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article imageNew evidence supports biological basis for 'gay brain' theories

By Sykos Masters     Jun 17, 2008 in Science
Newly published research uses modern imaging technology to compare sizes of specific brain structures between subjects, while past studies have relied primarily on brains of deceased persons, when studying the biology of sexual preference.
While there have been a number of studies investigating possible biological 'causes' for sexual preference / orientation, past research has been met with criticism from a broad spectrum of interested parties: e.g., scientific peers, ultra-conservative religious groups, gay and lesbian activists. Those that disagreed with previous findings have often focused on the methodology involved, as non-human – rat, monkey, pig – brains or those of deceased humans have generally been used in past research. This new research by Ivanka Savic and Per Lindström of the Stockholm Brain Institute will likely not suffer from similar attacks as they used modern imaging of live subjects for their study.
One of the most notable previous reports, published in 1993 by Simon Levay, studied differences in specific brain anatomy between deceased homosexual (n=19) and heterosexual men (n=16) and heterosexual women (n=6). Detractors of his findings have emphasized his use of non-living brain matter—of note are the manner of death (AIDS) for 25 of the 41 brains, the hypothalamic structure itself, and the inability to confirm the sexual orientation of the 'subjects'. Even with these defaults, LeVay's research has often been cited by adherents on both sides of the 'Nature v. Nurture' argument in determining sexual preference.
In contrast, by using both Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET), Slavic and Lindström were able to accurately observe activity deep in the interior of the brain—from the abstract available online:
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 participated in PET measurements of cerebral blood flow, used for analyses of functional connections from the right and left amygdalae.
There are major differences in the methodology of this new research (other than the obvious use of living subjects).
• there are more than twice as many subjects in this new study – 41 v. 90
• the interior structures – LeVay's hypothalumus (involved in hormonal responses) v. Salvic and Lindström's amygdala (center of deeply ingrained fight / flight and other emotional responses / bonds)
• the use of MRI and PET to directly measure the size of the structure as a result of its function between subjects v. LeVay's positing of size differences based on past (living) hormonal activity
Process Connection Between Brain Structures
Process Connection Between Brain Structures
Stanford.edu
However, both studies are important in that they measure the size of brain structures that have a relatively fixed and predictable growth rate. Both the hypothalamus and amygdala are located deep at the base of the brain and are involved in the processing and effect of emotions, although the amygdala functions as the 'controller' rather than a contributor, and tend to reach their maximum size at the onset of full adulthood. The larger brain mass that we are most familiar with (frontal, parietal, temporal lobes, etc.) continues to change in size and density from birth through the 30's and 40's, as it acts as a storehouse for memory, motor function, and other life experiences that continue to effect its development.
This new study measures the number of connections made within the amygdala's left and right hemispheres (hence its overall size), thereby showing sex-differences between male and female subjects and atypical differences between sexual orientations within sexes: i.e., there is a strong correlation between the heterosexual female and gay male subjects. Given that the size of this structure is 'set' by adulthood and is less effected than other similar structures by experience, the authors strongly suggest that 'learned behaviour' cannot be identified as the cause for these differences.
It is clear that the 'Nature v. Nurture' argument as a basis for sexual expression is far from resolved. Hopefully the work of Salvic and Lindström will widen the scope for more responsible research into this scientifically important and socially contentious area of interest.
More about Gay brain, Sexual preference, Physiology
 
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