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Study 15 into the causes of homosexual orientation

More detailed coverage of a relatively
conclusive study involving brain scans

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This is a continuation of a previous essay

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  • Andy Coughlan: "Brain scans have provided the most compelling evidence yet that being gay or straight is a biologically fixed trait." New Scientist, 2008-JUN-16 1
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Study 15: Based on brain scans: Overview:

Swedish researchers led by Dr. Ivanka Savic of the Stockholm Brain Institute, at the Karolinska Institute used neural magnetic resonance imaging (MRI) on a group of both heterosexual and homosexual men and women. They found key similarities

  • Between the brains of gay males and heterosexual females, and

  • Between the brains of lesbians and heterosexual males. 2

Their findings were published in the Procedures of the National Academy of Science on 2008-JUN-16. 3

Their article abstract concludes:

"The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities." (Emphasis ours)

That is, homosexual and heterosexual orientations seem to be linked to pre-birth brain structures, and are not caused by factors after birth, including parenting styles, sexual molestation during childhood, etc.

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The study:

BBC News reports that:

"Scientists have noticed for some time that homosexual people of both sexes have differences in certain cognitive abilities, suggesting there may be subtle differences in their brain structure [when compared to heterosexuals]." 4

However, as the New Scientist describes, there have been weaknesses in previous brain studies:

"Previous studies have also shown differences in brain architecture and activity between gay and straight people, but most relied on people's responses to sexuality driven cues that could have been learned, such as rating the attractiveness of male or female faces. 1

Researchers in Stockholm conducted a study to search for sources of the cognitive differences between homosexuals and heterosexuals by measuring brain structure directly, thus bypassing the possibility of learned cues. They used MRI measuring techniques to scan the brains of 90 healthy adults. The subjects included lesbians, gay males, female heterosexuals and male heterosexuals.

The researchers compared the size of the left and right hemispheres of the subjects' brains -- features that are likely fixed at birth. They determined that gay males and heterosexual women had roughly equally sized hemispheres. However, lesbians and heterosexual men had asymmetric brains in which the right side was larger than the left side.

In a subsequent experiment, the researchers concentrated on the amygdala (medical name: corpus amygdaloideum). This is a structure in the lower part of the brain that directs the individual's response to emotional stimuli. It is located roughly halfway between one's eyeballs and the back of the head, and about halfway between the neck and the top of the head. It is active when danger is present and the person has to decide between whether to "fight or flight." It is also active when sizing up a potential love interest or mate, and in many other emotional situations.

The studies used PET scans to measure blood flow to the amygdala. Results demonstrated, again, that heterosexual men and lesbians shared a similarity: they had more nerve connections on the right side of the amygdala than on the left. Meanwhile, gay males and heterosexual women had more neural connections on the left side of the amygdala than on the right. That might be anticipated, because straight men and lesbians are both sexually attracted only to women, while straight women and gay males are both sexually attracted only to men.

New Scientist reports:

"In straight women and gay men, the connections were mainly into regions of the brain that manifest fear as intense anxiety. 'The regions involved in phobia, anxiety and depression overlap with the pattern we see from the amygdala,' says Savic. This is significant, she says, and fits with data showing that women are three times as likely as men to suffer from mood disorders or depression. Gay men have higher rates of depression too, she says, but it's difficult to know whether this is down to biology, homophobia or simply feelings of being 'different'.

In straight men and lesbians, the amygdala fed its signals mainly into the sensorimotor cortex and the striatum, regions of the brain that trigger the 'fight or flight' response. 'It's a more action-related response than in women,' says Savic. ... But as Savic herself acknowledges, the study can't say whether the brain differences are inherited, or result from abnormally high or low exposure in the womb to sex hormones such as testosterone." 1

Whatever the source of the differences, they occurr before birth and are thus totally beyond the individual's control.

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Excerpt from the article's abstract:

"Ninety subjects [25 heterosexual men (HeM) and women (HeW), and 20 homosexual men (HoM) and women(HoW)] were investigated with magnetic resonance volumetry of cerebral and cerebellar hemispheres. Fifty of them also participatedin PET measurements of cerebral blood flow, used for analyses of functional connections from the right and left amygdalae.

HeM and HoW showed a rightward cerebral asymmetry, whereas volumes of the cerebral hemispheres were symmetrical in HoM and HeW. No cerebellar asymmetries were found.

Homosexual subjects also showed sex-atypical amygdala connections. In HoM, as in HeW,the connections were more widespread from the left amygdala;in HoW and HeM, on the other hand, from the right amygdala.

Furthermore, in HoM and HeW the connections were primarily displayed with the contralateral amygdala and the anterior cingulate, in HeM and HoW with the caudate, putamen, and the prefrontal cortex.

The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects. The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities." 3

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Reactions by other scientists:

  • Dr Qazi Rahman of the University of London said:

    "As far as I'm concerned there is no argument any more -- if you are gay, you are born gay."

  • Sandra Witelson, a neuroscientist at McMaster University in Hamilton, ON, Canada said:

"There are now about three or four studies on gays that point to a different organization in the brain. And what I think this means ... is that these are differences that can't be explained by any environmental or learning factors."

She noted that the authors of the study could not say what causes the observed brain differences, but Witelson suggests that they are almost certainly present at birth. She continued:

"So in fact there's a (genetic) predisposition."

She predicts that these findings should erode the moral judgments often made against homosexuals and counter arguments that homosexuality is a mere lifestyle choice.

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Reactions by religious or social conservatives:

There does not appear to be any immediate reaction. We predict that some will ignore the study, while others will argue that the scientists have things backward -- it is homosexual behavior that alters the shape and functioning of the brain not vice-versa.

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More studies are described in the next essay

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The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.

  1. Andy Coghlan, "Gay brains structured like those of the opposite sex," New Scientist, 2008-JUN-16, at:
  2. Joseph Hall, "Gayness linked to brain: Study finds similarities in MRI scans of gay men and straight women; lesbians and straight men," Toronto Star, 2008-JUN-17, at:
  3. Ivanka Savic and Per Lindström, "PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects," Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0801566105. Abstract available at
  4. "Scans see 'gay brain differences'," BBC News, 2008-JUN-16, at:
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Copyright © 2008 to 2014 by Ontario Consultants on Religious Tolerance
Latest update: 2014-JUL-22
Author: B.A. Robinson

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