It’s the question that has fueled a thousand dinner table debates and even more scientific white papers. Honestly, if you search for what causes homosexuality in people, you’re going to run into a wall of conflicting noise. Some people want it to be a simple "gay gene" you can find under a microscope. Others cling to outdated ideas about how your parents raised you.
But science doesn't really care about our need for simple answers.
The reality is messy. It’s a mix of biology, timing, and things happening in the womb long before a baby even takes its first breath. We aren't looking at a single "on/off" switch. Instead, we’re looking at a complex mosaic.
The Myth of the "Choice" and the Reality of Biology
Let’s get the big one out of the way. No one wakes up on a Tuesday and decides to change their fundamental attractions. Most people, whether they are straight, gay, or somewhere else on the spectrum, report that their feelings just were. They emerged.
Research consistently points toward a biological foundation. Take the work of Simon LeVay, a neuroscientist who, back in the 90s, looked at the hypothalamus. He found that a specific node—the INAH3—was typically smaller in gay men than in straight men, more closely resembling the size found in women. It was a massive "aha" moment for the scientific community, even if it didn't tell the whole story.
It wasn't just about brain structure, though. It was about how those brains were built in the first place.
Why Genetics Isn't the Only Answer
If it were just down to a single gene, identical twins would be the "smoking gun." If one identical twin is gay, and they share 100% of their DNA, the other should be gay every single time, right?
Except they aren't.
Studies on twins show that if one identical twin is gay, the other has about a 20% to 50% chance of being gay. That’s way higher than the general population, which tells us genes are definitely invited to the party. But because it isn't 100%, we know that the environment—specifically the environment in the womb—is doing a lot of the heavy lifting.
What’s Actually Happening in the Womb?
This is where things get fascinating. And a bit weird.
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The "Prenatal Hormone Theory" is currently one of the strongest explanations for what causes homosexuality in people. Basically, your brain and your body develop at different times during pregnancy. Your physical plumbing is decided early on. But the "masculinization" or "feminization" of the brain happens later, triggered by surges in hormones like testosterone.
If those hormone levels shift—due to maternal stress, a mother’s immune response, or just random biological variance—the brain might develop in a way that doesn't "match" the physical body in terms of typical attraction patterns.
The Older Brother Effect
Ever heard of the Fraternal Birth Order Effect? It’s one of the most well-documented phenomena in human sexuality.
Ray Blanchard, a researcher at the University of Toronto, discovered that the more older biological brothers a man has, the more likely he is to be gay. It doesn't apply to adopted brothers or stepbrothers. Only biological ones.
Why? It’s likely an immune response.
The theory goes like this: When a mother is pregnant with a male fetus, her body perceives certain male-specific proteins (linked to the Y chromosome) as foreign substances. She builds up antibodies. With each subsequent male pregnancy, those antibodies get stronger and can cross the placental barrier, potentially altering the development of the fetal brain.
It’s not a "flaw." It’s just a biological process. It’s estimated that this effect accounts for about 15% to 29% of the gay male population.
Epigenetics: The Bridge Between Nature and Nurture
If genes are the blueprint, epigenetics is the contractor who decides which walls actually get built.
Epigenetic "marks" sit on top of our DNA and tell our genes when to turn on or off. Dr. William Rice and his team proposed that "epi-marks" usually protect a fetus from the natural swings in hormone levels during pregnancy. For example, they stop a female fetus from becoming "masculinized" if her mother has high testosterone.
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However, if these marks are passed down from a parent or occur at a specific time, they can lead to reversed sexual preference. It explains why homosexuality persists in the human population even though gay people historically have fewer children. These marks provide an evolutionary benefit in other areas—like making a woman more fertile—but result in same-sex attraction when expressed in a son.
The "Gay Gene" Study of 2019
In 2019, the largest study ever conducted on the genetics of sexual orientation was published in Science. They looked at the DNA of nearly half a million people.
The result? They found five specific genetic loci (locations) associated with same-sex behavior.
But—and this is a big but—these five markers only explained a tiny fraction of the variation. The researchers basically said that while genetics play a role, there is no single "gay gene." Instead, it's thousands of tiny genetic variants all interacting at once.
It’s a symphony, not a solo.
What About Women?
Interestingly, much of the research on what causes homosexuality in people has focused on men. Male sexuality tends to be more "bimodal"—people are usually either very straight or very gay.
Female sexuality is often described as more fluid. The biological markers, like the older brother effect, don't apply to women. For women, the research points more toward prenatal androgen exposure, but the data is less clear-cut. It reminds us that we can't just apply "male" science to everyone and expect it to fit.
It’s Not About How You Were Raised
For decades, the "refrigerator mother" or the "absent father" theories dominated psychology. People thought you became gay because your dad didn't play catch with you or your mom was too overbearing.
We now know that is basically nonsense.
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If parenting caused homosexuality, we would see clear patterns in different family structures. We don't. Children of gay parents are no more likely to be gay than children of straight parents. Children from broken homes aren't "turning" gay at higher rates.
Your orientation is essentially "baked in" long before you ever learn how to ride a bike.
The Evolutionary "Why"
If being gay means you're less likely to reproduce, why hasn't evolution weeded it out?
This is the "Kin Selection" hypothesis, often called the "Gay Uncle" theory. The idea is that having family members who are gay and don't have their own children provides a survival advantage to the family. These individuals can help provide resources and protection for their nieces and nephews, ensuring the family’s overall genetic line continues.
In some cultures, like the Fa'afafine in Samoa, this is exactly what happens. They are highly valued members of society precisely because they dedicate themselves to their extended families.
Actionable Insights and Moving Forward
Understanding the "why" behind sexual orientation helps strip away the stigma. When we see it as a natural biological variation—like being left-handed or having blue eyes—the "morality" debate starts to look a bit silly.
- Accept the Complexity: Stop looking for a single cause. It’s a mix of polygenic traits (many genes), prenatal environment, and epigenetics.
- Trust the Timing: Recognize that sexual orientation is generally fixed early in development. Efforts to change it (conversion therapy) are not only ineffective but deeply harmful because they try to "fix" something that isn't broken and is biologically ingrained.
- Support the Individual: If you or someone you know is navigating their identity, focus on the psychological well-being rather than the "cause." Knowing why you are the way you are can be validating, but it doesn't change the need for a supportive environment.
- Look at the Science, Not the Politics: Follow researchers like those at the Broad Institute of MIT and Harvard, who are doing the heavy lifting in genomic studies.
The quest to find out what causes homosexuality in people is really a quest to understand the incredible diversity of the human brain. We are far from having every answer, but we know enough to say this: it is a fundamental, natural part of the human experience, written into our biology before we even arrive.
To learn more about the specific genetic markers found in the 2019 study, you can review the data published by the American Association for the Advancement of Science (AAAS). Focusing on the biological reality is the first step in moving past outdated social myths.
Stay curious, and prioritize empathy over simple answers.
Next Steps for Understanding: 1. Review Prenatal Hormone Studies: Look into the work of Dr. Melissa Hines regarding how early testosterone exposure influences later behavior.
2. Explore the Epigenetic Model: Read up on the "Rice-Friberg" model which explains how sex-specific epi-marks can lead to same-sex attraction.
3. Differentiate Between Behavior and Identity: Understand that scientific studies often measure "same-sex behavior" (who people have sex with) vs. "identity" (how people label themselves), which can lead to different statistical outcomes.