Some 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.
FOUR WOMEN AGAINST CANCER
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
*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.