The Male Brain (18 page)

Are men gay because their brains are different? Studies have been conducted for two decades in order to answer this question. Some of them have found evidence of anatomical or functional differences between gay and straight brains. Others have established that genes play a part in determining gender orientation, which implies the existence of brain differences.

One of the early studies, by Dick Swaab, found that a part of the hypothalamus called the suprachiasmatic nucleus (SCN) is twice as large in gay
males as in straight males
. This difference was
difference in the way
developing brain
. Other
later shown to be caused by a

testosterone
reacts with the
researchers showed that the anterior commissure--a bundle of superfast cables that connects the brain's two hemispheres--is larger in gay
males than in straight males
. This structure, which is also larger in women than in men, is believed to be involved in sex differences related to cognitive abilities and language, and fits with the finding that gay males, like females, have better verbal abilities than straight males.

Recently Ivanka Savic and other Swedish researchers reported that an anatomical asymmetry in the size of the two brain hemispheres that is characteristic of straight male brains is not observed in gay male brains. Instead, their magnetic resonance imaging studies showed that in this respect gay male brains were more like female brains. With PET scans, the researchers also found that the connectivity of the amygdala of the gay male brain is more like that of the straight female brain than
of the straight male brain
. These studies suggest that there are differences between gay and straight male brain areas that are not directly involved in sexual attraction.

Savic has also reported a different pattern of activation in gay male brains in response to a pheromone that is excreted in male perspiration. She found that the hypothalamus in gay male brains is stimulated by the scent of male sweat, but in straight male brains it is not. This suggests that a difference in the brain's hypothalamic circuits for response to pheromones may attract gay males to the scent produced by the sweat glands of men and that this plays a role in their sexual orientation. Other studies have found anatomical differences in the structure of the hypothalamus
in gay and straight men
.

There is also evidence of differences between gay and straight men in performance on certain spatial tasks. It has been consistently shown that straight men outperform straight women on tasks requiring navigation. Recent studies have shown that gay men perform more like
straight women on such tasks
.

Brain scans have also been used to measure activity changes in the gay and straight male brain when pictures of men and women were shown. Viewing a female face produced a strong reaction in the thalamus and medial prefrontal cortex of straight men but not of gay men. In contrast, the gay male brains reacted more strongly
to the face of a man
.

Genetic studies have also contributed some evidence of innate differences between gay and straight males. In a recent study, Dr. Niklas Langstrom estimated the part that genes play in gay male behavior by studying sexual orientation in adult male twin pairs. He found that identical twin pairs, who have all the same genes, are more likely to share sexual orientation than fraternal twin pairs, who share only half of their genes. Based on this comparison, he concluded that about 35 percent of sexual orientation is attributable to genetic influences, whereas the rest is due to as yet unidentified factors.

So far, none of the specific genes that influence sexual orientation have been identified, and researchers believe that the combined actions of many genetic and environmental factors will be involved. Furthermore, research on the brain circuits and on hormonal effects related to sexual orientation in humans is only just beginning. Nevertheless, the evidence we already have does show that some human brain differences are related not only to gendered behavior, but also to sexual orientation.

What Makes a Man
: Certain brain areas and functions are built differently in the male and female brains and have evolved over time to produce the most successful versions of men and women. For example, the brain circuits that alert us to danger (the amygdala) and help us remember it (the hippocampus) are the sources of sex and individual differences in emotional memory. In Hamann 2005, the authors found sex differences in the amygdala response during emotion-related activities, such as formation of emotional memory and sexual behaviors. And for more on the evolution of male and female brain circuits, see Lindenfors 2007 and Dunbar 2007: they say "... there are striking differences between the two sexes in the social mechanisms and brain units involved. Female sociality (which is more affiliative) is related most closely to neocortex volume, but male sociality (which is more competitive and combative) is more closely related to subcortical units (notably those associated with emotional responses). Thus brain units have responded to different different selection

pressures." For more on cellular and genetic differences in the male and female brain, see Reinius 2008 and Arnold 2009b.

and the fix-it-fast emotional brain
: Coates 2009 found that testosterone sets the male brain up for faster visuomotor scanning, faster physical reflexes, and more risky behavior.

Male and female brains
: Penaloza 2009. The authors say, "Sex of the cell dictates its response." Malorni 2007 even found reduction-oxidation differences between male and female cells.

for later amplification by hormones
: For more on sex hormones, genes, and the brain, see Arnold 2009c and Neufang 2009.

differences between women and men
: For good reviews on sex differences in the brain, see Becker 2008b, McCarthy 2009, and Proverbio 2009.

role in shaping and reshaping our brains
: It is important to note that biological predispositions can be shifted over time through experience and that existing dispositions can be mitigated or even overridden by situational demands in both men and women. For both males and females, upbringing, experience, and the environment can make long-lasting biological and behavioral impacts via epigenetic changes to our DNA. See Merzenich 1983 for early work on brain reorganization in response to changed circumstances. For more on changes in brain architecture with experience, see Kozorovitskiy 2005, and on environmental experience and epigenetic effects, see Meaney 2005, McCarthy 2009, and Murray 2009.

wired into the male brain
: Coates 2009 found prenatal androgens promote increased risky behavior, movement, and physical reflexes. For more on the male brain, see Arnold 2009c, Van Nas 2009, Chura, 2010, Wu 2009, Field 2008 and 1997, Baron-Cohen 2003 and 2009, Pfaff 2002, Holden 2004, Eme 2007, and Becker 2008b; see also De Vries 2008 and McCarthy 2009a. In humans and most other mammals, a gene on the Y chromosome, the SRY gene, confers maleness. Studies suggest that the SRY gene directly affects the biochemical properties of the dopaminergic neurons of the nigrostriatal system and the specific motor behaviors they control. This means that a direct male-specific effect on the brain is caused by a gene encoded only in the male genome.

play at fighting off enemies
: Auyeung 2009b says, "... our data are the first documentation that androgen exposure prenatally relates to sexually differentiated play behavior in boys and in girls." For more on genes and hormones in boys and girls, see Wu 2009 and Berenbaum 2008.

seemed to find them fascinating
: Connellan 2000. For more on gender differences in newborns, see Ashwin 2009, Baron-Cohen 2009, Auyeung 2009, and Gilmore 2007.

without as much mutual gazing
: Leeb 2004. For more on maternal attachment and bonding, see Young 2008, Baron-Cohen 2003, Carter 1998, Nichools 1996, and Bowlby 1980.

"contact much more than girls"
: By six months old, boys gaze-avert more frequently than girls. Whether they are straining to look at something else that caught their eyes or away from the face is not entirely known. For further reading, see Byrd-Craven 2007, Knickmeyer 2006, Bayliss 2005, and Hittelman 1979. For more on autism and the male brain, see Baron-Cohen 2009, who says that his results suggest that prenatal exposure to high levels of testosterone influences some autistic traits and thus hormonal factors may be involved in vulnerability to autism. Ashwin 2006 says that, for still unknown reasons, Asperger's and autism affect four to ten times more boys than girls. Autism and Asperger's syndrome (AS) are genetic neurodevelopmental conditions characterized by social deficits, abnormal face processing, and amygdala dysfunction that are more common in boys.

"airplanes and other moving objects"
: Moore 2008. For an overview of moving objects and movement in brain circuitry in boys and girls, see Hampson 2008 and Field 2008.

weeks after he was conceived
: See Field 2008 for more on the development of sex differences in brain circuits for movement.

circuits that control male behaviors
: For a review of the formation of male brain circuits, see Wu 2009, Gagnidze 2009, Becker 2008b, Eme 2007, Breedlove 1983, and Archer 2006. Note: a failure of masculinization is called
pseudohermaphroditism
. See Tsunematsu 2008 on vasopressin increasing locomotion in males.

make others wither and die
: For more on hormones causing death or acting as growth factors, see Wu 2009 and Kimura 2008. For more on sex differences and brain development, see Penaloza 2009, Swaab 1985 and 2009, Ehrlich 2006, and Zuloaga 2008.

defeminized
David's brain and body
: Wang P. 2009. Wang and colleagues found that the foundations for the observed MIS-dependent increased movement and exploratory behavior in males is laid down in the brain circuits during fetal life. Wang P. 2005 found that in males, Mullerian inhibiting substance is secreted by the fetal testes and causes the death of the Mullerian ducts in order to prevent the growth and development of female reproductive organs in male bodies. Note: MIS is also called "anti-Mullerian substance."

off the female reproductive organs
: Normal development of the male brain involves two distinct processes, masculinization and defeminization. They occur during critical periods of Masculinization allows behavior in adulthood, and defeminization eliminates or suppresses the expression of female sex behavior in adulthood. Once inside the fetal brain, much of the testosterone is actually converted by the enzyme aromatase into estrogen. Ironically, then, it is the estrogen that helps masculinize and defeminize the male brain, working in concert with MIS. For more on sex and the brain, see Wu 2009, Wang P. 2009, and Becker 2008b. brain sexual differentiation.

spatial skills, and rough play
: Wang P. 2009 found that male mice lacking MIS exhibit feminization of their spinal motor neurons and of their exploratory play behavior. They hypothesize that, along with testosterone, MIS may be a regulator of the sex-linked behavioral biases in the nervous system and brain toward more movement, pursuit, rough play, and exploration in males.

effects of testosterone or MIS
: Fynn-Thompson 2003. MIS is
not
present in the female embryo but is induced in females only after birth. For a review of the association between testosterone and aggression that occurs in boys, but not girls, during childhood, see Becker 2008b, Eme 2009, and Archer 2006.

to appreciate her son's maleness
: Diamond 2006 found that the boy's sense of masculinity early on is shaped by factors like the mother's recognition and affirmation of her son's maleness, the role of the involved or uninvolved father, and the nature of the parental relationship.