by Dan Eden

Update: Are Lesbian's Brains Different?

New Primate Studies!--Androgens cause brain gender!--A must read.

Some interesting facts about being gay:

  • In heterosexual women, the index and ring fingers are usually about the same length. In heterosexual men, the index finger is shorter, on average, than the ring finger. It's one of several differences between the sexes that seem to be set before birth, based on testosterone exposure.
  • About 3 percent of American men and 1.5 percent of women describe themselves as gay or bisexual, according to the National Institutes of Health. Those percentages are three to five times higher among people who have a gay brother or sister.
  • Every older brother a man has increases his chances of being gay. A man with four older brothers is three times more likely to be gay than a man with none. (Blanchard)
  • Lesbians' finger lengths were, on average, more like men's. The same holds true for other traits, like eye-blink patterns and inner-ear function.(Breedlove)
  • 75 percent of young boys who dress up like girls, play with dolls and consistently choose stereotypical female pursuits will grow up to be gay. A similar, though less pronounced, pattern is found in girls who prefer trucks over tea sets.
  • Mothers and aunts of gay men had more offspring than female relatives of heterosexuals.
  • In animal studies, about 8 percent of rams never father offspring because they only have eyes for other males. Australian sheepherders call them "shy breeders." (Roselli).

So what's going on here...

Since my youth, I have been asking questions and seeking answers. "Why is the sky blue?" "Why do we have to die?"

These simple inquiries usually went unanswered and caused a distinct wrinkle in the forehead of my parents; nevertheless, as I grew older and wiser, the answers were available in the form of scientific explanations. As a young adult I began to ask other questions which seemed to allude scientific explanations. One of the most perplexing, for me, was why some people were homosexual.

I am not homosexual. I state this not as an excuse or out of fear for being labeled as such. It is simply a fact. But in my nearly fifty years of life I have known many homosexual people and have had many friends who were homosexual-- some secretly and some openly. As a mental health worker in my youth, I counseled many homosexuals and was able to witness the painful humiliation and blatant disrespect that most people afforded individuals who preferred their own gender. It always seemed an oxymoron to call them "gay." Despite their own acceptance of who and what they were, they continued to struggle against the name calling and ridicule that came from even the clergy and so-called "moral" people.

One of my closest friends, Michael, once confessed to me that he first "knew" that he was homosexual when he was just ten years old. He vividly recalled watching his mother put on makeup and style her hair in front of her boudoir and later he would play with her clothes. He was not interested with the things that usually amused little boys. When he was older and entered the military service, he became emotionally and physically attracted to a fellow recruit who was openly homosexual. This event validated his suspicions that he was somehow "different" from other males.

The fact that such feelings could be cognizant at the age of ten always bothered me. I had been taught that homosexuals made the choice to be such-- or that they were somehow coerced into homosexual identity by an adult at some delicate stage in sexual development. In subsequent conversations with other gay men, I learned that almost everyone had a similar revelation at some point in their early life. Hardly anyone actually made the free choice to be homosexual. In fact, studies confirmed that almost 95 percent of homosexual men were adamant that they were born gay.

I learned that there were homosexuals as far back as recorded history. Historians (Gore Vidal in particular) suggested that there has always been, more or less, a constant 10 to 15 percent of males that have preferred their own gender. Once again the question puzzled me: was homosexuality an aberration, an evil choice, a disease-- or could it be a natural phenomenon?

While watching a news story on brain research this year, I was surprised to learn that scientists had determined that the characteristics which make us "think" either as males or females was localized in a small portion of the brain called the hypothalamus. Autopsies showed that the relative size and configuration of this master gland is different in males and females. Further research indicated that the hypothalamus in homosexual men was significantly different from that of "straight" (heterosexual) men (see Science, 253: 1034-1037, 1991). To clarify this story, I contacted Dr. Barry Kosofsky, Chief of Neurology at Harvard Medical School's Massachusetts General Hospital in Boston.

Doctor Kosofsky [pictured right] warned me that this was a controversial issue and that he would only state what he knew to be true. He wanted it understood that his answers, although enlightening me on the neurological aspects of the hypothalamus, should not be taken as an affirmation of any theories relating to homosexuality. "Fair enough," I said. He then confirmed that my recollection of the scientific bulletin was correct. It does appear that brains, at least the hypothalamus, are gendered.

Quickly, I sent him another question regarding embryonic development of the brain. "Is it true that the hypothalamus is formed in the first few weeks of pregnancy?" His response was affirmative, "Most parts of the brain are formed in the first trimester..." (i.e. the first three months of pregnancy).

Embryology teaches that early embryos all start out as female. At some point in early gestation, if the chromosomes destine the fetus to be male, this female embryo is altered by the genetically programmed addition of certain hormones, called androgens. These androgens, especially testosterone, instruct the embryo to develop male characteristics. In their absence, the embryo continues to develop into a female.

An "XX" pair of chromosomes will yield a female; an "XY" pair will result in a male. The "X" is always contributed from the mother (since she has only "X's"), but the father can contribute either an "X" or a "Y"-- so it is the father's genetic contribution that determines the gender of the child. Homosexual men have "XY" pairs which are typically male in all respects.

If all homosexual men have the same chromosomes as heterosexual males, what makes their hypothalamus different? This inquiry received no definitive commitment from my learned friend at Harvard, but he did refer me to some studies where I could find the answer.

In a paper published almost a quarter of a century ago, a research psychologist at Villanova University was also puzzled about gender. Dr. Ingebog Ward was studying the sexual behavior of rats, years before the role of the hypothalamus was even suspected of gendering human brains. She divided a group of pregnant rats into three groups. Suspecting that something special might be happening in the early stages of pregnancy, she subjected the first group to stress during the first ten days of gestation by irritating the mother rats to bright lights, noise and annoying vibrations. Ten days in a rat's pregnancy corresponds to the first trimester (3 months) of a human pregnancy. The second group was subjected to stress towards the end of their pregnancy, just before birth. The third group was comprised of male offspring from both prenatal stressed mothers and unstressed mothers. These babies were subjected to the same stress producing stimuli.

Dr. Ward then allowed all the males to grow to adulthood without further interference. She then placed each group of males in cages with healthy females to observe if their ability and desire to mate with normal adult females. Here is what happened.

"Abstract: Male rats were exposed to prenatal (i.e. before they were born) or postnatal (after they were born) stress, or both. The prenatally stressed males showed low levels of male copulatory behavior and high rates of female lordotic responding. Postnatal stress had no effect. The modifications are attributed to stress-mediated alterations in the ratio of adrenal to gonadal androgens during critical stages of sexual differentiation. Specifically, it appears that stress causes an increase in the weak adrenal androgen, androstendione, from the maternal fetal adrenal cortices, or both, and a concurrent decrease in the potent gonadal androgen, testosterone."

Parental Stress Feminizes and Demasculizes the Behavior of Males, Science, January 7, 1972 (83-84).

Her findings showed that if a mother is stressed during the early stages of pregnancy, she will release an adrenaline related hormone into her own bloodstream and that of her unborn baby. This hormone, called androstendione, is structurally similar to testosterone, the male hormone. If the baby carries "XY" chromosomes and is destined to become a male, testosterone needs to be active when the Central Nervous System (including the hypothalamus) is being formed. This is the only way that the CNS "knows" to develop along male lines. Because the stress hormone seems to bind to the receptors that would normally be receiving testosterone, there is the delay or blockage of the effectiveness of testosterone, even if it is plentiful.

In 1972, Dr. Ward had no idea that androstendione in male pregnancies would prevent or inhibit the hypothalamus to develop into a healthy male brain, but this stress-related hormone now appears to do just that. The brain makes its gender committment very early in development and, once committed to either male or female, it can not change. The interference with testosterone in the later stages of pregnancy, or after birth, does little or nothing to inhibit primary gender development of the other organs of the body.

In Doctor Ward's own words:

"...The present data support the hypothesis that exposure of pregnant rats to environmental stressors modifies the normal process of sexual behavior differentiation in male fetuses by decreasing functional testosterone and elevating androstenedione levels during prenatal development. During stress conditions plasma testosterone emanating from the gonads decreases while adrenal androstenedione rises. The molecular structure of the two androgens, being very similar, it is postulated that the two hormones compete for the same receptor sites. Since androstenedione is a less potent androgen than testosterone, the decrease in male copulatory ability and increased lordotic potential seen in the prenatally stressed animals of the present study would be expected. The relative difference in potency between testosterone and androstendione has been repeatedly demonstrated.


It is therefore possible that while the body and organs of an animal can be a "male," the brain can coincidentally be "female." This extreme reaction to maternal stress even has a very logical and natural purpose. Sensing that a population is under the stress of crowding or poor living conditions, nature provides this hormonal mechanism as a means to limit population growth and thereby reduce the cause of the stress. Homosexual behavior results in less offspring than heterosexual behavior.

Again, in Doctor Ward's own words:

"The resulting alterations in sexual behavior provide the basis for an effective population control mechanism, since offspring so affected would not possess the behavioral repertoire necessary to contribute to population growth. Thus, the environment, by triggering an adrenal stress response, may control the reproductive capacity of successive generations of differentiating fetuses and, thereby, population size. "


Rats have been routinely used as psychological models for human brain studies for decades. Although no primate studies have attempted to duplicate this effect, it therefore seems fitting to apply these rodent findings to humans. Prenatal stress in early pregnancy seems to be a rational and plausible explanation for male homosexuality and should be viewed as a natural population limiting phenomenon. Personal choice in homosexuality appears to be an insignificant factor in those offspring who are born with a female hypothalamus, encapsulated in an otherwise normal male body.

The social implications of these findings have likely contributed to the lack of primate research or even the replication of Dr. Ward's work with rats. This theory, if accepted at face value, would mean that there are at least three naturally occurring human sexual genders: male, female and homosexual. The problematic explanation (or lack of one) for female homosexuality might well be hidden in some similar biologic mechanism.

Remember that the developing female fetus is expecting no prenatal testosterone. This molecule is only supposed to be present if the fetus is male. But androstenedione produced by maternal stress, closely resembles testosterone. Even a small amount of this molecule during the critical first trimester of pregnancy could be enough to make the developing hypothalamus masculine. So the same mechanism can explain both male and female homosexuality.

Readers should "google" the famous Dresden Study, conducted after World War II. It sought to prove this theory of stress related homosexuality by evaluating the number of homosexual offspring of women who were in their first trimester of pregnancy during the bombings of Dresden, Germany. The results showed that the percentage of homosexual children during this period was significantly higher. Perhaps modern researchers will want to look at Iraq in future years to see if a similar pattern emerges.

Dr. Harry Harlow's famous studies with Rhesus monkeys demonstrated that such things as love and the ability to nurture healthy children was a learned skill that could be altered by post-natal experiences. This non-biologic effect may play a role in female homosexuality and may also be a contributing factor in the degree to which male homosexuality is either expressed or repressed.

So another of life's perplexing questions has, for me, been resolved by science. This consistently occurring phenomenon has been labeled everything from a "terrible sin" to a form of mental illness. It should perhaps, in light of the hypothalamus discovery, be viewed as part of the natural variety of human biology. As such, it could be protected from slander by our strong admonitions prohibiting discrimination based on gender or a congenital disaiity. A violation of any human privileges against males with a "feminized hypothalamus" ought to be seen as an affront to basic civil rights.

As we evaluate new scientific understanding, antiquated prejudices need to be cast off, letting the functions of nature show us how to live in harmony, to foster compassion to devote our attention to more important moral issues. If you seek the truth, you will eventually find it. And the truth, however elusive, will ultimately set us all free.

We ask you to voice your opinion using the comments link at the end of this article.

Terms and Definitions

androgens: Any substance (usually hormones) that produces male characteristics, such as androsterone and testosterone.

chromosomes: In human cells, a linear thread of DNA which transmits genetic information. In man, there are 46 chromosomes, including two (XX or XY) which determine the sex of the organism.

copulatory behavior: In animals, this refers to the apparently innate knowledge of how to mate and is generally referring to the active role of the male. In female animals, this innate behavior is more passive (see lordotic).

embryonic: In animals this term refers to the rapid stages of growth and development that take place in the early weeks (usually from the second to seventh week) in the womb. It is during this important phase that the "blueprints" of the Central Nervous System are formed.

fetus: In humans, the unborn offspring in a stage of development immediately following the embryonic stage (from about the seventh or eighth week through the ninth month of pregnancy).

gestation: A term usually applied to the period of time starting when the egg is fertilized and ending when the organism is born and can survive independent of the mother.

gonadal: In humans, this refers to the testis in males and the ovary in females.

heterosexual: Refers to the attraction of either sex to those individuals of the opposite gender.

homosexual: Refers to the attraction of either sex to those individuals of the same gender.

hypothalamus: A small portion of the gray matter, located at the base of the brain, near the brain stem, this important part of the brain has many functions, including control of body temperature, water retention, food intake, hormone secretion, secondary sexual characteristics and is now believed to be the portion of the brain responsible for sexual characteristics associated with gender and gender preference.

lordotic: The curving of the lower spine and arching of the back usually characteristic of female mammals during copulation.

It should be noted here that in the rat studies conducted by Dr. Igebog Ward, the rats which were subjected to prenatal stress not only failed to show interest in the adult mating experiments but also, when placed with other normal male rats, did not resist being mounted, as normal rats usually do, but exhibited this lordotic posture. This suggests that the masculine behavior had been replaced by female behavior and showed that the rats were not merely rendered asexual or disinterested in sexual stimulation.

prenatal: Refers to the period of time before birth.

postnatal: Refers to the period of time after birth.

testosterone: The hormone usually produced in the male testes which is responsible for secondary male characteristics such as hair growth, muscle development and body shape. It has been suggested that another source of testosterone in embryonic development may reside in the placenta.

trimester: Three months, usually used in reference to the passage of time in pregnancy.

Other Research

harlowSome primate work has been done at the University of Wisconsin's Harlow Primate Research Center on the effects of stress and the importance of maternal care on the sexual behavior of monkeys; however, much of this work measured postnatal nurturing and was not experimentally designed to assess homosexual behavior. A review of this work does suggest that the expression of healthy sexual behavior and successful knowledge of mating can be environmentally inhibited by the lack of parental nurturing. This work indicates a potential source of aberrant sexual behavior and dysfunction rather than simple homosexuality.

Currently, Dr. Simon LeVay, a neurobiologist at California's Salk Institute and founder of the Center for Gay and Lesbian Studies in Santa Monica, California, has been the most vocal proponent for some type of gendered brain influence on homosexuality but his work has unfortunately remained unreviewed by many professionals.

Some of LeVay's works are listed here for recommended reading:

Albrick's Gold - New York: Richard Kasak Books, 1997.

Queer Science - Cambridge: MIT Press, 1996.

City of Friends - Cambridge Press, 1995.

The Sexual Brain - Cambridge: MIT Press, 1993.

Are Lesbian Brains Different?
Study Suggests Difference in Lesbians' Brains
From Dr. Alan Cantwell, MD

WASHINGTON (AP) - Lesbians' brains react differently to sex hormones than those of heterosexual women, new research indicates.

That's in line with an earlier study that had indicated gay men's brain responses were different from straight men - though the difference for men was more pronounced than has now been found in women.

Lesbians' brains reacted somewhat, though not completely, like those of heterosexual men, a team of Swedish researchers said in Tuesday's edition of Proceedings of the National Academy of Sciences.

A year ago, the same group reported findings for gay men that showed their brain response to hormones was similar to that of heterosexual women.

In both cases the findings add weight to the idea that homosexuality has a physical basis and is not learned behavior.

"It shows sexual orientation may very well have a different basis between men and women ... this is not just a mirror image situation," said Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine at McMaster University in Hamilton, Ontario.

"The important thing is to be open to the likely situation that there are biological factors that contribute to sexual orientation," added Witelson, who was not part of the research team.

The research team led by Ivanka Savic at the Stockholm Brain Institute had volunteers sniff chemicals derived from male and female sex hormones. These chemicals are thought to be pheromones - molecules known to trigger responses such as defense and sex in many animals.

Whether humans respond to pheromones has been debated, although in 2000 American researchers reported finding a gene that they believe directs a human pheromone receptor in the nose.

The same team reported last year on a comparison of the response of male homosexuals to heterosexual men and women. They found that the brains of gay men reacted more like those of women than of straight men.

The new study shows a similar, but weaker, relationship between the response of lesbians and straight men.

Heterosexual women found the male and female pheromones about equally pleasant, while straight men and lesbians liked the female pheromone more than the male one. Men and lesbians also found the male hormone more irritating than the female one, while straight women were more likely to be irritated by the female hormone than the male one.

All three groups rated the male hormone more familiar than the female one. Straight women found both hormones about equal in intensity, while lesbians and straight men found the male hormone more intense than the female one.

The brains of all three groups were scanned when sniffing male and female hormones and a set of four ordinary odors. Ordinary odors were processed in the brain circuits associated with smell in all the volunteers.

In heterosexual males the male hormone was processed in the scent area but the female hormone was processed in the hypothalamus, which is related to sexual stimulation. In straight women the sexual area of the brain responded to the male hormone while the female hormone was perceived by the scent area.

In lesbians, both male and female hormones were processed the same, in the basic odor processing circuits, Savic and her team reported.

Each of the three groups of subjects included 12 healthy, unmedicated, right-handed and HIV-negative individuals.

The research was funded by the Swedish Medical Research Council, Karolinska Institute and the Wallenberg Foundation.

Copyright 2006 The Associated Press.

Alan Cantwell M.D.


ScienceDaily (Oct. 26, 2009) -- Prenatal sex-based biological differences extend to genetic expression in cerebral cortices. The differences in question are probably associated with later divergences in how our brains develop. This is shown by a new study by Uppsala University researchers Elena Jazin and Björn Reinius, which has been published in the latest issue of the journal Molecular Psychiatry.

Professor Elena Jazin and doctoral student Björn Reinius at the Department of Physiology and Developmental Biology previously demonstrated that genetic expression in the cerebral cortices of human beings and other primates exhibits certain sex-based differences. It is presumed that these differences are very old and have survived the evolutionary process. The purpose of the new study was to determine whether they appear during the process of brain development or first upon the conclusion of that process. Identifying the initial genetic mechanisms that prompt the brain to develop in a female or male direction is a long-range research objective.

The Uppsala University researchers analysed data, on the basis of sex, from another extensive study of the prenatal human brain.

"The results show that many of the genes situated on the Y chromosome are expressed in various parts of the brain prior to birth and probably provide a developmental basis for the sex-based differences exhibited by adult brains," according to Elena Jazin.

More than a third of Y-chromosomal genes appear to be involved in sex-based human brain differentiation. Some of the genetic activity in question is evident in the adult brain, while other of it only appears at earlier stages of brain development. It is yet unknown whether the differences in genetic expression among female and male brains have any functional significance.

"The findings are consistent with other factors, such as environment, also playing a role in how we develop," emphasizes Elena Jazin.

Knowledge of the development of sex-based brain differences is of potential significance for the treatment of brain disturbances and diseases. A large number of psychiatric illnesses, including depression and autism, affect men and women differentially.

"Taking account of sex-based differences is crucial to the study of normal and abnormal brain activity," according to Elena Jazin.

Some further reading:

Symmetry Of Homosexual Brain Resembles That Of Opposite Sex
*Article in Salon Magazine
*Stressful events in prenatal life of bi- and homosexual men.
*Dresden, German & related studies
*Rat Studies
*Prenatal Stress Studies
*A test of the maternal stress theory of human male homosexuality

Recent Article from ScienceDaily

February 2010: Potential Evolutionary Role for Same-Sex Attraction

Male homosexuality doesn't make complete sense from an evolutionary point of view. It appears that the trait is heritable, but because homosexual men are much less likely to produce offspring than heterosexual men, shouldn't the genes for this trait have been extinguished long ago? What value could this sexual orientation have, that it has persisted for eons even without any discernible reproductive advantage?

One possible explanation is what evolutionary psychologists call the "kin selection hypothesis." What that means is that homosexuality may convey an indirect benefit by enhancing the survival prospects of close relatives. Specifically, the theory holds that homosexual men might enhance their own genetic prospects by being "helpers in the nest." By acting altruistically toward nieces and nephews, homosexual men would perpetuate the family genes, including some of their own.

Two evolutionary psychologists, Paul Vasey and Doug VanderLaan of the University of Lethbridge, Canada tested this idea for the past several years on the Pacific island of Samoa. They chose Samoa because males who prefer men as sexual partners are widely recognized and accepted there as a distinct gender category -- called fa'afafine -- neither man nor woman. The fa'afafine tend to be effeminate, and exclusively attracted to adult men as sexual partners. This clear demarcation makes it easier to identify a sample for study.

Past research has shown that the fa'afafine are much more altruistically inclined toward their nieces and nephews than either Samoan women or heterosexual men. They are willing to babysit a lot, tutor their nieces and nephews in art and music, and help out financially -- paying for medical care and education and so forth. In a new study, the scientists set out to unravel the psychology of the fa'afafine, to see if their altruism is targeted specifically at kin rather than kids in general.

They recruited a large sample of fa'afafine, and comparable samples of women and heterosexual men. They gave them all a series of questionnaires, measuring their willingness to help their nieces and nephews in various ways -- caretaking, gifts, teaching -- and also their willingness to do these things for other, unrelated kids. The findings, reported on-line this week in the journal Psychological Science, lend strong support to the kin selection idea. Compared to Samoan women and heterosexual men, the fa'afafine showed a much weaker link between their avuncular -- or uncle like -- behavior and their altruism toward kids generally. This cognitive dissociation, the scientists argue, allows the fa'afafine to allocate their resources more efficiently and precisely to their kin -- and thus enhance their own evolutionary prospects.

To compensate for being childless, each fa'afafine would have to somehow support the survival of two additional nieces or nephews who would otherwise not have existed. "If kin selection is the sole mechanism by which genes for male same-sex sexual attraction are maintained over time," the fa'afafine must be "super uncles" to earn their evolutionary keep, explains Vasey. Consequently, Vasey suggests "that the fa'afafine's avuncularity probably contributes to the evolutionary survival of genes for male same-sex sexual attraction, but is unlikely to entirely offset the costs of not reproducing."

Do these findings have any meaning outside of Samoa? Yes and no. Samoan culture is very different from most Western cultures. Samoan culture is very localized, and centered on tight-knit extended families, whereas Western societies tend to be highly individualistic and homophobic. Families are also much more geographically dispersed in Western cultures, diminishing the role that bachelor uncles can play in the extended family, even if they choose to. But in this sense, the researchers say, Samoa's communitarian culture may be more -- not less -- representative of the environment in which male same-sex sexuality evolved eons ago. In that sense, it's not the bachelor uncle who is poorly adapted to the world, but rather the modern Western world that has evolved into an unwelcoming place.

Viewzone|| Discussion || Comments? || Body Mind Spirit

New Primate Studies!--Androgens cause brain gender!--A must read.


Friday, June 5, 2009

Brains of gay men show similarities to those of heterosexual women, study reports The brains of gay men resemble those of straight women, according to research being published Tuesday that provides more evidence of the role of biology in sexual orientation.

Using brain scanning equipment, researchers said they discovered similarities in the brain circuits that deal with language, perhaps explaining why homosexual men tend to outperform straight men on verbal skills tests -- as do heterosexual women.

The area of the brain that processes emotions also looked very much the same in gay men and straight women -- and both groups have higher rates of depressive disorders than heterosexual men, researchers said.

The study in Proceedings of the National Academy of Sciences, however, found the brain similarities were not as close in the case of gay women and straight men.

Previous studies have found evidence that sexual orientation is hard-wired. More than a decade ago, neurobiologist Simon LeVay reported that a key area of the hypothalamus, a brain structure linked to sexual behavior, was smaller in homosexual men compared to heterosexual men.

The latest study, led by Ivanka Savic of the Karolinska Institute in Stockholm, was significant in that it looked at areas of the brain that have nothing to do with sexual behavior, suggesting there was a basic biological link between sexual orientation and a range of brain functions.

"The question is -- how far does it go?" said Dr. Eric Vilain, who studies human sexual development at UCLA and was not involved in the study. "In gay men, the brain is feminized. Is that limited to particular areas or is the entire brain female-like?"

Vilain said his hunch was the entire brain was not feminized because "gay men have a number of masculine traits that are not present in women."

Savic and colleagues used magnetic resonance imaging to measure brain volumes of two groups, each divided evenly between men and women: 50 heterosexuals and 40 homosexuals. They knew going into the study that in men the right cerebral hemisphere is largest but in women the left and right hemispheres are of equal size.

The results showed that gay men had symmetrical brains like those of straight women, and homosexual women had slightly asymmetrical brains like those of heterosexual men. Language circuits are thought to be more symmetrical in straight women than in heterosexual men, the report said.

The differences were pronounced. For example, the right cerebral hemisphere in heterosexual men was 624 cubic centimeters -- 12 greater than their left side. In homosexual men, the right hemisphere was 608 cubic centimeters -- 1 cubic centimeter smaller than the left.

In heterosexual women, there was no volume difference between right and left hemispheres. But in homosexual women, their right hemisphere was 5 cubic centimeters larger than the left.

Next, researchers used positron emission topography to measure blood flow in the amygdala, a brain area involved in processing emotions. The wiring of the amygdala in gay men more closely resembled that of straight women than straight men, researchers said. The amygdala of gay women looked more like those of straight men, according to the report.

Savic said she believed the brain differences were forged in the womb or infancy, probably as a result of genetic or hormonal factors. She said she could not explain why the differences were more pronounced in homosexual men than in homosexual women.

Marc Breedlove, a neuroscientist who studies sexual development at Michigan State University, said that in his studies with rats, changes in prenatal levels of testosterone caused the sort of brain alterations Savic observed in her study.