by Dr. Scot O'Grair @ isocrat.org	Related Items...
	The Fraternal Birth Order Effect... 2008-01-01 Nature vs. Nurture... 2008-09-15 How Homosexuals Are Different... 2008-11-24 Handedness as Orientation... 2008-12-11
	Find more related items by: author or by topic
Keywords:: Nature vs. Nurture, Science, Causes of Orientation, Fraternal Birth Order Effect, Twin Studies, Genetics, Gay Genes, Nurture, Nature

The causes of sexual orientation are certainly a subject of much social and political conflict, if the orientation is not heterosexual. People on both sides of gay rights issues have a stake in spinning this topic one way or another, and the potential to ignore data in favor of a desired end is ever present. We hope here, though, to focus first on the research, and include what we find from both nature and nurture.

This article will be split into nature and nurture sections. As was discussed in our introduction to the nature/nurture debate, there is inevitable overlap.

Contents:

• Evidence for Nature - Events before your conception.
  • Physical and Mental Differences - How homosexuals are different in both mind and body.
  • Causes of Homosexuality in Other Species - We aren't the only gay creatures.
  • Running in Families - Orientation flocks together.
  • Twin Studies - How much of orientation is genetic.
    • Non-Identical Identicals - What twin studies mean and do not mean.
  • Gay Genes - Where in the ACGT's are the GLTB's..
• Evidence for Nurture - Events after your conception.
  • Hormone Exposure - The chemicals that can nudge your orientation and much more.
  • Fraternal Birth Order Effect - Big brother can make you gay.
  • Developmental Instability - A lot of you depends on the toss of dice.
  • Sexual Abuse - Something terrible might change you.
• Proposed Causes - The causes that don't have much evidence to back them, but should be mentioned.
  • Distant Parents - Being gay can drive a wedge between parent and child.
  • Feminizing Environment - Can growing up with girls make you girly?
• Conclusions
  • Figure of the Web of Causes Behind Orientation.
• References

Evidence for Nature

I - Physical and Mental Differences -There are many subtle physical differences associated associated with homosexuals when compared to heterosexuals, from hair patterns to differences in brain anatomy. Furthermore there are differences in certain cognitive functions. Most all of these differences (though not all) point to a cause for homosexuality in the genes or in prenatal occurrences, such as differences in brain structure, body shape, and certain cognitive abilities. For details on these differences and as a companion article to this article, please see the isocrat.org article on how homosexuals are different; the information contained within will be useful as a primer for this article.

II - Causes of Homosexuality in Other Species - Please see this related article for information on homosexuality in other species. Through the added ethical freedom we afford ourselves in the use of non-human subjects, researchers have been able to determine several causes for homosexuality in several species. They have also been able create homosexual animals by genetic and surgical manipulation. Much of this research, for example the work of surgical alteration of the hypothalamus, has significant relevance to the causes of human sexual orientation, heterosexual and homosexual, despite the fact that such research on humans is rightly prohibited.

III - Running in Families - Like red-headedness and left handedness, homosexuality seems to run in families. Using three independent samples of gay men, it was found that the brothers of gay men were about 10% likely to identify as gay themselves (about double the background level), and gay men with gay brothers also reported having significantly more gay uncles and gay male cousins than gay men with no gay siblings (1, 2). Evidence also exists that homosexuality is elevated in the matrilineal line of gay men (3). One study found almost double the number of gay men on the matrilineal side of a gay man's genealogy than in the patrilineal line, causing researchers to look for an X-linked genetic cause (4), as discussed below. Additionally, Pillard et al. in 1986 found brothers of gay men had approximately 4 times the chance of being gay as brothers of heterosexual males (5). For lesbians it has been found that female homosexuality tends to be more prevalent in sisters of lesbians (6). It is important to note that, in the case where two siblings are gay, the vast majority seem to realize their orientation without knowing their sibling's orientation, diminishing support for a claim of inter-sibling influence (7).

Furthermore, there are traits other than orientation that seem to run in families which contain gay and lesbian members. It has been found that family members of gay men tend to produce more children than families that are not closely related to gay men (particularly in the matrilineal relatives of gay men), perhaps pointing towards a Darwinian benefit found in a cause of same-sex orientation (4, 8). These finding have recently been repeated by Lemmola and Camperio, and seem to be holding up (9). Along similar lines, Zietsch et al., in 2008, showed that psychologically masculine females and feminine males tend more often to be homosexual, but, when heterosexual, they tend to have more sexual partners, and, in twins discordant for sexual orientation, the heterosexual twin will have more opposite partners throughout their life (10). Their statistical analysis pointed to a genetic influence that caused both homosexuality and this mating advantage. Interestingly, genetic modeling has been used to show that a two-location genetic model, with at least one gene on the X chromosome which boosts female reproduction but also creates homosexual males, may account for such data and the existence of homosexuals within an evolving species (4, 11).

IV - Twin Studies - In this section the data are given in hereditability and concordance numbers. A hereditability of 100% would mean the trait is found to be completely genetically hereditable. Alternatively, the concordance for monozygotic twins (MZ), identical twins with identical genes (not actually identical, as discussed below), is the percentage of twins that share the same trait as their twin. Dizygotic (DZ) or fraternal twin concordances is the percentage of same-sex fraternal twins that share the same trait, representing different genes but genetic siblings (they have about 50% of the same genes). The difference between the two concordances should somewhat indicate the extent to which a trait is genetic and not the result of upbringing or womb environment. Thus, as a rough simplification, if the MZ concordance is 100% and the DZ concordance is 50%, hereditability should be near 100%.

When twins were found via gay publications, it has been found that lesbianism has a concordance rate of 48% for MZ twins, 16% for same sex DZ twins and 6% for adopted sisters (12). Similarly, for homosexual men, the concordance rate was found to be 52% for MZ twins, 22% for same sex DZ twins, 9% for non-twin male siblings, and 11% for adopted brothers (13). In a separate study by a separate group of researchers, both male and female orientations were analyzed; it was found that there was about a 66% concordance rate for MZ twins and 30% for same-sex DZ twins (14). However the self-selecting sample collection used in these studies is rightly suspect, due to the fact that similar twins may be more apt to respond.

In a twin studies in which the gay subjects were not self selected through gay publication, concordances are somewhat lower. Using a 25,000 twin pair registry, with the exact reasons for the research kept concealed, it was found that the MZ concordance rate was 35% for a non-zero Kinsey score (any non-heterosexual tendencies), whereas that concordance was 15% for DZ same-sex twins (15). Another study on this same twin registry found that hereditability of lesbianism was over 50% and was about 30% for gay men (16). Kendler et al. repeated such work, beginning with a representative sample of 50,000 households from the continental United States, 15% of which contained twins. They report a MZ concordance rate of 32% and a DZ concordance of 13% for homosexual orientation (17). Finally, in 2007 another twin study was conducted with 6,001 female and 3,152 male twins, and found that homosexual behavior was 37% hereditability in men and 46% in women (18). It may also be interesting to note that these concordances for orientation are in the same range as MZ and DZ twin concordances for various sex hormone levels (19).

It should be mentioned that one substantial study was unable to find a significant concordance in MZ twins, and actually found a correlation between having an opposite sex DZ twin and a homosexual orientation (20). However this study only included children between the ages of 12 and 18, when most all orientations are either small or in strong flux, and therefore had little to say in the realm of eventual adult orientation (Anecdotally, I thought I was heterosexual and then asexual up to about half way through this study's age range, while I know some of my straight friends had homosexual experiences before then). The more likely conclusion is that DZ male twins of females are more likely to admit homosexual feelings as teens, even if they end up as heterosexual adults.

Similar results have been found with regards to gender identity disorder (where an individual feels they are a sex different from their anatomical sex). In a study of 96 MZ and 61 DZ twins by Coolidge et al. the genetic influence of gender identity disorder was found to be 62% (21) . Repetition of these finding is needed, though.

All the evidence together suggests there is a strong genetic component to homosexual orientation, judging by the difference between MZ and same-sex DZ concordance. The case for a genetic effect is further bolstered by the the difference in concordance between DZ twins and adoptive siblings. Nevertheless, the fact that there is evidence that adoptive siblings of homosexuals have a greater rate of concordance in homosexual orientation than the general public suggests some environmental effect. Because a significant percentage (about 9%) of self-identified straight men in the general public seem to be having sex with men nonetheless (22, 23, 24), this is likely the effect of being more able to come out if you already have an out gay sibling. Stronger evidence for an environmental effect may be seen in the fact that concordance amongst MZ twins is not 100%, but there are complications in our genetics and biology that make that assumption incorrect, as will be discussed in the next section.

Non-Identical Identicals:
Before conclusions are made regarding what twin and other familial data could mean, it may be helpful to look at some less politically charged traits for comparison.

The first observation in looking for such data should be how difficult it is to find a human trait that is actually shared 100% between identical, MZ twins. There is actually no such thing as an identical twin, not even for traits that are far from psychological. The highest hereditability on which we could find data is height and eye color, but those traits are still below 90% hereditable, and there are several psychological conditions that no one chooses but also do not have a 100% MZ twin concordance, such as left handedness and autism. Handedness, in fact, is significantly less genetically influenced than sexual orientation. It's also interesting to note how many personality traits show strong hereditability when compared to same-sex fraternal twins.

The take home lesson should be that twin studies cannot give the whole story of biological determination. That is to be expected with orientation and all other traits. Identical twins have actually been shown to have distinct genetic codes and they may even have more copies of genes than their twin siblings (37, 38). Over their life, their genes may change in the degree of methylation, affecting gene expression and phenotype (39). There are differences between MZ twins in birth weight and congenital anomalies that may occur for multiple biological reasons (40) (Anecdotally, I have identical twin nieces, one with a cleft pallet and hare lip and the other with typical facial features). In fact some research has shown that identical twins that are discordant for sexual orientation show significant anatomical differences in finger length, which are thought to be related sex hormone exposure (see related isocrat.org article), whereas twins concordant for sexual orientation show no such physical difference (41).

Thus, contrary to what some anti-gay rights activists claim, just because so-called identical twins do not share orientation 100% of the time, that does not mean that homosexuality may not be 100% biological or even genetic. Simply, concordance in MZ twins represents a lower limit of biological determination of sexual orientation (or handedness or eye color or so on), not the upper limit.

V - Gay Genes - Because of the evidence for a significant genetic influence on sexual orientation, as described in the previous sections, researchers have begun looking for the actual genes that may be involved. For some perspective it should be noted that this research is in its early stages. Unlike the work in creating, for example, fruit flies that behave homosexually by gene manipulation (see related isocrat.org article), the human genome is considerably more complicated. It should also be kept in mind that there is no one gay gene. Much research has indicated that the genetic causes of sexual orientation, like traits such as handedness (see related isocrat.org article), are likely to be found in the interplay of multiple genes, and the inheritance model of orientation is not a simple Medelian model (42).

That said, several studies have had some success in pointing towards areas of the human genome that will deserve a closer look. Again, because there seemed to be a strong genetic cause running in the matrilineal line of gay men (4), the X chromosome has been the focus of several studies. The first linkage study was conducted by Hamer et al. and found that a certain region of the X chromosome was shared 75% of the time between gay brothers (3). In such studies of siblings pure chance would predict 50%. Hu et al. repeated this study, now including heterosexual brothers, and found this same region was shared only 22% of the time with a heterosexual brother of a gay man, and was shared 67% of the time amongst brothers of the same sexual orientation (43). The same percent sharing was found in another separate study by Sanders et al. (44).

However, all the above mentioned X chromosome studies looked at families in which homosexuality was found to run through the matrilineal line. When all families are considered, this one area of the X chromosome seems to have no detectable significance between gay brothers (45). Such finding suggest, once again, many genes are involved and those genes that may be important in triggering homosexual orientation in some families may be unimportant in other families.

In contrast to focusing solely on the X chromosome, Mustanski et al. conducted the first genome wide scan for genes related to homosexuality (46). This study involved 456 individuals from 146 families, most of which contained at least two gay brothers. They found a region of chromosome 7 was shared 63% of the time between gay brothers, and, on chromosome 8, another region was shared 60% of the time. They found several other areas of interest that seemed to be significant in causing same-sex orientation in men, but were unable to detect the same importance of the region in the X chromosome discovered in previous research. However, it was found, in further analysis, that this inability to repeat past research was due to their choice of genetic markers (See Mustanski's non-technical explanation of the research here: (47)). In short such findings are a starting place, but they should further drive home the point that multiple genes in a complicated interplay are likely involved in homosexuality (or heterosexuality).

Evidence for Nurture

I - Hormone Exposure - The effect of hormones may be on the border between nature and nurture. Much of what occurs in the human body and mind is controlled by our exposure to certain chemicals, from the time of conception. Levels of, for example, certain steroids can markedly increase our aggression and muscle mass. Hormones also have a profound effect on human sexual development, as any teenager will tell you. Though such experiments cannot ethically be conducted in humans, prenatal exposure to certain hormones in animal models has been shown to lead to homosexual behavior (see related isocrat.org article).

In humans though, there is still strong evidence that hormone exposure plays a significant role in creating both heterosexual and homosexual orientation in the neurological structures of the human mind. For example, some genetic men (experiencing the effect of male hormones through gestation) are raised female and are surgically altered to appear anatomically female just after birth due to a congenital defect in the penis or circumcision error. The vast majority of such men end up being sexually oriented towards females. Mustanski et al. have reviewed such research (42).

Several other lines of evidence exist, but some conditions that lead to intersexed humans should be introduced first. Congenital adrenal hyperplasia (CAH) occurs when a recessive genetic trait in an otherwise genetic female disrupts cortisol production and results in additional testosterone and dihydrotestosterone levels, both masculinizing hormones (48, 49). In essence, females with CAH are females who experience elevated male hormones. Such genetic females may remain anatomically female or the clitoris may be enlarged or she may even appear and be raised as a male in rare cases.

Androgen Insensitivity Syndrome (AIS) is a genetic condition that results in the body's inability to respond to androgens, male hormones. Genetic males with complete AIS will be born with female anatomy, but they are generally also raised as female and thus nurture and nature become too entangled to tease apart.

When CAH females (those who experience experience elevated levels of androgens) are compared with a control group of typical females (those who experience normal androgen levels) and AIS females (those who are insensitive to androgens altogether), the percentage who report sexual arousal towards other females is 48%, 15%, and 7%, respectively (50). Attraction to women, in women, increases with increasing effect of male hormones in females. Also, note that the average human female may have more incidences of homosexual orientation than females with certain genetic conditions, suggesting a cause for heterosexuality and a naturally elevated baseline for homosexuality.

Lastly there is the case of diethylstilbestrol (DES). This is a hormone that has masculinizing effects on a foetus, and it was prescribed for at risk pregnancies until it was found to have adverse side effects. Female children of women taking DES have been found to be significantly more homosexual and bisexual, with only 75% of them experiencing "complete" or "nearly complete" heterosexuality, once again pointing to a hormonal/neurological cause of sexual orientation (51).

II - Fraternal Birth Order Effect - One nurture cause of homosexuality that has strong empirical backing is the fraternal birth order effect. Simply, it has been repeatedly shown over many decades and populations that the more older brothers a male child has, the more likely he is to be gay. This seems to hold even if the older brother is adopted away and does not live with the younger brother. For a discussion of this significant cause of homosexuality, and the likely mechanism behind it, please see the related isocrat.org article.

III - Developmental Instability - Developmental instability refers to the effect of certain perturbations that occur during gestation. These perturbations may be anything from infection to chemical exposure and they result in an organism being nudged from one phenotype into a random selection of phenotypes. One may look at developmental instability as converting a genetically determined "nature" trait into a "nurture" trait that is effectively determined at random, as is a leading theory of how some humans end up to be left-handed (see related isocrat.org article). This cause may also account for some of the striking differences found between identical twins.

Evidence for such an effect in regards to homosexuality is found in the fact that homosexual people are significantly more likely to be left handed, and have a counterclockwise hair sworl (see the related isocrat.org article for details). Additionally, a high degree of fluctuating asymmetry, a physical measurement of how symmetrical a body is in its features from finger length to ankle width, is the result of noise in developmental instability. Recently, Hall and Schaeff found homosexuals, both men and women, have higher fluctuating asymmetry than heterosexuals (52). These results were replicated for gay men, but not for women; however, a relatively small sample was used (53). Such findings, again, suggest a biological basis for sexual orientation, but also may indicate possible involvement of developmental instability as a cause of homosexuality.

IV - Sexual Abuse - There are several studies that show a correlation between being sexually molested as a minor and having a same-sex sexual orientation. But, here particularly, it must be kept in mind that correlation is not causation. Sexual predators are well known to seek out children who seem vulnerable, have low self-esteem, and appear to be experiencing problems with their family, and they often target boys who appear to have more feminine qualities (54, 55, 56, 57, 58). Homosexuals, often become isolated and detached from typical support when they are realizing their orientation, making them more vulnerable and more willing to look for an accepting adult, who may have nefarious motives. Early age gender nonconformity and opposite sex characteristics (a risk factor for boys being targeted) more often correlate with adult homosexuality (59, 2)and that nonconformity often causes similar problems of alienation. Thus, a child may even find themselves to be an easier target for a predator before they realize their sexual orientation.

In short, there are several factors that may lead to a homosexual being targeted more often than a heterosexual for sexual abuse, just as is the case for being disabled, nonwhite, poor or from a divorced family (60, 61) . Orientation may clearly contribute to sexual molestation, but orientation may not contribute to, say, alteration of your genetic code.

That said, several studies do indicate gay men and women, on average, have been molested more often than heterosexuals, and it could be seen as one contributing factor to orientation in some people. Tomeo et al. found that gay men had about double the chance of having been molested than heterosexual men, and lesbians had about 1.7 times the chance compared to heterosexual women (62). Simari and Baskin found similar results, but reported that 96% of the victim viewed themselves as homosexual before the abuse took place (63), highlighting the problem of assigning causation to sexual abuse and the fact that gay teens are particularly vulnerable to predators.

Interestingly, in a study of 1,262 college students by Eskin et al., students were categorized by both their sexual behavior and self-reported sexual orientation (64). The researchers were unable to find any significant correlation between reported sexual abuse and describing one's self as homosexual or having homosexual attractions. They only found a significant correlation with homosexual behaviors in men. Many men occasionally have sex with men but do not feel they are gay or are out as gay, and they may otherwise have a heterosexual sex and family life (22, 23, 24) . It seems such men are more likely to have been abused, while abuse is less likely for people who feel they have a homosexual orientation. This may be the difference between having sex out of neurosis and sex out of the natural drives of one's orientation towards men or women. Alternatively, being sexually abused may make staying in the closet more likely. Regardless, judging by the numbers of heterosexuals that have been molested and homosexuals who have not, sexual abuse cannot be a sufficient or necessary cause of homosexuality, and more research needs to be conducted in this area.

Proposed Causes:

I - Distant Parents - At some point it was thought that absent fathers made male children gay. There is a common notion that gay men have greater odds of growing up without a father than heterosexual men. If true, the causation could be explained by the Fraternal Birth Order Effect (see here). The more older brothers a man has the more likely they are to be gay, but being the youngest of your siblings also means you have a greater chance of your father dying or leaving at a young age.

Some studies have also suggested that gay men are not as close to their fathers as heterosexual men (64). However, such distance is more likely to be the cause of the stigma behind homosexuality and the associated problems (e.g. substance abuse, depression (65, 66, 67)) than a cause of homosexuality. Many gay men come into unusual conflict with their family and particularly fathers when they come out as homosexual. Due to the fact that homosexual men tend to be more effeminate from early childhood (59), long before they have a sexual orientation manifest, some fathers may put distance between themselves and their effeminate sons even before the son admits his orientation. In fact, it has been found that negative relationships with and physical abuse by fathers is related to his son's femininity, not necessarily his orientation (68, 69).

II - Feminizing Environment - Similarly to distant fathers it was once proposed that men living with many females caused homosexuality. However, much research on the Fraternal Birth Order Effect (see here) has shown that a gay man is more likely to be raised in a home full of brothers rather than sisters.

Conclusion:

It is important to remember it is possible that none of these causes, alone, are sufficient or necessary to make a person homosexual, though they might all play a role in the population on whole. Personally, I have older brothers but other gay men, of course, do not. I lack every proposed post-birth nurture cause of homosexuality out there, from sexual molestation to distant fathers, but that does not mean that such "nurture" causes could not be behind another's same-sex orientation. A vast, unique, and complicated web of causes must come together to make humans everything from intelligent, to left handed, to Hindu, and that web seems to be no less complicated for sexual orientation.

As a means to organize this web of causes and associated effects, we have attempted to graphically display our understanding of sexual orientation's origins from the current research, in the figure below. Solid lines indicate fairly likely lines of causation, and dashed lines indicate significant uncertainty, but all possible causes are in need of further research. Approximate strengths of each cause are shown as a pie chart for both gays and lesbians (with a gray slice left for the unknown). This figure is certainly an oversimplified approximation for the average gay man or lesbian and not necessarily representative of the individual; your mileage may vary. You can click on most areas of the figure to be taken to the appropriate section. As always, please help us refine this figure if you can.

In the end, the answer to the question of what causes homosexuality is not what the politics of either side of the gay rights debate would prefer. There is far too much nature involved to support the goals of those fighting against equal rights for the GLBT population, and far too much complexity and nuance in the causes to fit into a digestible sound bite for use by the gay community. I hope though we've gone some way here towards making the relevant issues and the findings clearer.

 

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Created: 2008-11-30; Last Edited: 2008-11-30; (ID547)