Biological Determinants of Sexual Orientation

¶ … resolve conflicting evidence that male homosexuality is correlated with similarities in the genetic markers from region Xq28 of the X chromosome. While many studies suggest a strong genetic and biological basis to sexual orientation, there is little persuasive, compelling, causal evidence to support this claim. Specifically, attempts to find a genetic correlate to homosexuality have proven elusive, and several attempts to replicate the finding of similarities of region Xq28 of the X chromosome in male homosexuals have proven problematic. This study will attempt to correct the methodological problems that may have been problematic in earlier studies, including problems with the use of self-selecting samples, small sample sizes, and problems with the definition of homosexuality.

The ideas and definitions surrounding human sexuality and sexual orientation can be more fluid and confusing than appear on the surface. To the outside world, primary sexual characteristics (such as a penis or vagina) determine whether an individual is male or female. Secondary sexual characteristics like breasts and body hair further confirm sexual identity. In addition, we speak about someone with XY sex chromosomes as being chromosomally male, while "males" may actually take other chromosomal forms. In addition, gender identity is an important component of sexual orientation. LeVay writes, "Most men have a deep inner conviction that they are male, and most women that they are female," thus forming their gender identity (3).

However, many factors show that sexual orientation can be fluid. Transsexuals show a confused sexual identity, suggesting that gender identity can exists in opposition to biological sexual characteristics. Sometimes, chromosomal sexual identity can be confused with external sexual characteristics, such as in an individual who appears to be male externally, but does not have the usual XY chromosome pattern of a chromosomal male. In addition, many cognitive characteristics and behaviors that are commonly different between the sexes (or sexually differentiated), such as aggressiveness and parental behavior, can be seen in either males or females (LeVay).

In the past century, there has been a great deal of debate whether sexual orientation is a biological trait that is genetically determined, or whether sexual orientation results from factors such as upbringing or environment. Religious groups tend to favor the camp that says sexual orientation is caused by environmental factors, but a plethora or recent studies is in favor of the idea that sexual orientation is genetically determined. While the majority of research on sexual orientation has focused on nature vs. nature controversy, some other theories have been proposed. These include hormonal influences, demonic possession, parental influences, viral or bacterial factors.

Literature Review

In the general population, the rate of homosexual orientation is estimated at between 2 and 10%. This variance is due to the specific criteria used to define homosexuality. Commonly, 4-5% of males are thought to be homosexual, while 2-4% of females are homosexuals (LeVay, 1994).

While male homosexuality is often the focus of familial studies, both male and female homosexuality has been shown to be familial in nature. Specifically, homosexual females have more homosexual brothers than heterosexual females. Similarly, homosexual males have more homosexual siblings than heterosexual males (Bailey and Bell, 1993; Bailey and Benishay, 1993).

Studies of monozygotic (identical) twins reveal that there is a strong genetic component to human sexual orientation. Specifically, a study of thirty-eight pairs of monozygotic twins (34 male and 4 female pairs) found that the twins had a concordance rate of 65.8% for homosexual orientation, while twenty-three pairs of dizygotic twins had a concordance rate of 30.4% for homosexual orientation (Whitam, Diamond and Martin, 1993). Bailey and Pillard (1991) reported similar rates of concordance for homosexual orientation. In Bailey and Pillard's study, 52% of monozygotic cotwins, 22% of dizygotic cotwins, 11% of adopted brothers, and 9.2% of nontwin biological siblings were had a homosexual orientation. Interestingly, childhood gender nonconformity did not seem to be linked to homosexuality in adulthood (Bailey and Pillard, 1991).

Similarly, a 2000 study of monozygotic twins found a 32% concordance rate for non-heterosexual orientation in monozygotic twins, as opposed to 13% for dizygotic twins of the same sex. The study measured 3,000 people in a random national sample in the United States (Kendler et al., 2000).

Further, studies of the families of male homosexuals suggest that sexual orientation has a genetic basis. In Hamer et al.'s 1993 study of 114 families of homosexual men, increased rates of same-sex orientation were linked to the both the male cousins and maternal uncles of homosexual men. Sexual orientation was not linked to fathers or maternal relatives. In a study of 40 families with two gay brothers (and no indication of non-maternal transmission) a correlation between the inheritance of polymorphic markers on the X chromosome and homosexual orientation. This correlation occurred in close to 64% of sibling pairs tested (Hamer et al., 1993).

Hamer hypothesized that the genetic locus of sexual orientation may be found on the X chromosome within these families. In fact, he discovered significant similarities in the genetic markers from region Xq28 of the X chromosome in 33 of the 40 families studied. Specifically, 82% of the sibling pairs shared DNA in this region. Given that males have two X chromosomes, the probability that both brothers would inherit the same part of the X chromosome is 50%, making the 82% similarity found by Hamer significant (Hamer et al., 1993). Further research using a smaller sample showed that 67% of sibling pairs shared DNA from the Xq28 region among homosexual brothers (Hu et al., 1995).

Interestingly, female homosexuals have not been shown to exhibit a similarity in a specific genetic locus. Later studies on the Xq28 region showed that the Xq28 region of the X chromosome was similar in only male homosexuals and not female homosexuals (Hu et al., 1995).

Other lines of evidence fail to support a genetic link in homosexuality. Biological and adopted siblings tend to have similar rates of male homosexuality. Further, many twin and family studies have suffered from methodological uncertainties tied to small sample size or potential errors in reporting homosexual behavior. Specifically, Hamer's identification of the Xq28 gene may have resulted from the introduction of a Type 1 (false positive) error into his study. As such, while the genetic basis for homosexuality has a significant amount of evidence in its favor, a genetic basis for homosexuality has not been conclusively proven (Rice et al., 1999).

Importantly, a 1999 study of 52 gay male sibling pairs failed to link male homosexuality to the Xq28 position on the X chromosome. Specifically, four markers at Xq28 (DXS1113, BGN, Factor 8, and DXS1108) did not show an increased similarity between homosexual brothers (Rice et al., 1999).

A number of studies have linked physical characteristics like the number of fingertip ridges, and finger lengths to homosexuality. As such, these studies indicate that the occurrence of homosexuality may be linked to the development of characteristics that develop before or during pregnancy, or even at conception. Specifically, androgen levels influence finger length in the womb, making finger length an indirect measure of fetal androgen levels (Williams et al., 2000).

Sexual orientation has also been linked to hormonal changes during pregnancy. The probability that a male child will have a homosexual sexual orientation as an adult increases nearly 33% for each older brother that the child has, while older sisters have no effect on the sexual orientation of a younger, male sibling.

An immune response within the mother during pregnancy may account for this effect. Specifically, the mother may produce anti-H-Y antigens during pregnancy that affect aspects of sexual differentiation that occur during development. In addition, homosexual males who have older brothers weigh less at birth than heterosexual males with older brothers who in turn weigh less than heterosexual males with older sisters. Here, when the maternal production of anti-H-Y antigens is small, birth weights are only slightly reduced (as in heterosexual men with older brothers), while when the maternal production of anti-H-Y antigens is large, birth weights are markedly reduced (as in homosexual men with older brothers) (Blanchard, 2001).

Reports of anatomical differences in the brains of heterosexuals and homosexuals have also been reported. The third interstitial nucleus of the anterior hypothalamus (INAH1) has been reported as smaller in gay males than heterosexual males. Further, the cell density in the area was higher in gay males (Byne et al., 2001). In addition, Swaab and Hoffman (1990) reported that the suprachiasmatic nucleus (SCN) is larger in gay males than in heterosexual males. The SCN is not known to play a role in sexual behavior.

Environmental factors are likely play an important role in the development of sexual orientation. In studies of monozygotic twins, the highest rates of concordance for homosexuality approach 50%, indicating an important environmental component in the development of sexual orientation. Further, the rate of homosexuality in adopted brothers of homosexual or bisexual twins has been estimated at 11%, a significant amount over the rate of homosexuality in the general population, suggesting a role for environment in the development of homosexual sexual orientation (Bailey and Pillard, 1991).

In addition, a simple genetic explanation of homosexuality is difficult to envision, considering the argument that the selective pressure for homosexuality would not be strong. Homosexuals are likely much less likely to reproduce than are heterosexuals, thus leading to strong selective pressures against a simple Mendelian gene that controls homosexuality (Rice et al., 1999). Despite this, several theories have been proposed to account for the presence of gay genes, including the postulation that gay genes may help promote the reproductive success of the siblings of gay people (Bailey, 2003).

As noted earlier, there are often a number of methodological problems that undermine the existing evidence that sexual orientation has a genetic basis. In samples where participants are recruited using homophile publications, the concordance rates for sexual orientation among monozygotic and dizygotic twins are higher than when samples are taken from the general population. This can be explained easily by the fact that individuals who read homophile magazines and who respond to advertisements asking for volunteers on homosexual research may be biased.

However, it is important to note that studies based on random samples of the population (not selected using homophile publications) also show a significant familial link for sexual orientation. However, even in these studies, Kendler et al. (2000) note "sexual orientation was substantially influenced by genetic factors, but family environment may also play a role" (p. 1846).

The measurement of sexual orientation has also been problematic within the study of sexual orientation. Sexual orientation can be measured during a number of different scales. In some cases, homosexuality can be defined simply through subjects answering yes to questions that indicate sexual thoughts about homosexuality. In other cases, homosexuality is defined through self-identification or life experiences. Importantly, rates of familial homosexuality can differ depending on the type of criteria used to define homosexuality (Bailey and Benishay, 1993).

Part of the difficulty in defining sexual orientation comes from developing an understanding of the distribution of sexuality. The use of homosexual and bisexual people be used as one group to be contrasted with heterosexuals indicates that researchers think of sexuality can be an easily defined. In contrast, many anecdotal reports suggest that sexual orientation occurs along a wide spectrum, and thus it is often difficult to define homosexuality or heterosexuality at a specific point on the continuum. Further, homosexuality in men and women may be a different trait, and that the genetic influences on homosexuality in men will be different from that in women (Bailey et al., 1993).

Future lines of research point to attempting to determine different regions of DNA that may be linked to sexual orientation. In addition, the location of specific genes within the Xq28 region may be useful. Further, given the recent failure of Rice et al. (1999) to replicate Hamer's identification of the Xq28 region as similar within homosexuals, research that attempts to reasons for the inconsistent findings is crucial. It is interesting to note that both Rice et al. (1999) and (Hamer et al., 1993) used homophile publications to attract subjects, and yet had different results. As such, the problem with replicating the Xq28 findings may lie in the small sample sizes used.

In summary, current research seems to indicate that sexual orientation is shaped early in life, and that genetic factors play an important role. At an early age, biological, psychological and social factors interact to shape an individual's sexual orientation. However, a great deal of work remains to determine the exact nature of both genetic and environmental factors in the determination of sexual orientation.

The study proposed here will attempt to address many of the common methodological problems that have been associated with research into the genetic basis of homosexuality in the past. These problematic methodological issues include the common use of self-selecting samples, small sample sizes, and problems with the definition of homosexuality in studies that attempt to determine the genetic basis of human sexual orientation.

Proposed Research

Hypothesis: Male homosexuality is correlated with similarities in the genetic markers from region Xq28 of the X chromosome.

The proposed study will attempt to determine the relationship between male homosexuality and the Xq28 position on the X chromosome. Specifically, this study will attempt to determine if male homosexuality is correlated with similarities in the genetic markers from region Xq28 of the X chromosome. This study will attempt to correct methodological problems that may have been problematic in earlier studies. Specifically, problems with the use of self-selecting samples, small sample sizes, and problems with the definition of homosexuality will be addressed in this study.