Study 15 into the causes of homosexual orientation
More detailed coverage of
conclusive study involving brain scans
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
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
the brains of gay males and heterosexual females, and
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.
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
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.
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:
Whatever the source of the differences, they occurr before birth and are thus totally beyond the individual's control.
"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
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
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
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.
"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.
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.
The following information sources were used to prepare and update the above
essay. The hyperlinks are not necessarily still active today.
Andy Coghlan, "Gay brains structured like those of the opposite sex," New
Scientist, 2008-JUN-16, at:
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:
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
"Scans see 'gay brain differences'," BBC News, 2008-JUN-16, at:
Copyright © 2008 to 2014 by Ontario Consultants on
Latest update: 2014-JUL-22
Author: B.A. Robinson