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Religious Tolerance logo

Are the cause(s) of sexual orientation due to nature or nurture?
Three proofs that homosexual & bisexual sexual orientations
do NOT have a genetic cause, with rebuttals

Part 2: A brief description of three proofs. A
rebuttal to proof #1: based on "twin studies."

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This topic is continued from the previous essay.

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Frequently cited proofs used by people who believe that sexual orientation is not genetically caused:

People who reject genetic factors as the cause of sexual orientation typically offer one or more of these three observations as common sense proofs of their beliefs:

  1. By definition, Identical twins have precisely the same genetic makeup. If one is lesbian, gay, or bisexual, and if sexual orientation is determined by genes, then they assert that the other twin would always be of the same orientation.

    Studies show that this does not always happen in practice. It is not difficult to find identical twins in which one twin has a homosexual or bisexual orientation and identifies as gay or lesbian, while the other twin has a heterosexual orientation and identifies as straight. Many religious and social conservatives offer this observation as a convincing proof that homosexual and bisexual orientations are not genetically caused.

  2. Lesbians and gays have, on average, far fewer children than do persons with a heterosexual orientation -- commonly called "straights." Thus if a person were born with a an allele -- a variety of gene -- that made them lesbian, gay or bisexual, they would, on average, have fewer, if any, genetically related children during their lifetime. Most would not pass pass their genes on to the next generation. If a "gay gene" existed, copies of it would become far fewer within a few generations, and would eventually disappear. "Straight" couples would simply outbreed gay couples.

    However, lesbians, gays, and bisexuals are still with us. They appear to be present at about the same percentage in every country and in every era. Many religious and social conservatives offer this observation as proof that homosexual or bisexual orientations can not be genetically caused.

  3. Overwhelming numbers of homosexuals and bisexuals have sought help from reparative therapy counselors and/or transformational ministries. They have been reported as having successfully "changed." They are typically described, as having left the "gay lifestyle," and becoming "ex-gays." Many clients of these organizations have married persons of the opposite sex. Many religious and social conservatives believe that if homosexuality has a genetic cause, then few if any lesbians, gays, or bisexuals would be reported as having been able to make such major lifestyle changes.

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1: Rebuttal to the "identical twin" argument against a genetic cause of homosexual or bisexual orientation:

Various factors in a person's makeup such as hair color, eye color, skin color, left or right handedness, many diseases, and disorders, etc. are generally accepted as being genetically caused.

Consider multiple sclerosis (MS), whose cause is far less controversial than that of sexual orientation. There is a gene buried in human DNA that can cause this disease. It is found in two versions, called alleles. One is a very rare allele that may cause the individual to develop the disease. The other allele is much more common; if you have it, you definitely will not develop the disease.

Studies show that having the rare allele of the gene does not necessarily cause the disease to appear in that person's life. For example, in North America:

  • The probability of a randomly selected person developing MS is about one in 750. 1

  • If an identical twin has multiple sclerosis, one might think that because the two twins have identical genes, that the other twin is certain to develop the disease. In practice, the other twin does not usually have MS. The other twin only has a probability of one in four of developing MS. This is true even if the twins are separated at birth and raised in different places.

So, MS clearly has a genetic cause. Identical twins have identical genetic structure. Yet, if one identical twin experiences MS, the other twin will probably escape the disease.

Why does this happen? The answer is that alleles have a property called "penetrance." It is a measure of the allele's effectiveness -- its likelihood of being expressed or activated in a person's life. For example:

  • The Multiple Sclerosis allele that produces the disease has a penetrance factor of 25%. If you have this allele, then you have three chances out of four that you will remain free of the disease. If you have an identical twin, then your twin will also probably be free of the disease. But it is quite possible that one or even both twins will develop the disease. Note that identical twins have identical genes, and yet they can have different outcomes.

  • Huntington's Disease: It is casused by a rare allele that is 100% penetrant. If one identical twins is born with this allele, then the other will almost certainly have the same allele. Both are certain to develop the disease later in life.
  • Type 1 diabetes: The penetrance of the gene which causes early onset diabetes is only 30%. If one identical twin has the allele that causes diabetes, then the other twin will also have the same allele, and will have a 30% chance of developing the disorder.
  • Schizophrenia: The penetrance is 48%. So, if one identical twin develops schizophrenia, the other twin has about a 48% chance of also developing the disorder. 2
  • Bipolar Affective Disorder: This has about a 60% penetrance factor. If one identical twin develops bipolar affective disorder, (formerly called manic depression) the other twin's chances are about 6 in 10 of having it as well.
  • Brain neurotransmitter: A New Zealand study examined a gene that controls the production of an enzyme called monoamine oxidase A (MAOA). One allele can cause behavior problems. It is found in about one third of the male subjects tested. It appears to have a high penetrance factor of about 85%. Among this minority of males, 85% of the boys who were physically abused during childhood turned to criminal or antisocial behavior as adults; if they were not abused, they would have developed normally. More info.

In the examples shown, the penetrance factors have varied from 30% to 100%. The penetrance of the "gay gene" or "gay genes" appears to be about 67%. So if one identical twin is gay, lesbian, or bisexual, one would expect that the probability of the remaining twin being gay or lesbian would be about 67%. On the other hand, fraternal -- non-identical -- twins have fewer genes in common. One would expect that if one fraternal twin was gay that the other would also be gay about 22% of the time. This number agrees with the results of studies of families with fraternal twins. This observation comparing fraternal and identical twins is regarded by some researchers as the best indicator that a sexual orientation is genetically caused.

Biologist Chandler Burr concluded:

"There could be hundreds of millions of straight men walking around with this gay allele but who are straight simply because it didn't penetrate." 3

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To recap, some people believe that a homosexual or bisexual sexual orientation cannot have a genetic cause because although identical twins have identical genes, they may end up with different sexual orientations. They assume that the allele that causes a homosexual or bisexual orientation is 100% penetrant. But the alleles(s) that causes a homosexual orientation is like most alleles that cause different traits or diseases. It is less than 100% penetrant. Thus it is quite possible for identical twins to discover that they have different sexual orientations, just as most persons with multiple sclerosis who have an identical twin, find that their twin is free of the disease.

Observing that a set of identical twins discover they have different sexual orientations later in life does not prove that people are not born gay or lesbian. It merely proves that the penetrance factor of the "gay" allele is less than 100%.

In the above analysis, our intention is not to treat homosexual or bisexual orientation -- which are traits -- as if they are a disease. However, most genetic studies have been performed on diseases, like multiple sclerosis, and not on traits, like left handedness or sexual orientation.

What causes the "gay gene" to be activated in about 67% of people, while about 33% of people with the same allele turn out to have a heterosexual orientation? It appears to be something in the environment that triggers the allele.

Consider the common light bulb. In order for it to generate light, power must be available at the light switch and the light switch must be turned on. Similarly, in order for a person to have a homosexual orientation:

  • Their DNA must include the "gay allele(s)" and

  • Some unknown event must have happened after their conception and before their death to turn the allele on -- to make it penetrate.

Thus a homosexual orientation requires both a certain gene(s) and something in the environment. The latter could involve catching a virus, a fall, or some other event. The actual factor that causes the "gay" allel to penetrate is currently unknown. Thus a homosexual orientation in a person with the "gay allele(s) can neither be avoided nor chosen.

To summarize: For a homosexual orientation to occur, it needs a genetic setup and a trigger in the environment. The nature of the trigger is currently unknown. It is not something that a person can freely choose. It is not something that a person can avoid. It just happens and is beyond the person's control.

Unfortunately, this statement will probably cause anger in close to 100% of the population:

  • Back in 1999, which was when the most recent national data became available, only about 10% of the population believed that a homosexual orientation is caused by both genetics and something in the person's environment or upbringing.

  • The vast majority of the Lesbian, Gay, Bisexual and Transgender/transsexual community (LGBT) believe that an newborn's eventual sexual orientation is totally determined at or before birth genetically. i.e. they believe that the environment and upbringing have no effect.

  • Most religious and social conservatives believe that genetics have little or nothing to do with causing sexual orientation. Rather homosexuality is a choice or perhaps an addiction.

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This topic continues in the next essay

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References used:

The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.

  1. "Who gets MS? (Epidemiology)," National Multiple Sclerosis Society, at:
  2. "Strong genetic component to Schizophrenia," Undated, at:
  3. "Gay gene?, identical twins?, and choice," Liberated Christians, 2002, at:

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Home page > "Hot" religious topics > Homosexuality > Cause > here

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Copyright © 2014 by Ontario Consultants on Religious Tolerance
Original posting: 2014-DEC-13
Latest update: 2014-DEC-27
Author: B.A. Robinson

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