Countless studies have been published in recent years studying the factors that form individuals' sexual orientation and gender identity. Here's some of the most groundbreaking research into the genetics, brain activity, and arousal patterns of gender and sexual minorities.1. Prenatal Bloodstream Effects
A study published in December tried to explain why gay men on average have a greater number of older brothers than heterosexuals. The work, published in PNAS and authored by Brock University psychologist Anthony Bogaert, found an explanation of this trend in the bloodstream. Some pregnant mothers create antibodies in their bloodstream in response to a protein linked to the Y chromosome, but if there's a buildup of such protein because of a mother carrying more than one boy, it can pass through to the fetus and impact brain development, which in turn affects sexual orientation.2. Fraternal Birth Order
Bogaert's recent work furthers 2006 findings published in PNAS that showed fraternal birth order provided a significant indicator of whether male children were ultimately gay. That study found that it didn't matter if brothers were raised separately for the findings to bear out.3. The Right Genes
A study published by Nature last year found the presence of two genes in the human genome of gay men (and their mothers). The study, by Alan Sanders of NorthShore University HealthSystem Research Institute, focused on men of European ancestry. The work offered some of the most sound evidence yet that sexuality gets determined in utero.4. Separation Anxiety
A study by University of Lethbridge researchers released early in 2017 found common genetic similarities among gay men across cultural lines. The study, which focused largely on a particular population in Mexico, also found that gay cisgender men and gay trans men disproportionately shared feelings of childhood separation anxiety, something that further indicates biological similarities among homosexual men, according to Scientific American.5. Female Steroids to the Brain
The exposure of female-typical levels of sex steroids to male fetus can "feminize" multiple areas of the brain, including those related to sexual orientation, according to a 2009 study published by University of New Mexico researcher Marco Del Guidice.6. Twinsies
A study published in Nature in 2015 by Eric Vilain's lab at the University of California, Los Angeles, led by Tuck Ngun, looked at twins and family trees for gay subjects and found that if one twin is gay, the other has a 20 percent to 50 percent likelihood of also having a same-sex orientation, which shows genetics play a major part in explaining sexuality but may not be the only factor, as twins share a genome. In search of explanation, the study found five epigenetic markers that were more common among the gay men than their straight counterparts.7. Antagonistic genetics
A 2012 study published in the Quarterly Review of Biology looked at those epigenetic markers that get passed between generations (most don't get passed on) and found that when markers are sexually antagonistic, it mosaically feminizes offspring with XY chromosomes (males) and masculinizes those with XX chromosomes (females). This in turn influences same-sex attraction.
8. Unlocking the Gay Gene
In a widely publicized TED Talk in 2016, Dr. James O'Keefe presented his own research and posited a slightly different theory, that most human beings have a "gay gene," but the variable will be if a mother's epigenetics unlock that in response to overpopulation concerns, which might explain why a mother with several other children would see her youngest offspring come out as gay. He compared it to other natural phenomena, such as when a queen ant produces more warrior ants or worker ants based on the needs of the colony.
9. Why Straights Have Gay Kids
A study by Giorgi Chaladze at the Ilia State University in the republic of Georgia, published by Archives of Sexual Behavior in 2016, found a likelihood that a gay gene in fact does get passed on through generations even if it doesn't mean every carrier ends up feeling same-sex desire. In fact, Chaladze found that about half of all straight men and women likely carry the gene.10. It's Literally In Your Head
The Stockholm Brain Institute in 2008 published work in PNAS that showed similarities in the brains of gay men and lesbians that were not prevalent in straight counterparts. Specifically, the study found the same "sex-atypical" amygdala connections, strongly indicating a neurobiological explanation for sexual orientation.11. Female Sexual Fluidity
University of Utah researcher Lisa Diamond, who has studied female sexuality for decades, found evidence that particularly among women, sexuality can change significantly over time. Her work has noted women are more likely than men to be aroused by members of both genders. The lesbian academic's controversial research was explained in her 2008 book Sexual Fluidity: Understanding Women's Love and Desire.12. Straight Women Not the Norm?
A controversial study published in 2016 by the Journal of Personality and Social Psychology found that the majority of women are either lesbian or bisexual, and that those who identify as lesbian exhibit more masculine traits. The findings showed a contrast with men, who were more likely to exclusively feel sexual attraction toward a particular gender.13. Yes, Bisexual Men Exist
Even with bisexuals standing as the largest segment of the LGBT community, the notion that bisexuality doesn't exist persists. A 2011 study published in Biological Psychology arguably debunks this by studying the physical arousal of gay, straight, and bisexual men in reponse to various types of pornography. Indeed, it found that gay men physically reacted to gay porn and straight men to straight porn, but not vice versa. And of course, it found that bisexuals were aroused to varying degrees by porn with straight sex or gay sex. Perhaps most significantly, bisexual men reacted more strongly than straights or gays when watching pornography where individuals had sex with both men and women, such as threesomes.14. Phantom Sex Parts
And what of the science behind being trans? A groundbreaking study by Dr. Laura Case, published in 2017 in the Archives of Sexual Behavior, found evidence that gender dysphoria may be tied to neural representation of the body and altered white matter connectivity in the brain, which causes a discomfort with existing sexual organs or a sensation of missing nonexistent organs that's similar to the phantom sensation amputees feel after losing a limb.15. Transgender Brainwaves
There's evidence as well that individuals who identify with a different sex than their birth gender do so because of brain activity -- transgender women have brains similar to those of cisgender women, and trans men show similarities to cis men. Psychobiologist Antonio Guillamon of the National Distance Education University in Madrid and neuropsychologist Carme Junqu Plaja of the University of Barcelona published a paper on the subject in 2013 after conducting MRI scans of trans individuals, male and female, both before and after transition and gender-confirmation hormone treatment, according to Scientific American.16. Scent of a Transition
Gender identity extends to all of the senses, according to a 2014 study by Sarah M. Burke of VU University Medical Center in Amsterdam and biologist Julie Bakker of the Netherlands Institute for Neuroscience. Researchers studied the reaction by young subjects diagnosed with gender dysphoria to an odorous steroid known to cause a different pheromone-like reaction in males and females. Data shows that responses lined up with the experienced gender of individuals, as opposed to birth gender.