The Neuroscience of Same Sex Sexual Partner Preference
Morgan Levy, MD
Levystien Productions
Parker, Colorado
Copyright 2012
2,100 words 

     Hello, I'm doctor Morgan Levy.  I have no personal agenda with regards to homosexual men and women.  This is an educational essay that is designed to further our understanding about brain science and its relationship to human behavior. 

     Most heterosexuals, myself included, feel some degree of disgust when we hear, or even read, the word homosexual.  This feeling typically leads to rationalizations about why homosexuality is wrong and an attempt to get others to agree.  The conscious brain evolved to rationalize instinctual behavior that the sub-conscious brain does, not to logically analyze the universe for the purpose of discovering the truth.  To utilize our newly evolved frontal lobes for the purpose of discovering what is real we must slow down our initial judgments.  The human brain has the unique ability to take a minute and think things through.  We are then able to use some degree of deductive reasoning to get a hint at reality.  Let's try to do that now.

     OK, let's begin.

     If God did not create Adam and Steve and if evolution favors procreation and if the pieces simply fit better between a man and a woman, then why is it that when physiological recording devices are connected to humans 3-4% of boys and 1-2% of girls are sexually aroused by same sex stimuli?


     Perhaps, little boys who want sex with mommy are scared of a jealous daddy cutting off their peckers.  So scared, in fact, that they never want to have sex with any woman.  Yeah, probably not. 

     Conrad Lorenz says, "If I am the first thing that a newborn duck sees, then he will accept me as his mother."  So, the first thing that tickles your pecker will become your soul mate?  No, I would have married my hand.


     Maybe we don't know how the environment that we grew up in causes a few of us to prefer same-sex partners but we know that it does, right?  I mean, isn't our gender identity and partner preference learned from our parents, siblings, and community?  Wait a minute, we grew up with gay parents, gay siblings and in gay communities?  Yeah, right.

     In fact, the scientific literature reveals that gender identity and sexual preference in homo sapiens are determined primarily by four things:  1. genes,  2. hormone exposure at critical periods of development, 3. random variability of human characteristics, and 4. environmental factors that affect gene expression.   We now know with relative certainty that gender identity and sexual preference are not determined by learning and upbringing. 

     The thing of it is, is...Moms, gay siblings, and older gay men don't lure innocent young boys into being gay. 

     As an aside, when I was in college I noticed that, for whatever reason, I am particularly attractive to gay men. In an attempt to increase the amount of sex I was having I thought to myself. "Hey, maybe I should try and be gay?"

     To my chagrin, this is when I realized that homosexuals are not faking it.  They are legitimately aroused by men.  And I am not.  Sorry, Bruce.

     So, back to the four things that are etiologically associated with homosexual behavior.

     First, same sex sexual partner preference in humans is heavily influenced by genetics.  Twin studies reveal that it occurs in 50% of identical twin brothers of homosexual men, 25% of fraternal twin brothers, and 10% of ordinary brothers.  This is in contrast to 3-4% of males in the general population.  This is a pattern that indicates that a trait is genetically inherited, or familial, in a similar manner as is heart disease, eye color, height, IQ, athletic ability, etc. 

     Also, it seems to be passed through maternal lines, at least in males, which would indicate that it may be a sex-linked trait.    

     When you study large numbers of individuals it turns out that numerous gender non-conforming behaviors are highly correlated with sexual preference.  You inherit a position along the continuum of male to female behavioral traits, the exact traits varying considerably, AND homosexual behavior together. 

     That is not to say that you can't be a macho man and still be gay.  We are talking about bell curves in large populations so there is extensive overlap. 

     The flavor of gender non-conformist behavior also tends to be similar in blood relatives and especially in twins.  By "flavor" I mean the exact constellation of gender non-conforming behaviors.  For example, you could say that gay twins are not only both gay but they are both gay in the same way. 

     Finally, same sex sexual partner preference can be induced in animals through genetic manipulation.  For example, Fruit Flies are a strictly heterosexual species but you can easily induce them to do opposite sex fruit fly specific sexual behaviors through specific gene deletion. 

     Second, testosterone exposure at critical times during development is directly related to future sexual orientation.

     There are four critical periods for testosterone exposure.  These are the second trimester, the peripartum period, puberty, and adulthood.  Incorrect exposure to testosterone at any of these times results in period specific changes.  Low "T" in adulthood causes libido to be low but sexual preference and gender identity are usually unchanged.  If it happens at puberty, then gender identity can become effeminate but sexual preference often remains the same.  If it happens in the second trimester or peripartum, then sexual preference and gender identity are frequently both changed. 

     All fetuses start out as female but if you are genetically male then testosterone exposure induces hyperproliferation and/or selective pruning in certain cell groups that result in masculine characteristics such as male genitalia.  In the brain these cell groups are called sexually dimorphic nuclei.  For example, a sexually dimorphic nucleus that is directly involved in the expression of male sexual behavior is called the third interstitial nucleus of the anterior hypothalamus.  It grows 2 to 3 times larger in men than in women. 

     In gay men, the third interstitial nucleus of the anterior hypothalamus is smaller than in straight men but larger than in women.  In gay women, it is larger than in straight women but smaller than in men.  This shift towards the opposite sex is a frequent theme in neuroscience research into homosexual behavior.

     In the lab, mice and guinea pigs have been repeatedly under, and over, exposed to in-utero testosterone at, and not at, critical periods resulting in predictable changes in the size of their sexually dimorphic nuclei that correlate with changes in their sexual behavior.  For example, you can easily induce male guinea pigs to wiggle their butts (the lordosis reflex) at other males and be less aggressive and forage for food like a female and you can easily induce females to mount other females and be more aggressive and forage for food like a male.


The sexually dimorphic area labeled "Reproduction" is the pre-optic area
and the one labeled "Thermoregulation" is the anterior hypothalamus.


     In rams, 8% are dedicated homosexuals and they have the same reduced cell population in their sexually dimorphic nuclei as human male homosexuals do.


     Third, most human characteristics exhibit random variation along a bell-shaped curve.  Thus, homosexuality may not only occur due to a specific disruption at a critical period for testosterone.  It may result from an individual's random position at the far end of the bell curve of testosterone exposure at a critical period. 

     For example, if there is a bell shaped curve for masculine behavior, including sexual attraction to females, next to a bell shaped curve for feminine behavior, including sexual attraction to males, and they cross over so that 20 percent have varying degrees of both behaviors, then some individuals might find themselves in a grey area.  For those individuals things that have a much weaker effect, such as imprinting or other random events, could tip the balance. Or, they could randomly be at the really extreme end of the bell curve.

     Fourth, and finally, environmental factors may affect sexual preferences but not in the way you may have been thinking.  I'm not talking about your mom fiddling with your penis while changing your diaper.  I'm talking about your mom experiencing an illness or other stress while you were at a critical period during your stay in her belly.  This may have affected your exposure to testosterone. 


     Now...some of you may not believe that the human brain is biologically pre-programmed to do certain behaviors.  You might  think that genetics, hormonal exposure, random variability and environmental factors don't determine complex behaviors.  You may think that we have free will and can do whatever we want.  It may be your opinion that we are the only animal that does homosexual behavior and, therefore, this behavior goes against nature.  You may even believe that non-procreative sex does not occur in the animal kingdom and is, therefore, not ordained by the natural order of things...or by God.  I have a doctor friend who is convinced that same sex attraction is not only unnatural but is actually an illness. 

     I totally appreciate that homosexual behavior causes us to experience gut level disgust and that our brains are programmed to collectively rationalize why it is bad.  However, if you can pause with me for a minute and examine the evidence you may alter your opinion.

     The reality is that most domesticated animal species routinely engage in homosexual sex and, despite the fact that it is less common in the wild, dedicated homosexual pairs exist in multiple species.  It turns out that increasing amounts of homosexual behavior in the wild occur in species that are increasingly social.  Of course, homo sapiens are both domesticated and social.   

     For example, Mallard ducks are a very social species and they form pair bonds with up to 19% of these pairs being male-male?


     Highly social penguin colonies usually have multiple homosexual couples that mate for life even when available females are plentiful. 


     Giraffes have homosexual sex in 90% of their sexual encounters.  However, they are polygamous and we can't be certain that any individuals prefer same sex partners. know...I would think.


     Deer do homosexual sex.  However, they are also highly promiscuous.


     Genetically, the closest species on the planet to homo sapiens are the bonobos and the common chimps.  Chimps do homosexual behavior when in captivity but not so much in the wild, or maybe the wild chimps are embarrassed to do it in front of Ms. Goodall. 

     Bonobos are highly sexual in captivity and 50% of sex is between same sex partners.  They seem to interact sexually as a means to avert conflict.  Anytime there is the slightest opportunity for conflict they manipulate each other's genitals.  In this way, male-male sex in bonobos clearly reduces intra-species violence.  When males have conflict they settle it with "bat in hand," so to speak. 


Two female bonobos at the San Diego zoo.

     Notice, I have said nothing about whether or not homosexual behavior in humans is OK.  Sociopathy is just as "natural" in that it is due to a brain alteration but we still can't allow it. 

     When we first grew large agrarian societies we had to set up some rules because our brains were still programmed to do hunting and gathering.  We set up a list of taboos and, for a lot of societies, homosexual behavior was front and center on the list.  Not because it logically belonged there but because of how our brains work when evaluating things.  It caused our collective guts to feel bad so we rationalized that it was bad.  Reality is irrelevant to a human brain but our guts got it right on a lot of other things.  Maybe we should "go with our guts" most of the time but slow down and think about it once and a while.

     Additionally, homosexual behavior may actually have an evolutionary advantage.  There is some evidence that sisters of gay men are more fertile and there are a variety of hypothetical scenarios in which homosexuality may increase survivability. 

     Remember, individuals do not evolve, gene pools do.  Male non-breeding individuals perform functions that increase herd survivability in many species.  For example, in our species homosexuals frequently perform shaman-like functions.  They tend to be teachers, dancers, hair stylists, fashion designers...and priests.  I'm just sayin'.

     Thus, homosexuality is biologically acquired through genetics, hormone exposure, random variation, and environmental factors.  Humans do not choose to be homosexual nor are they influenced by others to be homosexual.  Specific brain structures across species are affected in individuals who prefer same sex sexual partners.  Finally, same sex sexual partner preference is one of a group of behaviors that are altered by these neurological changes.




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