Simply put, the average gay person has been shown to have many subtle physical and mental differences when compared to heterosexual counterparts. Such differences are often similarities with the homosexual's so-called opposite sex, with lesbians having more masculine characteristics and gay men having more feminine characteristics (with some interesting exceptions). These differences are also very useful in illuminating the reasons behind human sexual orientation, as they are likely to be the result of the same causes and most are present at birth, or long before sexual orientation is realized.
Here we will describe what the research has found regarding both physical and mental differences between homosexuals and heterosexuals, though there will be necessary overlap. For the possible reasons behind these differences see
this companion isocrat.org article on the causes of homosexuality.
Contents:
http://1. Physical Characteristics
http://I - Hair Sworl - The hair on a person's crown almost always lays naturally to form a clockwise or counterclockwise pattern. Klar, a researcher who had an interest in finding the cause behind human handedness, has conducted several studies on hair sworls. Because handedness is not shared 100% between identical twins, there exists a strong nurture vs nature debate in that field of study as well (see the
related isocrat.org article). One theory behind handedness is that certain genes make a person right handed but a change in those genes make a human randomly right or left handed, with about a 50% chance (explaining why identical twins do not always have the same handedness). Klar found backing evidence for this theory is his data on hair sworls. It was found that, in the general public, a counterclockwise hair sworl occurred about 8% of the time, whereas for non-right handed people it occurred 45% of the time (
1).
With this understanding and the findings that non-right handedness is also associated with homosexual orientation in humans (
2,
3,
4,
5,
6), Klar looked at the hair sworls of gay men. He found gay men were about 4 times more likely to have a counterclockwise hair sworl (30% for gay men vs. 8% for straight men) (
7). As a person's hair sworl is present at birth, this is significant evidence linking a congenital, nature cause of homosexuality to both scalp patterns and handedness.
http://II - Finger Length - One significant difference between men and women, though slight, is the ratio of the lengths of our index to our ring finger. Women have an average 2D:4D ratio (length of the index finger divided by the length of the ring finger) of about 1, and men average at about 0.98. Though a small difference, it is outside the error bars with significantly large samples. A study by Manning et al. found this ratio is established at least as early as the age of two and "is probably established in utero" (
8). It has been determined that this ratio runs in families, and is different for various races (
9). A recent twin study found that the cause behind this ratio was approximately 60% in the genes, with about a 70% concordance with identical twins and 35% concordance with fraternal twins (
10). In short, it's thought that low 2D:4D ratios indicate high prenatal testosterone and low estrogen, and high 2D:4D ratios indicate low prenatal testosterone and high estrogen.
In 2000, Robinson and Manning, who were initially studying the correlation between 2D:4D and sperm production, published a study looking at 2D:4D relevance to sexual orientation (
11). This research suffered with relatively small samples, and the control group of "heterosexuals" was chosen "without regard to sexual orientation." They found no reliance of the ratio on birth order and found that gay men were masculinized in finger length, though they did confirm the
Fraternal Birth Order Effect for gay men. A larger study by Williams in the same year found gay men had a more feminine ratio and lesbians had a more masculine ratio (
12). The results of that study were repeated by McFadden and Schubel, in 2002 (
13). This study, though, was quite small too (35 gays, 29 lesbians and about twice the number of heterosexuals) and is not conclusive.
Brown et al. found this difference may be a function of masculinity, as well as orientation. In their research, self-identified "femme" lesbians did not show a diminished 2D:4D ratio from heterosexuals, but "butch" lesbians did (
14). Furthermore, women with gender identity disorder, lesbian or heterosexual, seem to show a masculinized finger ratio as well (
15).
Lippa, in 2003, used a much larger sample than previous work: 351 heterosexual men, 461 gay men, 707 heterosexual women, and 472 lesbian women (
16). Many races were used and corrected for in his calculations. His findings confirmed McFadden's results and bolstered, in part, Williams, though contradicted Robinson's again. He found gay men were more feminized in this ratio, and in the length of the ring finger alone. These results were found for all races, save for Asian (for which he had only a small number of participants, 19). But, he found no significant effect with lesbians, compared to straight women; they were still within the anticipated error, possibly because he did not correct for the Butch-Femme effect. Nevertheless a smaller 2005 study by Rahman found this effect for lesbians again, but was unable to find it for gay men (
17).
In summary, much more research and repetition is needed to get past the small differences in finger length between men and women. Nevertheless, such data does suggest a possible hormonal cause to both orientation and, of course, human anatomy.
http://III - Penis Size - In a couple studies, both the penis length and circumference has been found to be greater for gay men than heterosexual men. A study by Bogaert and Hershberger used a very large sample size (935 gay men and 4,187 heterosexuals) (
18) and confirmed the earlier work of Nedoma and Freund (
19).
http://IV - Brain Differences - As is discussed in
the isocrat.org article on homosexuality in the animal kingdom, differences in hypothalamus structures between mammals that behave heterosexually and homosexually have been found. The hypothalamus is an area of the brain known for controlling our behavior and emotional life from hunger to anger to sexual activity. Differences between homosexually behaving animals and others have been found in sheep (
20), ferrets (
21,
22), rats (
23), and macaques (
24). Furthermore, surgical alteration of such structures has been shown to alter an animal's sexual orientation (
21,
22), making the theory that homosexual behavior causes these brain differences, and not the other way around, quite unlikely. Such findings in the animal kingdom, have led researchers to look for similar structures in humans. A recent review may be found here (
25).
Brain anatomical difference between that average man and woman are well known and are present from birth (
26). LeVay, in a blind study, looked at numerous brain areas of 19 gay men, 16 heterosexual men, and 6 heterosexual women (
27). One region of the hypothalamus (the drably labeled INAH-3) was significantly smaller in gay men than in heterosexual men; it was in fact about the size of the same structure in women. This is a region of the human mind that has a well documented difference between sexes (
28,
29) , which were confirmed in this research as well. One common criticism of LeVay's work is the fact that all the gay men had died of AIDS. Nevertheless, some of the heterosexual men in the study had AIDS as well and the difference remained if only they were used in comparison. Furthermore this research was repeated by Byne et al. (
30). This research group also found that INAH-3 in heterosexual men contained more neurons than it did in homosexual men or heterosexual women, and found that HIV infection did not seem to affect INAH-3's size.
Building on this work, Kinnunen et al. conducted a double-blind study using PET scans of gay and heterosexual men (all physically and mentally healthy) while under the influence of a serotonin reuptake inhibitor (
31). They found gay men exhibited a "significantly (p<0.01) smaller reduction in hypothalamic glucose metabolism in response to fluoxetine than the heterosexual group", indicating that the differences between the homosexual and heterosexual hypothalamus are not merely structural, as one should expect. They also found differences in brain activity in areas outside the hypothalamus which may warrant further study.
Most recently the hemispheric asymmetry and the functional connectivity of the
amygdala (another brain structure which is known to influence the hypothalamus) of the brains of heterosexual and homosexual men and women were measured with PET and MRI scans (50 heterosexuals and 40 homosexuals) (
32). Again it was found homosexual men had the same brain structure as heterosexual women, and homosexual woman had the brain structure of heterosexual men. As the authors conclude, these anatomical differences "cannot be primarily ascribed to learned effects."
In summary, much more research and replication is needed, particularly with regards to homosexual women. Nevertheless, as in the animal kingdom, there seems to be significant brain differences between human homosexuals and heterosexuals (see also the studies on pheromone response,
below).
http://V - Puberty Onset - The average male reaches puberty later in life than the average human female (
33) , making the onset of the physical changes of puberty a promising correlate of orientation. Several separate studies have shown homosexual males enter puberty significantly earlier than heterosexual males (
34,
35,
36,
37), more like the average female. However, even when studied by the same researcher groups, a similar effect has not been found in women (
38,
39,
40,
37), highlighting the fact that some of the causes of homosexuality for men and women are likely quite different.
http://VI - The Ear - Several lines of research indicate a correlation between inner ear structure and orientation. Click-evoked otoacoustic emissions (CEOAE) are echoes produced by the inner ear in response to brief sounds. Men generally have lower magnitude CEOAEs than women, and magnitudes are greatly stable throughout life, and are highly hereditable (
41). When homosexuals are studied it has been repeatedly found that lesbians have significantly more masculinized CEOAEs than heterosexual women, while gay men show little to no difference with heterosexual men (
42). Interestingly, in more recent work (200
, this is exactly what is seen when sheep are given excess testosterone prenatally: female sheep show masculinized CEOAES and male sheep show no difference (
43). Such evidence suggests prenatal exposure to testosterone may contribute to the eventual homosexual orientation of females.
http://VI - The Face - Research has shown that, by viewing short (1 second) movies of homosexuals and heterosexuals simply speaking, most people could accurately determine another person's sexual orientation (
44). Such judgments, though, could likely be the cause of slight and learned mannerisms. However, research conducted in 2008 by Rule and Ambady has shown that the average person is able to correctly pick out, better than chance, who is gay and who is straight simply by viewing still photographs of their faces (a head on, unaltered view, without makeup, or jewelry, etc.) (
45). When chance would predict a 50% success rate, people can find the gay individual between 60% and 70% of the time. Such a finding suggests there may be other, yet to be determined, physical structures that people can pick up on that go hand in hand with sexual orientation. Interestingly, people can select the gay men 60% of the time when they see the face for only 50 milliseconds, faster that it takes to consciously process an image, half the time it takes to judge weather a face is attractive or not (
46).
http://2. Mental Differences
http://I - Orientation - This may seem trivial but it deserves emphasis due to our propensity to distrust our differences and ignore our commonalities. On this page there is much evidence showing how sex differentiation is visited differently upon homosexual humans, both physically and mentally. The defining difference, however, between heterosexuals and homosexuals is also their most obvious similarity: sexual orientation. Gay men have the sexual orientation that shows up in heterosexual females, and lesbians, by definition, have the orientation of a typical heterosexual man. If we were blind to anatomy, we really wouldn't be all that different in our orientation.
http://II - Pheromone Response - Much research has focused on the human response to various pheromone triggers. It is well established that mate selection in mammals is partially guided by such chemical (
47), as well as visual signals (see
related isocrat.org article), and the same holds true for humans (
48,
49,
50). Such chemicals have been shown to affect a human mind even when they are at concentrations below conscious detection (
51).
Savic et al. have looked at the effect of an androgen-like pheromone (a chemical that a male would emit) and an estrogen-like pheromone (a chemical that a woman would emit) on the human mind in MRI studies. Both pheromones have tediously long names and so we will simply refer to them as AND for the male pheromone and EST for the female pheromone. In 2001 this research group found distinct differences between men and women in their responses to these orders (
52). They found women smelling AND had increased activation in their hypothalamus, relative to their control orders, but showed no such response when smelling EST. Men exhibited the opposite response.
After finding this sex difference, this same group sought to determine if such a difference could also be a function of sexual orientation. In 2005 they published findings that repeated their 2001 results, but also found gay men and heterosexual women had a response to AND in the preoptic and anterior hypothalamus, whereas heterosexual men did not (
53). These finding again, are supported by the anatomical discoveries regarding the differences between a homosexual and heterosexual male brain (
see above). Most recently this group has shown a similar effect in the brains of lesbians: their brain activity while responding to EST is typical of a heterosexual male (
22).
In all, perhaps none of this research should be surprising, as sexual orientation undoubtedly exists and it must have some causation. A finding in brain activity that gay men and heterosexual women react uniquely to the sensory cues of men is hardly a surprise.
http://III - Cognitive Abilities - On the whole it should first be noted that no significant difference between homosexuals and heterosexuals has been found in such measures as IQ (
54,
55). The differences found are mainly regarding the mental differences typically seen between men and women, in verbal ability and spatial perception. It has long been know that, on average, men have greater spatial abilities and women have greater verbal fluidity, though there is significant overlap in the distributions. These differences are thought to be the effect of hormone exposure (
56,
57), and they have been observed to fluctuate with the hormonal fluctuations of menstruation in women (
58,
59). They are also the function of prenatal hormonal exposure, as demonstrated by animal studies (
60).
Gay men have been shown to perform significantly better than straight men (and identically to females) on their ability to remember individual objects in a group of random objects (
61). In tests of verbal fluency, gay men and heterosexual women have been shown to outperform heterosexual men and lesbians (
62,
63,
55,
64,
57). Such findings indicate that gay men are feminized in their prefrontal and temporal cortices of their brains, while lesbians are masculinized. In many tests of visio-spatial ability (the ability to correctly judge and predict spatial relationships) heterosexual men have repeatedly been shown to out perform heterosexual women and gay men, who have identical abilities on average in this area (
65,
63,
66,
67,
55,
61,
68,
57). It should be mentioned that one study failed to find such a difference for gay men, but, statistically, with so many studies that is to be expected (
69). Such results seem to also hold for males who are diagnosed with gender identity disorder; their mental abilities are significantly more feminine (
54).
http://IV - Startle Response - Most all mammals show a reflexive startle response to strong stimuli, such as a loud noise. Most mammals also show less of a startle response if the strong stimuli is preceded by a moderate stimuli. This is called a prepulse inhibition (PPI) and PPI has been shown to be stronger for the average man than the average women (
70). Rahman et al. found this same relationship for heterosexual men and women but they also found that lesbians had a significantly masculinized PPI, and gay men showed no difference with heterosexual men (
71). Such a finding is in need of repetition but it does indicate one more difference in the cause of lesbianism verses male homosexuality, and supports the findings on the relationship between CEOAEs and sexual orientation (
see above).
http://V - Gambling - Risk taking behavior is tied to several biological factors in humans, and is dependant upon sex (
72). In a large study, Hershberger and Bogaert found that gay men do indeed have gambling habits akin to heterosexual women, and lesbians tend to gamble more, like heterosexual men (
73).
http://VI - The Effects of Social Stigma - It should be clear that being gay or lesbian can be traumatic. Gay teens are more likely to be abused, physically mentally and sexually (
74,
75). GLBT youths typically discover their orientation at a very vulnerable time, in adolescence, and experience higher rates of violence for anti-gay sentiment in school and community settings (
76,
77,
78). They are placed in a minority group, one which they may even have biases towards and one they often do not share with their family. They are often pitted against family and one-time friends, leading to being cut off from normal support mechanisms and depression (
79). Also, their orientation is often taught to be an impulse to sin against their faith, causing a potent source of conflict. In some countries they could face criminal punishment for building a relationship; in others they may have their attempts to build and protect their family legally thwarted. Even in friendly jurisdictions, realizing you are different from most people, and knowing that difference may limit your plans for future family can be traumatic. In short, the additional stress has some measurable consequences.
Between 7% and 13% of US teens report making a suicide attempt (
80,
76). Several studies have shown gay and lesbian teens are significantly more likely, though, to consider and carry out suicide. Russell and Joyner found, using a representative random sample of US teens, gay and lesbian teens were about 2.5 times more likely to attempt suicide (
81). The US secretary of health and human services found similar results (
82), while a study by Garofalo et al. found the rate to be about 3 times more likely (
77). Parental rejection has a strong effect here, with suicide attempts increasing 3 fold when parents show a high degree of rejection of their child's orientation (
83) .
On whole there also exists several other average differences in the GLBT community regarding a couple negative mental conditions associated with stress. We will not go through all such research here as such findings were recently reviewed as part of a meta-analysis by King et al. (
84). In addition to the doubled chance of attempted suicide, gays and lesbians are 1.5 times more likely to experience anxiety and depression, and lesbians are notably at higher risk for alcohol abuse. These effects are significantly more striking for children of parents who strongly reject homosexuality (
83) . In 2009, Friedman et al. also found, that it was the harassment and homophobia experienced by gay children that caused them to be more likely to contract HIV and experience depression as adults (
85). With increasing acceptance and integration into families and societies such conditions are less and less a part of what it means to be gay, but they are, nevertheless, a real part of being gay in need of address for many individuals.
http://VII - Handedness - Multiple studies have been conducted on the handedness of individuals in regards to their orientation. We will not go through them all here (much more information on the causes of handedness and their strange similarity to the causes of homosexuality may be found in
this related isocrat.org article). Fortunately and in summary, Lalumiere et al. conducted a large metal analysis, combining such studies. In all, their analysis involved 6,987 homosexuals and 16,423 heterosexuals and found that homosexuals were 39% more likely to be non-right-handed (
86). Such evidence, along with the data on hair sworls (
presented above), points to
developmental instability--a cause of handedness and counterclockwise hair sworls--as being a cause of homosexuality.
http://VIII - Gender Behaviors - The stereotypes are clear. Gay men are said, for example, to have an effeminate walk and manner of speaking, and lesbians are tom boys who'd rather be at Home Depot and so on. While these generalizations are untrue for many gays and lesbians, they are not without some empirical backing or expectation. If the human mind is made more like its opposite sex in sexual orientation, one should expect most of the possible mechanisms presented above to also alter their mind towards the other sex in multiple ways. This difference has been shown in numerous studies and we'll not go into each; a review was conducted by Bailey and Zuxker (
87). For one example, Reiger et al took childhood videos of gay and heterosexual men and presented them to a panel blinded to the subjects orientation, and asked them to judge the children in the videos on their gender conformity (
88). As expected the homosexuals were picked out as being those more unlike their genetic gender in their childhood behaviors.
http://3. Conclusions
In the end, the differences between homosexuals and heterosexuals are not vast, they are in fact not much greater than the differences between men and women in general. The obvious lesion to be learned from all this research is that the mind and body of a gay person share more with their heterosexual opposite sex counterpart than their sexual orientation. The causes that result in a typically female orientation in gay men seem to also result in other typically female characteristics in areas such as cognitive abilities, puberty onset, and more. Lesbians are similarly masculinized, beyond with whom they best form a couple.
Simply, humans are not easily placed into comfortable black and white categories of "man" and "women", biologically, or psychologically. There is, in fact, a biological continuum between the two categories with gays, lesbians, transsexuals, bisexuals, and the intersexed occupying any fraction of that middle ground.
