Historical Timeline of Scientific Research on Sexual Orientation

Science Research

• In 460-370 BCE: Hippocrates (Greek physician known as the “Father of Medicine”), studied sexuality, but never mentioned same-sex behavior. Which is interesting, since same sex behavior was common and considered part of social norms in Greece at the time, especially between adult males and young boys. Same Sex Behavior was just considered a “preference” or “taste”.

• In 129-216 BCE, Galen (Greek physician of the Roman Empire) was considered to be one of the most accomplished of all medical researchers of antiquity, Galen influenced the development of various scientific disciplines, including anatomy, physiology, pathology, pharmacology, and neurology. While he did not study same sex behavior, his writing supported the beliefs of his time, which were that the male body was the norm and superior, while the female body was a deviation and inferior. The male was “dominant” male, and female was “passive”. In male-male sexual encounters, one would take the dominant role (the superior one), while the other took the passive role (the inferior one, which was acceptable for slaves, women, or young boys).

• In 1758 Samuel-Auguste Tissot (Swiss Physician), studied and wrote about sex and stated that masturbation could lead to a range of illnesses, including epilepsy, insanity, gout, rheumatism, and blurred vision. He also did not study same sex behavior.

• In 1864, Karl Heinrich Ulrichs (gay writer, lawyer, and pioneer of the scientific study of human sexuality), is known to be the first person to publicly write about same-sex relationships. He coined the term “urning” to describe men who were sexually attracted to other men and stated that it was innate and part of nature. His ideas laid groundwork for later understandings of sexual orientation as an identity rather than merely an act. It’s interesting to note that this is the first time there is any research on the subject, even though there is evidence of same sex relationships since 2500 BC.

• In 1869, Karl-Maria Kertbeny (Hungarian journalist, translator, memoirist, and human rights campaigner) was the first to coin the word “homosexuality“, shifting the understanding of same-sex desire from an action to an identity. It also created a new binary understanding of sexuality, with “homosexuality” and “heterosexuality” as opposing polarities. He based his conclusions on Hungarian nationalism, which was created out of culture and language rather than race and biology. 

• In 1870, Karl Friedrich Otto Westphal (German psychiatrist) coined the term “sexual inversion,” believed to be an inborn reversal of gender traits. A time when homosexuality was a criminal offense in most jurisdictions, the emergence of the theory of sexual inversion marked a turn in the conceptualization of same-sex sexual behavior from vice to congenital disposition.

• In 1886, Bénédict A. Morel (French psychiatrist) developed the Theory of social degeneration, and included homosexuals in the list of psychiatric disorders.

• In 1886, Richard von Krafft-Ebing (Austro-German psychiatrist),  was the first to explore sexual perversity, He was the first to use the word  “heterosexual”, a term that referred to emotion, desire, and acts, but not yet to a person or their identity. He proposed that homosexuality was an innate hereditary disease, a neuro-psychopathic disorder, or nervous disease. He believed that homosexuals were not responsible for their “malformation” and that homosexuality was not contagious. His work influenced Freud, who went on to treat this “abnormal” sexuality.  

• In 1897, Havelock Ellis (English Physician), argued that “sexual inversion” (his term for homosexuality) was a natural variation of the human sexual instinct, and that it should be treated as a natural phenomenon, not a disease or a sin. He used examples from history and anthropology to show that homosexuality was present in many cultures. He also cited studies that found that children can experience a wide range of sexual aptitude. 

• In 1912, Ernst Haeckel (German biologist) is known to have provided scientific evidence for the idea of latent bisexuality in all people, that same sex attraction was an innate condition, and argued that same-sex sexual acts should be legalized.

• In 1914, Magnus Hirschfeld (German physician and sexologist), published his findings that sexual orientation was innate and not a deliberate choice, and he believed that scientific understanding of sexuality would promote tolerance of sexual minorities. He conducted extensive research on homosexuality, gender identity, and transgender. He believed that scientific research could help reduce prejudice and promote acceptance.

• In 1921, Edward Kempf (American psychiatrist, psychologist, psychopathologist), coining the term “homosexual panic” as the state of acute anxiety and distress experienced by individuals when realizing they have same-sex attraction.

• In 1933-1945, the Nazi Regime ransacked the Institute for Sexual Science in Berlin (which provided counseling for homosexuals) and burned most of its 12,000 books. Gay men had no place in the Nazi vision as they did not enable growth of the Aryan population. Hitler criminalized homosexual acts and an estimated 50,000 received severe jail sentences in brutal conditions (hard labor, torture, castrated, experimented upon or executed). An estimated 10-15,000 men went to concentration camps. During the 1935 redrafting of Paragraph 175 in Germany, there was much debate about whether to include lesbianism, which had not been recognized in the earlier version. Ultimately lesbians and trans people were not included in the legislation, and they were subsequently not targeted in the same way as gay men.

• In 1935, Sigmund Freud (Austrian neurologist and founder of psychoanalysis), believed that everyone was bisexual at birth, and that homosexuality was a variation of what would be a normal progression towards heterosexual (due to a child’s unresolved issues of desire for the opposite sex parent, or jealous and anger towards same sex parent). He believed that it was difficult to change it once it happened. He wrote: “it is nothing to be ashamed of, no vice, no degradation; it cannot be classified as an illness.” His ideas were both influential and controversial in shaping 20th-century attitudes towards sexual orientation.

• In 1935, Louis Max (UK psychiatrist ) was the first to use chemical and shock therapy to reverse homosexuality (explained below). This went on for years. The scientific community, medical and mental experts now widely agrees that all conversion therapy is ineffective and can cause significant psychological harm. Many jurisdictions around the world now have passed laws against conversion therapy. 
  
  • In chemical therapy:  an emetic was injected by the nurse, and as nausea took hold, the subject was confined for days, tormented by tapes, and given pictures of an unclothed male. The patient was confined to his room and left surrounded by his vomit, his latrine bucket un-emptied too, so that the link could be forged in his subconscious between his sexual preference and physical squalor.  
    
  • In shock therapy: electrodes were administered to wrists, calves and feet and shocks were given when a picture of an unclothed male was displayed and the subject was asked to fantasize about a male. Alternatively, the entire floor would be covered with an electric grid, with the current passing through the feet when arousal was detected. In both types of treatment, it was thought that sleep deprivation and provoking emotional crises made subjects more easily persuadable.

• In 1940, Sandor Rado (Hungarian psychoanalyst), claimed homosexuality was mental “phobic avoidance” and not an innate trait. He believe it was caused by childhood experiences, particularly a fear of the opposite-sex parent.

• In 1944, Edmund Bergler (American psychoanalyst), wrote that homosexuality was not caused by hormonal or other biological factors. Instead, he believed it involved unsolved conflict with the mother during infancy. 

• In 1947, Alfred Charles Kinsey (American sexologist, biologist), founded what is now known as the Kinsey Institute for Research in Sex, Gender, and Reproduction. Kinsey believed that sexuality is a continuum that ranges from heterosexual to homosexual, rather than being limited to those two categories. He is best known for his Kinsey Reports, as well as for the Kinsey scale. Kinsey’s research on human sexuality, foundational to the field of sexology.
  
  • The first Kinsey report: “Sexual Behavior in the Human Male” (1948), based on interviews with around 5,300 men and presented findings that challenged conventional views about sexuality.
  • The second Kinsey report: “Sexual Behavior in the Human Female” (1953), based on interviews with about 5,900 women.

• In 1948, Kurt Freund (Czech-Canadian physician and sexologist) is best known for developing in his research a measurement of sexual arousal in males. After trying to treat 67 men of homosexual tendencies, he advocated against conversion therapy and in favor of the decriminalization of homosexuality.

• In 1952, the American Psychiatric Association listed homosexuality in the Diagnostic and Statistical Manual of Mental Disorders (DSM) as a “sociopathic personality disturbance”. This was immediately criticized by many professionals in the medical, mental health, and social sciences due to a lack of scientific data and they replaced it in 1974 for “sexual orientation disturbance”, then in 1980 they changed it to a diagnosis of ego-dystonic homosexuality”. Evelyn Hooker’s groundbreaking research contradicted the notion that homosexuality was a mental illness, ultimately removing it from the DSM.

• In 1961, Thomas Szasz (Hungarian-American academic and psychiatrist), argued that the same persecution that targeted Jews, gypsies, and homosexuals was used as scapegoats of the community.  Szasz argued that psychiatrics were created in the 17th century to study and control those who erred from the medical norms of social behavior.

• In 1973, Judd Marmor (psychiatrist), is known for his role in permanently removing homosexuality from the list of mental illnesses from the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders, changing a position it had held for nearly a century.

• In 1981, the first reports appeared about a new immunodeficiency disease happening, mostly in homosexual men. A year later, the U.S. Centers for Disease Control adopted the name Acquired Immune Deficiency Syndrome (AIDS). They found it was a virus originated from chimpanzees in West Africa, which was transmitted through unprotected sexual contact, non-sterile needles, and childbirth. The virus was affecting mostly people of color and gay men. In 1986, the name Human Immunodeficiency Virus (HIV) was adopted by consensus. HIV spread through Africa and to other parts of the world over decades.  

• In 1990, Dick Swaab and Michel Hofman (Dutch neuroscientists at the Brain Research at the University of Amsterdam), first report neuroanatomical and brain differences between homosexual and heterosexual men. They found that the suprachiasmatic nucleus (a small cluster of brain cells located in the hypothalamus) was twice as large in homosexual men and contained twice as many neurons. The homosexual brains contained 2.1 times as many cells as the reference group. The same study was done on heterosexual men that died of AIDS, and the conclusion was the same, so AIDS had nothing to do with the results.

• In 1991, Simon Le Vay (British-American neuroscientist) conducted a similar research on 40 brain, but on a different part of the hypothalamus (the “third interstitial nucleus of the anterior hypothalamus (INAH 3).” He also found a difference between homosexual and heterosexual brains. This study found a group of cells in this area of the brain was typically larger in males than in females (both in humans and other mammals) and in gay men, that region of cells was smaller than in straight men (the same as the structure in straight women). The brain theory was confirmed when studying the brains of sheep, gerbils, ferrets, and rats. The team also found that they could manipulate this structural difference by changing the hormone levels that the animal was exposed to when it was a fetus. It did not work when the hormones were manipulated when the animal was an adult. There is a crucial developmental period in the womb when the brain is first assembling itself.

• In 1991, J. Michael Bailey and Richard C. Pillard conducted a research on homosexuality in twins found that: 52% of identical twin brothers were gay (a genetic trend). The study’s findings led Bailey to conclude that genetics likely play a role in sexual orientation to homosexuality between 30% and 70%. This was later traced to actually be connected to the epigene rather than the gene itself(see in 2012).

• In 1993, Dean Hamer of the National Cancer Institute in Bethesda, USA, performed a genome-wide scan of 409 independent pairs, and reported a significant linkage between male homosexuality and a specific region on the X chromosome, called Xq28.

• In 1997, The American Psychoanalytic Association made a statement that due to scientific findings that homosexuality is a normal part of human sexuality, they support gay marriage. The association concluded that homosexuality is not associated with psychopathology and that therapy cannot change sexual orientation. They stated that gay and lesbian relationships are as stable and committed as heterosexual relationships and that same-sex couples are just as fit to raise children as heterosexual couples.

• In 1998, the American Psychological Association officially stated that homosexuality is a normal expression of human sexuality, is not a mental disorder, and is harmful to try to change. 

• In 2002, American Academy of Pediatrics issues position statement that due to research findings, they support adoptions for same-sex couples; APA follows suit with a similar position statement that same year.

• In 2004, Italian researchers conducted a study of about 4,600 people who were the relatives of 98 homosexual and 100 heterosexual men. The researchers concluded that there was genetic material being passed down on the X chromosome which both promotes fertility in the mother and homosexuality in her male offspring.

• In 2005, Ivanka Savic-Berglund (Neurologist at the Karolinska Institute in Stockholm, Sweden) led a team to study the relationship between pheromones and homosexuality and found that the homosexual and heterosexual brain reacted reacted differently to these chemicals compared to common odors, and that the chemicals activate the brain in different ways depending on sexual orientation, suggestion a link between sexual orientation and hypothalamic neuronal processes. 

• In 2008, Ivanka Savic-Berglund (Neurologist at the Karolinska Institute in Stockholm, Sweden) led her team in another study of brain scans of 90 gay and straight men and women, and found that the size of the two symmetrical halves of the brains of gay men more closely resembled those of straight women than they did straight men. In heterosexual women, the two halves of the brain are more or less the same size. In heterosexual men, the right hemisphere is slightly larger. The number of nerves connecting the two sides of the brains of gay men were also more like the number in heterosexual women than in straight men. They found both male and female homosexual persons in her samples had brain activation with pheromones, and amygdala’s connections which resembled those of the opposite sex. Brain scans in utero show a difference between heterosexual brains and homosexual Brains. 

• In 2012, William Rice (researcher of genetics and professor at the University of California) and Urban Friberg (Senior Associate professor at Linköping University), co-authored a study that suggested epigenetics may underlie homosexuality (chemical tags on DNA that turn it off or on). Epigenes are the ones that can activate or deactivate the genes. So it’s not a genetic issue, it’s epigenes that are controlling the sexual orientation in the brain. Epigenetic effects are chemical modifications to DNA that affect gene activity without changing the DNA sequence. These “epi-marks” can be inherited or altered by environmental factors. 

• In 2014, Ray Blanchard (American-Canadian sexologist and professor of Psychiatry, University of Toronto), led a team of research that found birth order has an effect on sexual orientation, due to antibodies that a mother starts producing after having her first born male. Repeated studies showed that every time a woman has a boy, it increases the likelihood that the next son she has will grow up to be gay. To confirm this theory, researchers analyzed mother’s blood of mothers of both gay and straight males.  If a mom has an immunological reaction against the male specific proteins in the womb, the persons sexual orientation in their brain may be diverted from heterosexuality to homosexuality. The study showed that there is an antibody that is realized when the mother is pregnant with a male son, that increases by each pregnancy. This antibody affects the release of hormones that influence the probability that each younger brother is same sex attracted, increasing it by 33 percent per child. They coined it “the older brother effect”.

• In 2015, Tuck Ngun (molecular biologist and researcher), studied the relationship between DNA and sexual orientation. He presented research at the American Society of Human Genetics (ASHG) Annual Meeting that identified epigenetic marks in the human genome that are linked to male homosexuality. Ngun’s research examined DNA methylation patterns in pairs of identical male twins, and found that an algorithm could predict sexual orientation with up to 70% accuracy. 

• In 2016, J. Michael Bailey ( a psychologist and professor at Northwestern University) shared his research regarding Congenital adrenal hyperplasia, the discovery that when girls are exposed in utero to elevated levels of testosterone, which leads to masculinization of their genitals and sometimes to homosexual orientation.

• In 2019, Brendan Zietsch (professor at University of Queensland in Brisbane, Australia), and his colleagues have assembled the largest dataset ever used for this type of studies that showed a clear role of genes in determining same‐sex behavior.

• In 2019, Andrea Ganna (group leader at Institute for Molecular Medicine Finland and a research associate at Harvard Medical School) researches epidemiology, genetics, and statistics. He is a winner of an ERC starting grant and the Leena Peltonen Prize for Excellence in Human Genetics. He wrote an article for Science magazine saying they found that genes account for 8–25% of same-sex behavior. The study also identified five new genetic locations that correlate with same-sex activity. 

• In 2021, Christoph Abé led a team of researchers in affiliation with the Department of Clinical Neuroscience at Karolinska Institutet in Sweden, to study using MRI brain imaging and genetic data from over 18,000 UK Biobank participants. The study showed brain differences between heterosexuals and homosexuals. It found that brain patterns typically used to distinguish between males and females were less pronounced in non-heterosexual individuals, with their regional brain volumes tending to be more similar to those of the opposite sex, supporting the idea that certain brain structures in non-heterosexual individuals may show characteristics closer to the opposite sex than to their own.

• In 2023, Miguel Verdu (researcher at University of Valencia, Spain), led a team In 2023 to study homosexuality in the animal kingdom and found that it has been documented in over 1,500 species. It was most common in carnivores, ungulates, marsupials, rodents, and primates. Also found in birds, reptiles, fish, arthropods, mollusks, and mammals. Research with laboratory mice found that disabling the fucose mutarotase (FucM) gene caused female mice to behave as if they were male as they grew up.  They found that the “behavior was not primarily driven by establishing dominance within a group, but likely serving other social functions like bonding, alliance building, or conflict resolution.”