The Science on Sexual Orientation: A Review of the Recent Literature

Kenneth B Ashley. Journal of Gay & Lesbian Mental Health. Volume 17, Issue 2, 2013.


The “nature versus nurture” debate and discussions on homosexuality have continued for decades. The question as to whether homosexuality is due to some biological or cultural factor remains charged. Some wonder why the focus is predominantly on homosexuality, and not the entire continuum of sexual orientation. As much is understood (or not understood) about the determinants of heterosexuality, yet it has not been an area of significant investigation. There is also the question about who the stake holders are in such research. The idea has taken hold that if homosexuality is biologically mediated, there is greater support for human rights. Another thought is that there might be fewer attempts to try and “cure” same sex attraction. When looking at this area of study, one must decide how to define homosexuality versus heterosexuality; most of the studies use the Kinsey scale which includes both behavior and fantasy or imagery—but it is interesting to see how the scale was typically separated.

The biological research looks at several areas: neurohormonal, behavioral genetics, and immunologic (the most popular current theory). There has been little recent research on the sociocultural/postnatal environment related to the etiology of sexual orientation.


The neurohormonal studies implicate a relationship between the exposure of the fetus to sex-atypical hormones and subsequent development of homosexuality. These differences may manifest: neuropsychological, anthropometric, or neuroanatomic. One of the most studied situations of prenatal exposure to sex atypical hormones is that of congenital adrenal hyperplasia (CAH). CAH is an autosomal recessive disorder that causes a deficiency in the enzyme 21-hydroxylase. This results in cortisol precursors being shifted to the androgen pathway leading to the female fetus being exposed to significantly increased levels of androgens. These girls are born with some degree of genital virilization. A review by Hines (2011) of numerous studies looking at the sexual orientation of women with CAH found elevated rates of non-heterosexuality. A similar review of studies by Hines of the exposure of males in utero to ovarian hormones found largely negative results.

Otoacoustic emissions (OAEs) are sounds produced in the cochlea which spread out into the outer ear canal where they can be measured. There are several different types of OAEs, including spontaneous OAEs (SOAEs) and click-evoked OAEs (CEOAEs). Both SOAEs and CEOAEs exhibit moderately large sex differences, being typically stronger and more frequent in women. It is proposed that levels of prenatal androgens play a role in these sex differences. A review of studies by McFadden (2011) found nonheterosexual women exhibiting “masculinized” OAEs, that is, weaker and less frequent. No such differences were noted between heterosexual and nonheterosexual men.

Several anthropometric characteristics, often sexually dimorphic, have been hypothesized to be the result of nonspecific effects of prenatal sex atypical hormones. It is thought that if prenatal hormone exposure is involved in the development of sexual orientation, this alternation in hormone levels would be manifested in sexually dimorphic traits. In essence, the thought is that findings would show that the homosexuals would exhibit traits more similar to those found in the opposite sex: homosexual/nonheterosexual men more like heterosexual women, homosexual/nonheterosexual women more like heterosexual men. Some of the traits investigated are finger length ratio—second to fourth fingers, body motion and morphology, dermatoglyphics, and handedness. Hair whorl direction, a physical feature that is organized early in neurodevelopment which does not change with age, has also been studied as a potential biological marker for homosexuality in men.

The ratio of the lengths of the human second/index and fourth/ring fingers (2D:4D ratio) is a sexually dimorphic characteristic (Manning, 2002), with the ratio typically being smaller in men. Several studies investigated the ratio, some found that the ratio was smaller in lesbians when compared with heterosexual women; the differences in the ratio between homosexual and heterosexual men were even more inconsistent than with the women (McFadden et al., 2005). The ratios in the studies were found to vary by ethnicity, handedness, and nationality.

Several studies were performed to assess how body shape and motion affect perceived sexual orientation (Johnson et al., 2007). The studies used either computer generated animations or dynamic outlines of real people. Gender atypical combinations of body shape and motion were thought to generally be homosexual, especially among men.

Dermatoglyphics or skin ridges are determined by the 16th week of fetal life and the number of fingerprint ridges on the hand are thought to differ based on sex. A recent study (Mustanski, Bailey, & Kaspar, 2002) noted the inconsistency in the literature regarding dermatoglyphic directional asymmetry and sexual orientation. In their study, it was found that total ridge count varied significantly based on sex, no such difference was found based on sexual orientation. They also found no difference in directional asymmetry relative to sexual orientation.

Handedness is thought to be determined by some combination of prenatal factors, including genetics and androgens, as well as perinatal factors (Mustanki, Bailey, & Kaspar, 2002).

Handedness has not been found to be consistently associated with sexual orientation. A meta-analysis by Lalumiere, Blanchard, and Zucker (2000) found that nonright handedness was related to homosexuality in both men and women (greater association in women), although the factors for this have not been determined. In the Mustanski, Bailey, and Kaspar (2002) study the association of homosexuality and nonright handedness was only found in female homosexuality, but not male homosexuality.

Direction of hair whorls and sexual orientation in men received significant attention in the media when differences were found by Klar (2004). Subsequently, more rigorous studies (Rahman, Clarke, & Morera, 2009; Schwartz et al., 2010) found no significant differences in the direction of hair whorls and sexual orientation.

The studies on neuroanatomic differences were an important aspect of the search for biologic differences associated with sexual orientation in the 1980s and 1990s and will be included for their historic significance. Mustanski, Chivers, and Bailey (2002), Jannini, Blanchard, Camperio-Ciani, and Bancroft (2010) review these studies and note the findings and issues with methodologies. The size of anterior commissure of the hypothalamus was found to vary with sex as well as sexual orientation in men, although the findings were not consistent. The suprachiasmatic nucleus of the hypothalamus in homosexual men was found to be almost twice the size of that in heterosexual men, although these findings have not been replicated (Levay, 1991). The most significant findings involve the third interstitial notch of the anterior hypothalamus (INAH3) which has been found to be two to three times smaller in homosexual men compared with the size in heterosexual women. These findings have been replicated in several studies.

These studies are detailed further in Mustanski, Chivers, and Bailey (2002). Some concerns about these studies were that: in many cases the brains were from individuals with AIDS, and it is unclear how this might have influenced the findings; in some circumstances sexual orientation was presumed; and no homosexual/nonheterosexual women were included.

Behavioral Genetics

There are two types of genetic studies relative to sexual orientation: twin studies and genes/molecular research. These studies have shown that familial factors, including a genetic component, influence sexual orientation.

Earlier twin studies (Bailey & Pillard, 1991) indicated a high concordance on nonheterosexual orientation in male monozygotic twins (52%), lower rates in dizygotic twins (22%), and even lower in adoptive brothers (11%). More recent studies with more sophisticated design found higher rates of concordance of homosexual/nonheterosexual orientation, averaging 65% in monozygotic twins, 15% in dizygotic twins. (Bailey, Dunne, & Martin, 2000; Kendler, Thornton, Gilman, & Kessler, 2000; Langstrom, Rahman, Carlstrom, & Lichtenstein, 2010). Women were included in these studies and those rates of concordance for homosexual/nonheterosexual orientation were less than for the men.

With the advances in gene mapping there has been the search for the “gay gene.” Hamer and his colleagues (1993) reported the first linkage studies, discovering that pairs of gay brothers shared chromosomal region Xq28 at a higher than expected rate. These findings have been replicated and expanded upon, as noted in reviews by Mustanski, Chivers, and Bailey (2002) and Jannini et al. (2010). This should more appropriately be called the “gay male gene” since these results have been limited to men.

Fraternal Birth Order Effect

The fraternal birth order (FBO) effect is the most replicated and robust finding that is reliably associated with homosexuality in males. In 1996, Blanchard and Bogaert found that homosexual men have more older brothers than heterosexual men. Subsequently, numerous studies have replicated the findings as noted in reviews by Mustanski, Chivers, and Bailey (2002), Blanchard (2008), and Bogaert and Skorska (2011). Among these studies it is noted that the FBO effect may account for a homosexual orientation in up to 15% of men and that the estimated odds of being homosexual increased 20–33% with each additional older brother. The number of older sisters, younger brothers/sisters had no effect. The finding was also observed in brothers raised in different homes. The FBO effect included all genetic men; two studies found that the probability of male-to-female transsexuals reporting sexual attraction to men increased with the number of older brothers (Blanchard et al., 1996; Green, 2000). The effect was not seen in lesbians. While this effect has been recognized, the mechanism remains unknown; one that is in favor is the “maternal immune hypothesis,” which is discussed in several of the aforementioned articles (Mustanski, Chivers, & Bailey, 2002; Blanchard, 2008; Bogaert & Skorska, 2011). This hypothesis postulates that there is a maternal immune response which increases with each pregnancy with a male fetus which somehow changes the prenatal hormonal environment leading to a change in brain development.

Nurture-Cultural/Postnatal Environment

While there have been numerous articles on the neurobiological determinants of same sex/gender attraction, there are not a significant number of rigorous articles delineating cultural/postnatal environmental origins of sexual orientation.


Despite years of investigation and a multitude of studies the determinants of sexual orientation remain elusive. One might question the reasons behind such studies, especially given that the studies are almost exclusively focused on homosexuality—most often male homosexuality. Some might argue that discovering the factors resulting in a homosexual orientation will also provide information on heterosexual orientation. Others might consider heterosexuality “normal” and in no need of study; that homosexuality is the result of some dysfunction in the developmental process; something has gone wrong. It does not, however, have to be an either/or dichotomy; homosexuality could be viewed as a normal variant.

What these studies report are associations/correlations, but none indicates causation. There is the concern that if the “cause” of homosexuality is discovered, there will then be an attempt to identify a “cure,” even though all major professional mental health organizations have stated that homosexuality is not a disorder.

There is also the use of the data on the cause of sexual orientation in the struggle for, and against, human rights. Some say that if a homosexual orientation is biological or “natural,” then various human rights should be granted. The other side of the argument is that if homosexuality is a “choice,” then various human rights can, and should, be denied. This is in spite of the fact that there are a host of “choices” (e.g., religious affiliation) that are afforded nondiscrimination protections under the law. Also, when people argue that homosexuality is not a choice, it seems to imply that if they could they would have chosen otherwise.

There is a question of who and what is being measured. It is important to note that while many of the studies use the Kinsey scale to assess sexual orientation, which includes both behavior and fantasy or imagery, not all of the studies do. Many of the studies also differentiate the groups as heterosexual (either exclusively heterosexual or only incidentally homosexual) versus nonheterosexual (ranging from more predominantly heterosexual but more than incidentally homosexual to exclusively homosexual). This dichotomization of the groups may have been necessary to provide sufficient numbers for statistical analysis and positive findings, but it also increases significant diversity, especially in the nonheterosexual group.

There is an issue with the study participants, since homosexuality is still stigmatized. How does one get a random sample? Individuals who are willing to enroll in such studies can be expected to be “out,” more comfortable with their homosexuality/nonheterosexuality which is not representative of the entire population of homosexuals/nonheterosexuals.

The studies typically assume that sexual orientation is static, and may not account for fluidity of sexual orientation. These studies also presume that homosexuals are a homogeneous group, yet the findings indicate that there are significant differences, particularly related to gender. The data indicate the likelihood that the origins of sexual orientation in men and women are different, as the study results are generally gender specific. It also may be that different models of sexuality and sexual orientation are needed for different genders.

It also may be that there are different types of homosexuality, as some of the studies attempted to include gender atypicality as another variable (i.e., effeminacy in men and masculinity in women). Perhaps there are different types of homosexuality/nonheterosexuality related to the determinants of sexual orientation, that is, genetic versus fraternal birth order versus underlying cause of the variation in fetal hormone exposure.

Even though many of the studies on the “causes” of homosexuality are inconclusive or contradictory, there is evidence that biological factors do play some role in the development of sexual orientation, but it is not clear to what extent. Since no single proposed biological or genetic model accounts for 100% of homosexuality/nonheterosexuality there must be some combination of factors. Perhaps the effects of different biological factors interact to varying degrees in each person, and sexual orientation is determined when several predisposing factors (including psychosocial/postnatal environmental factors) are combined.

There is much more work which needs to be done to understand the origins of sexual orientation. What is known at this point is that sexual orientation represents a highly complex behavioral trait with multiple determinants involved, including genetic, hormonal, possible immunological factors interacting with one another and the sociocultural postnatal environment.