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the scientific evidence


Is there a biological basis for homosexuality?

According to all the evidence I can find (summarized on this page), the answer is no. But we should be very cautious, especially when someone says "this is proof!"

Background issues

'Learned behavior'

If there is one overarching theme of this web site it is of free will. Ever since the fall of Adam, we have been free to choose our own destinies. Even membership of the church is based of principles of freedom, and personal understanding and responsibility. In this context, I am not comfortable with the phrase 'learned behavior.' It seems to imply that the individual is passive, and accepts something from outside. It suggests that sexual orientation can be taught. It can be a way of blaming outside forces, and is thus just as bad as blaming biology.

All human behavior is a matter of free choice. Usually we do not realize the significance of our thousands of day to day choices - especially when our attitudes subtly harden and become redefined over many years. But choices are involved at every stage. It is the job the church to identify ideas and behaviors that, though they seem innocent right now, can have serious long term effects if pursued continuously.

In summary, sexual orientation, like any other orientation, is decided by free will and choice. It is influenced of course by other factors - outside forces, and perhaps even genetic predisposition - but it would not be how it is without free choice. Sadly, most of our choices are made in ignorance.

'Occam's razor'

Most scientists are familiar with Occam's razor - the principle of shaving away all but the simplest and most essential elements of any explanation. It is named after William of Occam (1285-1349), a theologian and Franciscan monk. He got the rule from Durant de Saint pourcain (1270-1334), a theologian and bishop. The rule is useful for choosing between theological arguments, and its a basic tenet of modern science. It states:

"non sunt multiplicanda entia praeter necessitem," or,"entities should not be multiplied beyond necessity."

In this case, even the proponents of biological causes of homosexuality accept that free choices are involved to some degree, though they would suggest that their effect is only minor. My argument is this: we agree that free choice is involved. Free choice is enough to explain the whole phenomena of homosexuality. So why look for any additional cause?

I believe we should re-enthrone free will as the ruling principle of our lives, and help each other to understand and live according to our choices, rather than being slaves to blind forces beyond our control.

No man knows how the brain works

The following was written by Chandler Burr, a man who is well informed in this field, and gay. It is from the Atlantic Monthly; March 1993; p. 47 - 65. The first part of this page is largely based on the evidence he presents.

"It is undeniably true that neurobiological research is often pursued in a context of great ignorance. The brain remains an organ of mystery even in general, not to mention with regard to specific functions. "We don't know" may be the most frequently used words in neurobiology, and they seem to be used with special frequency when the subject of sexual orientation comes up. Once, I mentioned to a researcher how often I heard these words on the lips of her colleagues, and she replied, "Good -- then they're saying the right thing." In this context, and also considering that the subject matter is politically charged, professional rivalries are inevitable and occasionally bitter. Some of those involved in the research are motivated not only by scientific but also by personal concerns. Many of the scientists who have been studying homosexuality are gay, as am I."

One approach, to make things simpler, is to just look at the sizes of parts of the brain. That's easy to measure, right? Wrong. Even here, people get different results. "Part of the difficulty is methodological, involving whose brains are being compared, and how. Dead people or living people? Old or young or mixed? Healthy or sick? By means of brain sections or magnetic-resonance imaging? LeVay [the author of a famous study] calls studies of the corpus callosum 'the longest-running soap opera in neurobiology.'"

Against homophobia

In the 1980s, the term "sexual orientation" became more common that "sexual preference." It became normal to look on homosexuality in terms of "being gay" rather than "being ordinary and doing gay things." It became fashionable to look at homosexuality as a biological issue instead of a psychological issue.

In my opinion, this illustrates how science is the slave of belief.

The reason for the shift was not because something new was discovered, or because psychology had run out of ideas, but because people had come to see prejudice against homosexuals as a bad thing. There are stories of cruel and horrific things being done to gay men to "cure" them, and everyone has seen examples of gay men being treated with hatred. People just got tired of being nasty to gay men. Besies, heterosexual norms had become so free and easy that homosexuality no longer seemed particularly bad. So it is now fashionable to just say "God made him that way" and look for evidence.

Homophobia is wrong. Hatred is wrong. Taking someone's freedom away (without a very strong reason) is wrong. But this is not the same as saying "science has proven that some men are born gay." The scientific case for being 'born that way' is no stronger now than it ever was.

The historical evidence

The people who were in the best position to judge homosexuality were perhaps the ancient Greeks - they did not see anything wrong with it. For hundreds of years they accepted it as quite normal. So there were no aggressive 'straight' men trying to prove their side was right, and no defensive "gay" men trying to prove their side instead. Did the Greeks recognize that gay and straight men were fundamentally different? No. Most (most?) men practiced both kinds of sex at some time. It was just seen as a matter of choice and culture.

In fact, it was not until very recently - especially the 1980s - that people started to look on sexual orientation as a condition rather than a choice. This is a result of social changes, not biological discoveries.

Attempts to treat homosexuality as a pathology

People read a lot into the fact that psychiatry has failed to find a 'cure' for homosexuality. Of course it hasn't! You cannot change how someone feels unless (1) the behavior is unimportant, or (2) the person is easily changed, or (2) they want to change. In any case, psychiatry is a relatively young science, and deals with topics that are far more complex than anything in physics or biology. Psychiatry has failed in plenty of other areas too. It says more about psychiatry than it says about sexual orientation.

For example, homosexuality had for years been treated as a pathology - it was assumed that gay men were badly adjusted or were hiding something evil. But this is nonsense. If homosexuality is wrong (and I believe it is) it is because it has bad effects on family life and society. But so do a lot of things. To identify these by observation you would need to study whole societies for many years, and eliminate all other influences. Clearly that is not practical. The psychiatric approach was always going to be a dead end.

In 1973 the American Psychiatric Association removed homosexuality from its official 'Diagnostic and Statistical Manual.' In other words, being 'gay' was no longer treated as a disease. This was because "to be minimally scientific, pathology must be defined in a way that is objective and empirically observable." This was clearly impossible in the case of homosexuality, for the reasons I have indicated.

Notice the subtle but crucial shifts in attitudes:

Evidence for an environmental cause

It seems clear that environmental influences can influence people to become homosexual. For example, it is often reported that people in prison are far more likely to become gay than people outside of prison. At the risk of being adding even more offense, I will quote the famous zoologist Desmond Morris:

"Under normal circumstances, in their natural habitat, wild animals do not mutilate themselves, masturbate, attack their off-spring, develop stomach ulcers, become fetishists, suffer from obesity, form homosexual pair-bonds, or commit murder. Among human city-dwellers, needless to say, all of these things occur. Does this, then, reveal a basic difference between the human species and other animals? At first glance it seems to do so. But this is deceptive. Other animals do behave in these ways under certain circumstances, namely when they are confined in the unnatural conditions of captivity. The zoo animal in a cage exhibits all these abnormalities that we know so well from our human companions. Clearly, then, the city is not a concrete jungle, it is a human zoo.

"The comparison we must make is not between the city-dweller and the wild animal, but between the City-dweller and the Captive animal. The modern human animal is no longer living in conditions natural for his species. Trapped, not by a zoo collector, but by his own brainy brilliance, he has set himself up in a huge, restless menagerie where he is in constant danger of cracking under the strain." - Desmond Morris, 'The Human Zoo,' New York 1969 (Quoted in a letter to Sunstone).

It may be that some genes make homosexuality a little more likely, but some genes can also make baseball playing more likely, or taxi driving, or your favorite film, or any other learned behavior. For example, if I am born tall and strong, that is influenced by my genes, and it makes me more likely to take up sports. But all these things are still a matter of choice.

The dangers in focusing on a biological 'cause'

  1. The obvious and most serious danger is that this tells people that they 'cannot' do something that they can do. Thus, it reduces freedom and encourages feelings of helplessness.
  2. Second, the only practical reason to look for a biological 'cause' is if we wanted to make use of some biological pre-disposition. How would we use it, if not to strengthen it or remove it, or change it in some way? Since such tampering is generally agreed to be unethical, why pursue it?
  3. Third, it creates divisions. We cannot assume that this will lead to more tolerance. If you class homosexuals as a fundamentally different class of person, this give extra ammunition to those who hate gays. It is one thing to think someone as having chosen a 'bad' thing. It is quite another to believe that this person is fundamentally and irretrievably 'bad'.

The evidence for a biological cause

Evidence 1
"I feel like I cannot change"

This is the most common reason for saying that homosexuality has a biological basis. But the same argument could be used for any learned belief or behavior that becomes deeply ingrained. People often say "I cannot change how I feel" about a person or a topic. But when they find out more about that person or topic, their feelings often do change.

Evidence 2
Twin studies. Result - so far, "no".

The obvious way to find if gays are genetically programmed to be gay is to look at identical twins, who are (of course) genetically identical. If being gay has a simple genetic cause like eye color, we would expect each pair of twins to be either both gay or both straight (given the choice). That is not the case. So the only thing we can say with absolute certainty is your genes alone did not make you gay.

But perhaps certain genes make being gay more likely? To find out, you would need to find a large number of identical twins, who were all separated at birth (to eliminate the possibility that upbringing makes someone gay). And you would need enough gay men to make the study statistically significant. It is unlikely that anyone will find enough sets of gay identical twins who were separated at birth to make any kind of general conclusions, so this avenue of study is not very helpful.

Identical twins tend to choose the same things

If you look the same as someone else, have the same level of physical strength and agility, and think of the other person as your extra-special friend, you are more likely to choose the same kind of thing. For example, if one twin likes sports, goes to the movies a lot, and thinks he looks good wearing denim, an identical twin is more likely to think the same, compared with a non-identical twin who may be bad at sports, prefer music to movies, and looks better in leather. All these things are affected by whether of not the twins share the exact same genes. But they are all the result of choice.

Naturally, since identical twins are more likely to make the same choices, if one chooses to be gay, the other is a little more likely to choose that as well (compared with a non-identical twin). There are also other possible explanations - such twin studies usually have very small sample sizes, and are thus easily biased by random fluctuations.

In conclusion, twin studies are consistent with the idea that sexual orientation is a matter of free choice.

Evidence 3:
Brain sizes. Result - so far, "no".

In 1991 and 1992, research was published which suggested that two parts of the brain were slightly different sizes in gay men as compared with straight men.

It seemed pretty conclusive at he time - gays ARE different. The gay lobby got excited. But this was dealt a serious blow in March 2000, when research was published that showed that taxi drivers also had different brain structures. In taxi drivers, the part that dealt with navigation - the hippocampus - was larger than in non-taxi drivers. Does that mean that there is a taxi driving gene too? That some babies are pre-programmed to drive taxis?

The researchers concluded that if you use your brain differently from other people, it will grow differently. It has nothing to do with being born that way - it is down to freedom of choice.

Evidence 4:
Fruit flies and rats?
Result - so far, yes, but it favors the 'choices' model in humans.

It has been discovered that a single transplanted gene can cause fruit flies to display homosexual behavior. However, when you experiment with moving genes, you are in effect damaging an otherwise normal brain. The easiest thing is to cause some kind of harmful damage. You can induce some very strange behavior (or strange mutations) by changing genes in fruit flies (although by careful trial and error you could in theory also create neutral or beneficial changes). This kind of research could most easily be interpreted as saying that homosexuality is a kind of brain damage, which I am sure is not what was intended. (Actually, in this case no conventional illnesses result, but the 'bisexual' fruit flies are unable to perform sexually.)

"This 'bisexual' fly strain has existed behaviorally unchanged through hundreds of generations." (Not because the bisexual flies have offspring, but because two normal flies can carry half the gene, which becomes complete in a percentage of their offspring. Some of their other offspring will have half genes, ready for the next generation, and so on.) If this is he mechanism in humans, we should expect homosexuality to appear consistently in some families and not at all in others. But this is not the case. So if the fruit fly results mean anything, they show that, even if you are 'born to be gay', you can quite happily choose otherwise.

Dysfunctional rats

Similarly it has been found that if you castrate a male rat and pump it it full of female hormones, it makes some of the sexual movements seen in female rats. But this to me is the sign of a very confused rat having dysfunctional behavior. An even bigger problem is assuming that humans act like lab rats - as if we just respond to chemical stimuli without the conscious ability to choose and interpret. Are we to believe that gay men have no more freedom of thought than a damaged fruit fly or a drugged rat?

Perhaps referring to rats and flies is unfair. Perhaps similar behavior can be prompted in monkeys. But the same principle applies. We are comparing a creature of instinct to a creature of free thought. It is true that people, when we are young, lazy, or misled, can give up freedom to drugs or biological urges. But is that how we want to classify ourselves? Is this the self image we are choosing? "Hi - my name's Chris. I have decided to shut down my freedom of choice and become a slave to whatever forces happen to push me around. Why not join me?"

Evidence 5:
Some animals are homosexual? "Yes, and some animals also eat their young".

See http://www.newscientist.com/ns/19990807/queercreat.html for details. Researchers have documented a wide variety of homosexual behaviors across the animal kingdom. One prominent researcher of the subject "stresses that animal homosexuality is not a single, uniform phenomenon. His mission is to document its sheer diversity." Given that homosexual behavior in animals is essentially counter-productive (it does not result in passing on more genes) the easiest explanation is to note that animals exhibit all kinds of behaviors - some beneficial, some less so. It is only humans who group some bahaviors together and call them all "homosexual." It seems clear that a proportion of all animals will show these behaviors. That is how "natural selection" works. In the absence of sufficient intelligence, variety is the only way to cope with the unexpected. Those with extremely counter-productive lifestyles (e.g. those that get themselves killed, do not eat, or mate with the same sex) do not pass their genes on to the next generation.

It could be argued that because animals do it, that makes it "right," or at least "normal." But many animals also kill their own young, and certainly kill eachother. Sexual behavior in general in the animal kingdom is closer to what we would call "rape." So I would suggest that animals are not a good guide to how life "should" be. Humans have a great advantage over animals. We have a greater degree of free choice.

What is "natural"?

We need to be clear what we are advocating when we say something is "natural." The naural world operates by the law of the jungle - the strong survive and the weak starve. Life is pleasant only to the extent that animals do not think about the dangers. Homosexuality, like all other animal behaviors, exists in this context. From a human point of view, natural life is "nasty, brutish and short." Humans have the opportunity to rise above this by developing rules. "Natural," on its own, is not a good argument.

Could it be beneficial in any way?

It could also be argued that homosexual behavior may have some social role (this was argued in the case of bonobos (pygmy chimpanzees) who exhibit these behaviors more than most. But even if proven, we need to be vey careful. A strategy that works for a relatively simple society that also relies on relatively low intelligence and a certain amount of killing, may not be so ideal for human society.

With the bobobos, it has been suggested that male-male sexual behavior is a result of pressure - they live in large groups, and the potential for stepping on each other is great. They need some way to diffuse the situaton, and sex seems to work. This seems to support Desmond Morris's statement that such behavior is a result of being forced together. But there is another alternative, a better way to live in groups. It is to devise structures and institutions that allow individual freedom. Heerosexual marriage is one such nstitution. Homosexual marriage has been suggested as another, but (as noted in the page on "what's wrong with it?") this would tend to decrease freedom, decrease variety, and increase tension.

Finally, it should be remembered that homosexual behavior in animals is still in a minority pursuit (which is one reason it has taken so long to be recognized). The dominant (and therefore more successful) strategy is heterosexuality.

Evidence 6:
A "gay gene"? Result - so far, "no".

The best known evidence was published in 1993 (plus a couple of follow-up studies) by Dean Hamer. He claimed to find a correlation (not a cause, but a correlation - a coincidence) between a certain gene and gay men. The media started talking of a "gay gene," even though the researchers did say that many people had this gene who were not gay, and some gay men did not have the gene. Even so, it looked hopeful, although statisticians pointed out that, to be certain, the sample group would have to be much, much larger. But in 1999, a different researcher carried out the same experiment, and found no evidence at all that this so-called gay gene existed. He found that the occurrence of the so-called "gay gene" was no greater in gay men than in straight men. (See "Where Did the Gay Gene Go?" on the ABC News site at http://abcnews.go.com/sections/living/DailyNews/gaygene990422.html )

Evidence 7:
Hormones? Results - no.

Men have more testosterone, and women have more estrogen. So homosexual men are somewhere in between? Twenty years ago, the research here was like the research into hormones in the womb (see below). It all looked very promising. But as more and more studies were completed, the evidence began to look weaker and weaker.

"Three studies did show that homosexuals had significantly lower levels of testosterone, but [ a scientist who studied all the results together] believed that two of them were methodologically unsound and that the third was tainted by psychotropic drug use on the part of its subjects. Two studies actually reported higher levels of testosterone in homosexual men than in heterosexual men, and one unhelpfully showed the levels to be higher in bisexuals than in either heterosexuals or homosexuals."

Researchers have just about given up on this idea. But a similar idea - that hormone levels were different in the womb, before a baby was born - are still popular.

Evidence 8:
Hormones in the womb? Results are easily explained by the environment.

This is the 'The prenatal-hormone hypothesis" - that gay men and lesbians are the result of unusual amounts of male and female hormones acting on the unborn child. But the evidence is weak (as I shall show), and the whole theory is pretty shaky:

"The evidence from hormonal research may circumstantially implicate biology in sexual orientation, but it is far from conclusive. William Byne raises a warning flag: 'If the prenatal-hormone hypothesis were correct, then one might expect to see in a large proportion of homosexuals evidence of prenatal endocrine disturbance, such as genital or gonadal abnormalities. But we simply don't find this.'"

The rest of this section looks at the indivdual evidences for the hormone theory. (Sources: Most quotes are from the Chandler Burr article. The remainder are from articles in a San Francisco newspaper, Newsweek, Reuters, and NewScientist, all available on the web.)


Congenital adrenal hyperplasia (CAH) is a condition where a baby in the womb is flooded with far too much male hormone. A baby girl can then look physically a little similar to a boy. But this is superficial - internally and by chromosones she is indeed a girl. Corrective surgery then allows her to be raised as a girl. If hormones determine sexual orientation, surely all such women would be lesbians? In fact, the majority are heterosexual.

But about a third describe themselves as either 'lesbian' or 'bisexual.' This is much higher than average, but entirely consistent with a 'free choices' model of development. Imagine that you were a child, a girl. All children feel they are different and look to their environment to help them decide who they are. Imagine discovering that you really were physically different at birth - a little like a boy. Wouldn't you grab on to that as an important proof that you really were different? Wouldn't you identify strongly with bisexuals and others who also appeared to cross the gender gap? It would be 'only natural' to feel that way, and we do not need to look to a different brain structure for a cause.

Older brothers

"Ray Blanchard [has] shown that boys who have greater numbers of older brothers have a higher likelihood of being gay. Blanchard says he thinks Breedlove's study furthers the theory that hormones in the womb have an effect on the eventual sexual orientation of the fetus."

Note that there is no direct evidence for the hormones - they are only inferred as a possible cause. "Breedlove said it's a complete mystery as to how a mother's body could 'remember' how many male children she had borne, where this signal was kept and how it could influence hormone levels of a later- born child."

As usual, the effect, if real, is only slight. There are plenty of younger brothers in the world who are not gay. And many men who consider themselves gay have no older brothers. "Indeed, when asked how he accounted for men with no older brothers who are still homosexual, Breedlove replied that 'we have no explanation for their sexual orientation." And once again, this can easily be explained in terms of upbringing. If you are surrounded by older brothers, you are more likely to either (a) be bullied by them, which might lead you to reject conventional aggressive heterosexual norms, or (b) admire and respect them, which admiration might be transferred to males in general. Neither effect would be enough to make every young brother gay, but each could be enough to distort the statistics as observed.

Finger length

Newsweek summed it up like so: "Scientists have long believed that finger lengths may indicate the levels to which a fetus was exposed to male sex hormones, such as androgen, while in the womb. ... A person's finger-ratio is the index finger's length divided by the ring finger's length. In men, the average ratio is 0.95. For women, 0.97. Lesbians who participated, the Berkeley team found, scored 0.96" (It should be emphasized that these were averages - some heterosexual men have ratios above 0.97, and some lesbian women have rations below 0.95.)

This only applies to women. In gay men there was no overall difference. There may have been some effect linked to having older brothers or not, but the sample was not large enough to say.

I don't know how this was received in the US, but the next day, one of the best selling newspapers in the UK (which was not known for being anti-Gay) published a double page spread on the subject. Researchers has searched newspaper archives for clear photos of the hands of the world's most famous gay and straight celebrities. All the famous gay men and lesbians had "heterosexual" hands, and all the famous heterosexuals (such as those famous for heterosexual desire) had "gay" hands. Of course, this wasn't a scientific study - it was a piece of mischievous fun. But it was all genuine.

Plainly, it is silly to say "my finger length means I was born to be gay." As usual, personal choice is far more important. One observer comments wryly: "The researchers caution - in a disclaimer that probably will be widely ignored - that these differences apply only as broad averages in large samples, not to individuals. One stubby-fingered father of two, who ignored the business about averages over large populations, was surprised to discover he was lesbian."

Similar research has suggested that lesbians are more likely to have more masculine-looking inner ears.

But how do we explain the effect, no matter how small? Perhaps because having more masculine hands would suggest a more masculine body in general. Even as young children, peers and parents would notice this and make subtle (usually unconscious) judgements. This affects the person's self image. So a more masculine looking person is less likely to identify with the feminine norm. They are thus more susceptible to choosing an orientation that accepts and values their more masculine outlook.

Is the evidence really so strong?

At the time of writing, the finger length study is hot news. Some newspapers (the New York Post online edition, for example) describe the research with words like "stunning," and write as if it was all proven.

But another possibility is that it is all a statistical fluke in a single study. The recent finger study suggested that gay men had more male hormones than normal. It goes against all the other theories, where gay men are more like women. "John Manning of the University of Liverpool has found the opposite in gay men: that their finger ratios veer more towards the feminine. ... Manning also wonders if Breedlove's data may have been slightly muddied because the research did not take account of ethnicity. He has found big population variations in 2D:4D ratios. 'The geographical differences swamp the sex differences,' says Manning. 'There's more difference between a Pole and a Finn than between a man and a woman.'"

Sex-typical play behavior

"Robert Goy [at the University of Wisconsin at Madison] has done many studies over the years showing that you can reverse the sex-typical play behavior of infant monkeys by hormonal manipulations in prenatal life. [Play] is an example of a sex-reversed trait in gay people that's not directly related to sex. It's not sex, it's play. When you get to adulthood, these things become blurred. It's easier to tell a gay kid than a gay adult--kids are much of a muchness. Most gay men, even those who are very macho as adults, recall at least some gender-atypical behavior as children."

The assumed cause - hormonal changes in the womb - is pure speculation. But even if true, this supports the 'free choice' theory of sexuality. When we are young, we are less aware of our choices. (In the Church for example, children are not considered accountable until the age of eight. In the legal system it is often later.) So it is notable that young children could be manipulated by their hormones, but adults show more freedom of choice.

Damage in the earliest stages

"[One theory is] what Green refers to as male "vulnerability" during the process of sexual differentiation. A considerably larger number of male embryos come into existence than female embryos, and yet males and females come into the world in about the same numbers. Therefore, phenomena linked to sex must reduce the number of males who survive to term. Many disorders are, in fact, more common in men than women, and some of these could result from problems originating in masculine differentiation. Although good statistics do not exist, it appears that there may be two gay men for every gay woman, which would be consistent with the vulnerability theory."

Is this the publicity that gay men want? That they are basically damaged, babies who narrowly escaped the 'reject' bin?



What if, one day, a biological influence was discovered?

This has not happened yet, but who knows? Perhaps one day something will be discovered. But if it exists, it must be very weak - or else all identical twins would be always both gay or both straight. Clearly the biggest factor is choice.

If a biological predisposition to homosexuality was discovered, it would be no different in principle from a pre-disposition to be athletic, or obese, or alcoholic, or musical. It would be easier to go one way and more difficult to go another way, but the most powerful influence is still free choice. As someone who is currently on a diet (I need to lose 42 pounds - and keep it off) I know what it is like to have a deep-felt desire to put on weight. Since everyone in my family seems to be the same, I assume there is some biological influence. But I also know that by changing my lifestyle I can choose for myself. Even though dieting can be psychologically damaging, being fatter does not necessarily mean being happier.

The bottom line

The important question is not "is there a biological influence?"
The important question is "is anything wrong with it?"


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