There is substantial research showing that sexual orientation is
probably due to a combination of genetics, in utero influences, and
environment:
“Psychologists used to believe homosexuality was caused by nurture ?
namely overbearing mothers and distant fathers ? but that theory has
been disproved. Today, scientists are looking at genes, environment,
brain structure and hormones. There is one area of consensus: that
homosexuality involves more than just sexual behavior; it?s
physiological.”
http://www.cbsnews.com/stories/2006/03/09/60minutes/main1385230.shtml
60 Minutes: The Science of Sexual Orientation
Brain differences between homosexuals and heterosexuals:
In 1990, D.F. Swaab found in his post-mortem examination of homosexual
males’ brains that a portion of the hypothalamus of the brain was
structurally different than a heterosexual brain. The hypothalamus is
the portion of the human brain directly related to sexual drive and
function. In the homosexual brains examined, a small portion of the
hypothalamus, termed [the suprachiasmatic nucleus (SCN), was found to
be twice the size of its heterosexual counterpart.
Also in 1990, Laura S. Allen made a similar discovery. She found that
the anterior commissure (AC) of the hypothalamus was significantly
larger in homosexual subjects than in heterosexuals.
In 2005, Wysocki asked 80 homosexual and heterosexual men and women
(20 of each) to sniff two samples of human sweat and choose the odor
they preferred. Wysocki?s team made four comparisons: sweat from gay
men versus straight men, gay men versus straight women, straight women
versus lesbian women, and gay men versus lesbian women. Wysoci found
that homosexual men had a strong preference for the natural scent of
other gay men, which heterosexuals found unattractive. Wysocki
concluded: “Our findings support the contention that gender
preference has a biological component that is reflected in both the
production of different body odors and in the perception of and
response to the body.”
In a second study published also in 2005 (Proceedings of the National
Academy of Sciences), a different team of researchers used positron
emission tomography scanning to examine the brain?s response to two
hormone derivatives, AND and EST, which have been proposed as human
pheromones - powerful scents known to convey sexual signals in many
species. The scans showed a different pattern of brain activity in
response in heterosexual and homosexual men, particularly in a brain
region known as the hypothalamus that is involved in sexual arousal.
The brain activity of gay men turned out to be much more similar to
that of straight women, suggesting that sexual orientation rather than
gender was the determinant.
In 1990, researchers at McMaster University in Ontario, Canada
examined the occurrence of left-handedness in heterosexual and
homosexual women. Brain organization in left-handed people is known to
be slightly different than in right-handed people. For example,
left-handed people tend to have their language area centered in the
right hemisphere of their brain; it is generally accepted that the
area for language is in the left hemisphere. Even though only
thirty-five percent of the general population is totally left-handed,
the scientists found that sixty-nine percent of homosexual women were
totally left-handed . As a result, they suggested that homosexuals
have a different brain organization than heterosexuals. Referring to
studies in which women with higher than normal levels of masculinizing
hormones such as testosterone were more likely to be left-handed and
gay, they hypothesized that atypical sex hormone levels during
pregnancy may have affected lesbians’ early fetal development.
In 1994, Hall and Kimura studied the fingerprint ridges heterosexuals
and homosexuals. After the sixteenth week of pregnancy, fingerprints
are known to be unchangeable, so if there were any significant
fingerprint differences in the two groups, one could argue that sexual
orientation may be determined before birth. In fact, Hall and Kimura
did find that the difference between the number of ridges on the left
hands of homosexual men was greater than that of heterosexuals. Citing
that individuals with higher left-hand ridge counts perform
differently on sexually dimorphic cognitive tasks than do those with
higher right-hand ridge counts, the researchers concluded that
there must be an ?early biological contribution to adult sexual
orientation?.
In 1991, Bailey and Pillard studied three all male groups: identical
twins, fraternal twins, and men with adoptive brothers.Of the 170
relatives examined, 52% of the identical twins were both gay, 22% of
fraternal twins were both gay, and 11% of the adoptive brothers were
both gay.
In 1992, Bailey and Pillard followed-up their experiment on homosexual
men by studying identical twin, fraternal twin, and nongenetically
related adopted sisters. As expected, their results mirrored those
found in their gay brother study. Whereas only six percent of
adopted sisters were both lesbian, sixteen percent of fraternal twin
sisters and forty-eight percent of identical twin sisters were both
lesbian . Clearly, the basis for a similar argument for
predetermined homosexuality in women has been laid.
In 1993, Whitam, Diamond, & Martin found that 65% of identical twins
were both gay, whereas only 29% of fraternal twins were gay.
In 2004, Camperio-Ciani studied 98 homosexual and 100 heterosexual men
and their relatives, which included more than 4,600 people overall.
The female relatives on the mother’s side of the homosexual men
tended to have more offspring than the female relatives on the
father’s side. This suggests that women who pass on the gay trait
to their male offspring are also more fertile. In comparison, the
female relatives on both the mother’s and the father’s side of the
heterosexual men did not appear to be as fertile, having fewer
offspring.
In 2006, research published in the journal “Human Genetics” found that
the genetics of mothers of multiple gay sons act differently than
those of other women. Scientists looked at 97 mothers of gay sons and
103 mothers without gay sons to see if there was any difference in how
they handled their X chromosomes. They found that almost one fourth of
the mothers who had more than one gay son processed X chromosomes in
their bodies in the same way. Normally, women randomly process the
chromosomes in one of two ways – half go one way, half go the other.
The research “confirms that there is a strong genetic basis for
sexual orientation, and that for some gay men, genes on the X
chromosome are involved,” said study co-author Sven Bocklandt, a
postdoctoral researcher at the University of California at Los
Angeles. “When we looked at women who have gay kids, in those with
more than one gay son, we saw a quarter of them inactivate the same X
in virtually every cell we checked,” Bocklandt said. “That’s extremely
unusual.”
In 2005, Dr. Brian Mustanski and his colleagues at the University of
Illinois at Chicago, in the first-ever study combining the entire
human genome for genetic determinants of sexual orientation,
identified several stretches of DNA that appeared to be linked to
sexual orientation on three different chromosomes. The bottom line,
according to Mustanski, is that “genes play an important role” in
determining whether or not men are gay or straight.
A recent study conducted by Canadian researcher and psychologist
Anthony Bogaert reported that there was “no evidence that social
interactions among family members played a role in determining whether
a man was gay or straight.” What he found was that having one or more
older brothers increases the likelihood that males will be gay - not
based on social or environmental factors but based on biological
events that occur in the womb.
http://www.nlm.nih.gov/medlineplus/news/fullstory_35335.html
