 At the same time, an unpublished study by Alan Sanders (University of Chicago) corroborated the Rice results.28 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."29

A study of 13,000 New Zealand adults (age 16+) examined sexual orientation as a function of childhood history.30 The study found a 3-fold higher prevalence of childhood abuse for those who subsequently engaged in same sex sexual activity. However, childhood abuse was not a major factor in homosexuality, since only 15% of homosexuals had experienced abuse as children (compared with 5% among heterosexuals).30 So, it would appear from this population that only a small percentage of homosexuality (~10%) might be explained by early childhood abusive experiences.

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.31 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.32 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).33 Other studies have confirmed that sexual orientation is not fixed in all individuals, but can change over time, especially in women.34 A recent example of an orientation change occurred with The Advocate's "Person of the Year" for 2005. Kerry Pacer was the youngest gay advocate, chosen for her initiation of a "gay-straight alliance" at White County High School in Cleveland, Georgia. However, four years later, she is raising her one year old daughter, along with the baby's father.35 Another former lesbian, British comedienne Jackie Clune, spent 12 years in lesbian relationships before marrying a man and producing 4 children.36 Michael Glatze came out at age 20 and went on to be a leader in the homosexual rights movement. At age 30, he came out in the opposite direction, saying, "In my experience, "coming out" from under the influence of the homosexual mindset was the most liberating, beautiful and astonishing thing I've ever experienced in my entire life."37 A 2011 study of Christian gays who wanted to change their sexual orientation found that 23% of the subjects reported a successful "conversion" to heterosexual orientation and functioning, while an additional 30% reported stable behavioral chastity with substantive dis-identification with homosexual orientation.38 However, 20% of the subjects reported giving up on the process and fully embraced a gay identity, while another 27% fell in between the two extremes.38 Obviously, for at least some individuals, being gay or straight is something they can choose.

The question of nature vs. nurture can also be seen by examining children of homosexual vs. heterosexual parents. If homosexuality were purely biological, one would expect that parenting would not influence it. Paul Cameron published a study in 2006 that claimed that the children of homosexual parents expressed a homosexual orientation much more frequently than the general population.39 Although claims of bias were made against the study, another study by Walter Schuum in 2010 confirmed Cameron's results by statistically examining the results of 10 other studies that addressed the question.40 In total, 262 children raised by homosexual parents were included in the analysis. The results showed that 16-57% of such children adopted a homosexual lifestyle. The results were even more striking in daughters of lesbian mothers, 33% to 57% of whom became lesbians themselves. Since homosexuals makeup only ~5% of the population, it is clear that parenting does influence sexual orientation.

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.

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.41 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 scanGWAS) 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 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. The first attempt to do genome wide scans on homosexual males was done by Mustanski et al. in 2005.42 The results suggested possible linkage near microsatellite D7S798 on chromosome 7q36. However, an attempt to repeat the finding (along with ~6000 well-defined SNPs spread comparatively evenly across the human genome) failed to find any significant SNPs.43 However, a third study using Chinese subjects found a weak association at the SHH rs9333613 polymorphism of 7q36.44 A more general study, examining mate choice among different populations, found no genetic link, prompting the investigators to speculate that such choices were "culturally driven."45 The largest genome wide scan was conducted by 23andMe. 7887 unrelated men and 5570 unrelated women of European ancestry were analyzed by GWAS. Although unpublished, the data was presented at the American Society of Human Genetics annual meeting in San Francisco, showing that there were no loci associated with sexual orientation, including Xq28 on the X chromosome.46 So, the preliminary studies on possible genetic causes of homosexual orientation tends to rule out any dramatic genetic component to sexual orientation.

Why are people gay? 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. Early childhood abuse has been associated with homosexuality, but, at most, only explains about 10% of those who express a homosexual orientation. 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 has been recently examined using DNA microarray technology, although, the results have largely failed to pinpoint any specific genes as a factor in sexual orientation.

La Gentica y la Homosexualidad: Nace la gente, homosexual?

http://www.godandscience.org/evolution/genetics_of_homosexuality.html Last updated November 25, 2013

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Are People Born Gay? Genetics and Homosexuality

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