For Gay Men, an
Attraction to a Different
Kind of Scent
By NICHOLAS WADE,
NYTimes on the Web, May 10, 2005
Using a brain imaging technique,
Swedish researchers have shown that homosexual and heterosexual men respond
differently to two odors that may be involved in sexual arousal, and that the
gay men respond in the same way as women.
The new research may open the way to studying human pheromones, as well as the
biological basis of sexual preference. Pheromones, chemicals emitted by
one individual to evoke some behavior in another of the same species, are known
to govern sexual activity in animals, but experts differ as to what role, if
any, they play in making humans sexually attractive to one another.
The new research, which supports the existence of human pheromones, is reported
in today's issue of The Proceedings of the National Academy of Sciences by Dr.
Ivanka Savic and colleagues at the Karolinska Institute in Stockholm.
The two chemicals in the study were a testosterone derivative produced in men's
sweat and an estrogen-like compound in women's urine, both of which have long
been suspected of being pheromones.
Most odors cause specific smell-related regions of the human brain to light up
when visualized by a form of brain imaging that tracks blood flow in the brain
and therefore, by inference, sites where neurons are active. Several years
ago, Dr. Savic and colleagues showed that the two chemicals activated the brain
in a quite different way from ordinary scents.
The estrogen-like compound, though it activated the usual smell-related regions
in women, lighted up the hypothalamus in men. This is a region in the
central base of the brain that governs sexual behavior and, through its control
of the pituitary gland lying just beneath it, the hormonal state of the body.
The male sweat chemical, on the other hand, did just the opposite; it activated
mostly the hypothalamus in women and the smell-related regions in men. The
two chemicals seemed to be leading a double life, playing the role of odor with
one sex and of pheromone with another.
The Swedish researchers have now repeated the experiment but with the addition
of gay men as a third group. The gay men responded to the two chemicals in
the same way as did women, Dr. Savic reports, as if the hypothalamus's response
is determined not by biological sex but by the owner's sexual orientation.
Dr. Savic said that she had also studied gay women, but that the data were
"somewhat complicated" and not yet ready for publication.
The finding is similar to a report in 1991 by Dr. Simon LeVay that a small
region of the hypothalamus is twice as large in straight men as in women or gay
men. The brain scanning technique used by the Swedish researchers lacks
the resolution to see the region studied by Dr. LeVay, which is a mere
millimeter or so across. But both findings suggest that the hypothalamus
is organized in a way related to sexual orientation.
The new finding, if confirmed, would break ground in two important directions,
those of human pheromones and human sexuality.
Mice are known to influence each other's sexual behavior through emission of
chemicals that act like hormones on the recipient's brain and so are known as
pheromones. Hopes by the fragrance industry, among others, of finding
human pheromones were dashed several years ago when it emerged that a tiny
structure in the nose through which mice detect many pheromones, the vomeronasal
organ, is largely inactive in humans, having lost its nervous connection with
the brain.
Researchers interpreted that to mean that humans, as they evolved to rely on
sight more than smell, had no need of the primitive cues that pass for sexual
attractiveness in mice. But a role for human pheromones could not be ruled out,
especially in light of findings that women living or working together tend to
synchronize their menstrual cycles.
Some researchers see Dr. Savic's work as strong evidence in favor of human
pheromones. "The question of whether human pheromones exist has been
answered. They do," wrote the authors of a commentary in Neuron about Dr.
Savic's report of 2001.
Dr. Catherine Dulac, a Harvard University biologist who studies pheromones in
mice, said that if a chemical modified the function of the hypothalamus, that
might be enough to regard it as a pheromone. She said the Swedish study
was extremely interesting, even though "humans are a terrible experimental
subject." She noted, however, that the researchers used a far higher dose
of the armpit chemical than anyone would be exposed to in normal life.
If human pheromones do exist, Dr. Savic's approach may allow insights into how
the brain is organized not just for sexual orientation but also for sexuality in
general.
"The big question is not where homosexuality comes from, but where does
sexuality come from," said Dr. Dean Hamer, a geneticist at the National
Institutes of Health.
The different pattern of activity that Dr. Savic sees in the brains of gay men
could be either a cause of their sexual orientation or an effect of it. If
sexual orientation has a genetic cause, or is influenced by hormones in the womb
or at puberty, then the neurons in the hypothalamus could wire themselves up in
a way that permanently shapes which sex a person is attracted to.
Alternatively, Dr. Savic's finding could be just a consequence of straight and
gay men's using their brain in different ways.
"We cannot tell if the different pattern is cause or effect," Dr. Savic said.
"The study does not give any answer to these crucial questions."
But the technique might provide an answer, Dr. Hamer noted, if it were applied
to people of different ages to see when in life the different pattern of
response developed.
Dr. LeVay said he believed from animal experiments that the size differences in
the hypothalamic region he had studied arose before birth, perhaps in response
to differences in the circulating level of sex hormones. Both his finding and
Dr. Savic's suggest that the hypothalamus is specifically organized in relation
to sexual orientation, he said.
Some researchers believe there is likely to be a genetic component of
homosexuality because of its concordance among twins. The occurrence of
male homosexuality in both members of a twin pair is 22 percent in nonidentical
twins but rises to 52 percent in identical twins.
Gay men have fewer children, meaning that in Darwinian terms, any genetic
variant that promotes homosexuality should be quickly eliminated from the
population. Dr. Hamer believes that such genes may nevertheless persist
because, although in men they reduce the number of descendants, in women they
act to increase fertility.
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