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Biological basis of sexual orientation
STANFORD -- Research into the biological basis of sexual orientation
"presents a clear double message. Yes, genetics plays a part. No, it is not
all genetics," Dora B. Goldstein, professor emeritus of molecular
pharmacology, told the audience that attended the first in a series of public
lectures sponsored by the Medical Center's Lesbian-Gay-Bisexual Community on
"This shouldn't be too surprising because that is what all kinds of
behavioral studies indicate. Genes determine everything. The environment
affects everything. Then there is this big area where the two interact," she
In her noon presentation, Goldstein reviewed relevant research in a number
of different areas: population studies, family studies, twin studies, genetic
research, childhood behavior, and brain differences.
- The often used statistic that about 10 percent of men are homosexual
comes from a study performed by Kinsey. But the subjects were selected in a
non-representative fashion, Goldstein said. More recent studies have found
that the proportion is between 3 and 5 percent. There is less data on
lesbians, but they appear to make up about 1 to 2 percent of the female
population. "Of course these are lower bounds, because many people will not
admit that they are gay," she pointed out.
- Several studies also have shown that homosexuality tends to run in
families. The probability that the brother of a gay man is gay is about four
times higher than normal. Similarly, the odds that the sister of a lesbian is
also a lesbian is significantly higher than normal. However, male
homosexuality and lesbianism tend to run in different families: sisters with
gay brothers are not more likely than normal to be lesbian. A 1993 study that
traced the pedigree of pairs of gay brothers found that homosexuality tends
to run on the maternal side of the family tree: the brothers had a higher
than average number of maternal nephews and uncles who are gay.
- Identical twin studies shed additional light on the genetic
underpinnings of sexual preference. If there are differences in preference
between identical twins, who share the same genes, then that difference
cannot be genetic. Here, the research indicates that in cases where one
identical male twin is gay, about half the time the other twin is gay as
well. "This is way above 4 percent, so it's got to be genetic, but it is
nowhere near 100 percent," Goldstein said. The percentages for lesbian twins
are slightly lower, but generally comparable.
- DNA studies have identified the general location of at least one "gay
gene." The maternal heritability of male homosexuality narrowed the region
where such a gene must reside to the X chromosome, because sons get this
chromosome from their mother. Analysis of DNA markers on the X chromosomes of
sibling pairs has further narrowed the search to a region called XQ28. It
consists of hundreds of genes and is located near the tip of the X
chromosome. However, there is some indication that genes located elsewhere
may have a similar effect on sexual orientation, Goldstein said.
- Fetal development studies suggest how such a gene might influence such a
complex behavior. The development of a fetus into a male is accomplished by
the development of the testes, which produce testosterone, which has a wide
range of physiological effects. During the perinatal period, a week before
and after birth, testosterone has an irreversible organizing effect on the
body and brain of males. If the hormone is absent during this period, the
individual's anatomy and behavior never can become wholly male. A
testosterone surge during puberty activates male sexual development and
- Differences between gay and straight sexual orientation appear at a very
early age. In a study, a group of openly homosexual men were asked when they
first became aware of their attraction to men and boys, when they realized
that they were gay and when they "came out" to others. The group reported
becoming aware of their attraction at a very young age, between 5 years old
and puberty. Self-awareness of their sexual orientation took place around
puberty, and coming out of the closet took place much later.
The results of this survey are supported by studies of "gender
non-conforming children." In little girls, this behavior, acting as tomboys,
bears no social stigma. In little boys, cross-dressing, playing with dolls
and behaving like girls is socially damaging. A larger than average number of
such "sissy boys" become gay adults, she said.
- Finally, there is some evidence that the brains of homosexuals may be
different from those of heterosexual men and women. The differences have been
found in the hypothalamus, which controls eating, drinking, temperature
regulation and sexual behavior. Studies done in the Netherlands and in
Southern California have found such differences in several areas within the
hypothalamus. One region, the midsagittal area of the anterior commissure, is
larger in females than in males, but also appears to be larger in homosexual
males. Another area, the suprachiasmatic nucleus, which controls circadian
rhythm, is larger in heterosexual males and females than it is in
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