Ask this question, and you will probably receive one of two responses: Yes. People choose to be gay. They are making an immoral choice, which government should discourage. Or No. Sexual preference is biologically determined. Government should protect gay people from discrimination because homosexuality is an unalterable aspect of their identity. These two answers have something in common: With both of them, the science conveniently supports the moral decision. "Being gay is bad. How wonderful it is that nobody has to be gay!" "Homosexual behavior should be allowed to take place. Isn't it fantastic that, by an amazing coincidence, there is no way to stop it?" What if neither answer is right? Perhaps sexual preference can be changed - and people have the right to engage in gay sex and have homosexual relationships if they choose to do so. (The fourth option, that gay people have no choice but to be gay, but should be punished for it anyway, is morally unthinkable.) What does science tell us about sexual preference? Genes We know, from many twin and adoption studies, that sexual preference has a genetic component. A gay man is more likely than a straight man to have a (biological) gay brother; lesbians are more likely than straight women to have gay sisters. In 1993, a study published in the journal Science showed that families with two homosexual brothers were very likely to have certain genetic markers on a region of the X chromosome known as Xq28. This led to media headlines about the possibility of the existence of a "gay gene" and discussions about the ethics of aborting a "gay" fetus. There have also been headlines about an "alcoholism gene", which makes people become alcoholics, and a "warrior gene", which makes people unusually aggressive. Genes can't control behavior completely, though. Genes regulate the production of amino acids, which combine to form proteins. The existence or absence of a protein can have an effect on things like alcohol tolerance or mood. Affecting something is not the same as having complete control over it. Environment, like genetics, plays an important role in how our behavior develops. Alcoholism runs in families not only because there is a genetic component to alcoholism, but also because children learn how to cope with stress by watching how their parents and their older siblings behave in stressful situations. If you come from a culture where alcohol consumption is forbidden, it will be difficult for you to become an alcoholic, no matter how your body metabolizes alcohol. There are factors besides a "warrior gene" that contribute to aggression. Children learn to behave aggressively when they witness aggression being rewarded. If you grew up in a family or as part of a culture where aggression was not well accepted, you would be less likely to be aggressive. You would learn, from an early age, how to control your aggressive tendencies. Your environment affects your sexual and romantic relationships. Throughout history, marriages have been influenced by family relations and by economic needs. People adhere to cultural constraints of monogamy despite being attracted to people other than their spouses. Your culture affects your views on homosexuality. In some societies, homosexuality is accepted, in others, it is frowned upon but tolerated, in yet others, it is a serious criminal offense, possibly punishable by death. Male homosexual behavior was expected in ancient Athens. Today, ritual male homosexuality plays an important role in some cultures in New Guinea. Your upbringing can influence what you find desirable and what you find repulsive. Most Americans would be probably be nauseated if they learned that, when they thought they had been eating beef, they were, in fact, eating dog, even though there is nothing inherently unhealthy about dog meat. What you have learned about homosexuality as you were growing up will affect whether you consider engaging in homosexual acts to be desirable or disgusting. Some people might argue that if you are "genetically gay" but the thought of homosexuality nauseates you, then you just haven't accepted the fact that you really are gay. That argument is based on the assumption that sexual preference is purely biological; therefore, it has no place in a discussion about the possible causes of homosexuality. The Brain The structure of the brain might influence sexual preference. In 1991, a study published in the journal Science seemed to show that the hypothalamus, which controls the release of sex hormones from the pituitary gland, in gay men differs from the hypothalamus in straight men. The third interstitial nucleus of the anterior hypothalamus (INAH3) was found to be more than twice as large in heterosexual men as in homosexual men This study was criticized because it used brain tissue obtained at autopsies, and all of the homosexual subjects in the study were believed to have died of AIDS. A later study, which was performed in 2001, showed that HIV status has no significant effect on the INAH3. This study, which also used brain tissue from autopsies, did not reveal any significant difference between the size of the INAH3 in gay men and straight men. It did, however, show that in gay men, neurons in the INAH3 are packed more closely together than in straight men. PET and MRI studies performed in 2008 have shown that the two halves of the brain are more symmetrical in homosexual men and heterosexual women than in heterosexual men and homosexual women. These studies have also revealed that connections in the amygdalas of gay men resemble those of straight women; in gay women, connections in the amygdala resemble those of straight men. The amygdala has many receptors for sex hormones and is associated with the processing of emotions. Some studies have shown that the corpus callosum - the main connection between the two halves of the brain- has a different structure in gay men than in straight men. However, other studies have found no difference. Gay women and gay men are more likely to be left-handed or ambidextrous than straight women and straight men, according to a number of different studies. Some researchers have suggested that this difference in handedness - preference for one hand over the other can be observed in fetuses - is related to differences in the corpus callosum. A 1992 study showed that the anterior commissure, a smaller connection between the brain's two hemispheres, is larger in homosexual men than in straight men. However, according to a study that was performed ten years later, the size of the anterior commissure is not affected by sexual orientation. We know from studying rats that exposure to sex hormones in the womb during a critical period in brain development affects future sexual orientation. By manipulating hormone levels during this time, scientists can make rats engage in homosexual behavior later on. So your brain was influencing your sexual preference even before you were born. This can explain why many gay people feel that they have always been gay. Brain development does not stop at birth, though. A large amount of brain development takes place during childhood, when you are learning many new things - including how your family and the adults around you believe you should feel about things and what they believe is acceptable behavior. The education you receive as a child strongly affects how your brain will develop as you grow. For example, children who are given musical training experience changes to areas of the brain associated with hearing and motor control. With the right experiences, your brain can change even after you have reached adulthood. Both London taxi drivers and professional piano tuners show increases in gray matter in areas of the brain associated with the skills needed for their professions. The size of the increase in gray matter correlates with the numbers of years of experience. In one experiment, elderly subjects showed increases in gray matter in certain parts of their brains after they were taught to juggle. With proper rehabilitation, people who have suffered brain damage from strokes can develop new neural connections and regain some of their old skills. It's important to point out that the regions of the brain that have been shown to change because of training and experience are not the parts of the brain that have been associated with sexual preference. However, women do experience changes to the structure of the hypothalamus - which is thought to be associated with sexual orientation - throughout the menstrual cycle. So far, attempts to "cure" homosexuality by operating on the brain - homosexuals were once given lobotomies - have never worked. (Attempts to eliminate homosexuality via hormone therapy haven't been effective either. While changes in hormone levels in the womb during a very specific time can have an effect on future sexual preference, hormone levels have no effect on sexual preference afterwards. Gay men and straight men have the same levels of sex hormones; sex hormone levels are the same in gay women and straight women.) Today, however, we know much more about the brain than we did when homosexuality was considered a disease that required treatment, and the amount of knowledge that we have about the brain is increasing. Perhaps one day we will be able to adjust sexual preference via surgery - focusing on the particular regions of the brain that are associated with sexual preference - or via neural implants or training. If Sexual Preference Can Be Changed Even if gay people can never stop being attracted to members of the same sex, they can learn not to act on their desires. People already learn to stop smoking, to give up certain foods, and not cheat on their husbands or wives. If we define being gay as engaging in homosexual behavior (the concept of "gay" as an identity is a Western cultural concept - people who have sex with both men and women may call themselves gay, straight or bisexual, depending on the rules of their culture or subculture), then people stop being gay as soon as they stop engaging in this behavior. Should they stop? If they could, should they change their brains (or have their brains changed) in order to make themselves straight? I believe that people have the right to engage in any behavior that they choose, as long as their actions do not harm others, and I believe that gay sex and gay relationships do not cause harm to anyone. Therefore, people who are gay by choice have the right to remain that way (Of course, there are abusive and unhealthy gay relationships that should not be tolerated, just as there are unhealthy heterosexual relationships that should not be tolerated.) If sexual preference can be altered, then people who support gay rights can't rely on the argument that gay people should be protected from discrimination because gay people have no choice but to be gay - an argument that seems like an apology for homosexuality, as if homosexuality is a disease for which there is no cure. There is an element of homophobia in that argument- the implication that gay people would become straight, if only they could. Supporting gay marriage becomes equivalent to supporting the construction of wheelchair ramps. The "gays can't help being that way" approach is reminiscent of the old view of homosexuality as a psychiatric illness. In a blog post for Slate, J. Bryan Lowder comments on Cynthia Nixon's claim that her lesbianism is a choice. Lowder agrees with Nixon that blaming biology "cedes a great deal of control to bigoted people." You don't have to defend a controversial action by arguing that you have no control over your behavior. In fact, when we you do so, you reinforce the belief that your behavior is undesirable. Nobody has to prove that biology forces them to vote for a particular political party, practice a certain religion or follow a particular diet. Just as gay people who are happy as they are should not be forced to change their sexual orientation, gay people who want to be straight should have the right to change if they can - and the correct word is "change" - not "cure". In his blog post, Lowder states, "Many critics will argue that appealing to biology is the only way to protect against the attacks of the religious right." It might make these critics unhappy to hear this, but that's not how science works. Science doesn't change in order to support political opinions. Scientific beliefs change as we gain new information, and sometimes science tells us things that we would rather not hear. Get used to it. References: Bailey, J.M. & Pillard, R.C. (1991). A genetic study of male sexual orientation. 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Annals of the New York Academy of Sciences, 1169: 182-186. Johannson, B.B. (2011). Current trends in stroke rehabilitation: A review with focus on brain plasticity. Acta Neurologica Scandinavica, 123(3): 147-159. LeVay, S. (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253(5023): 1034-1037. Maguire, E.A. et al. (2000). Navigational-related structural change in the hippocampi of taxi drivers. Proceedings of the National Academy of Sciences USA, 97(8): 4398-4403. Prinz, J. (2012). Beyond human nature: how culture and experience shape our lives New York: Penguin Group USA. Teki, S. et al. (2012). Navigating the auditory scene: an expert role for the hippocampus. Journal of Neuroscience, 32(35): 12251-12257. Whitam, F.L., Diamond, M. & Martin J. (1993). Homosexual orientation in twins: A report on 61 pairs and three triplet sets. Archives of Sexual Behavior, 22(3): 187-206. Photo credits: Vancouver Gay Pride Parade 2008 by ecodallaluna on Wikimedia Commons; DNA by ynse on Wikimedia Commons; Brain fMRI by NASA.