These are twins that resulted from the splitting of a single fertilized egg
-- the zygote -- into two separate zygotes with identical genetic structure. It
can happen at any time prior to about 14 days after conception. They are
commonly called "identical twins." Studying these twins, researchers can
determine whether a trait such as homosexuality is determined by the environment
or by genetics or by a combination of both. The technique involves the study of
pairs of identical twins who were separated at birth and raised in different
families without later contact. Being identical twins, they would have the same
genetic structure. But being raised independently in different families (often
in different states), they would mature (at least after birth) and experience
totally unrelated environments, family types, family sizes, parenting methods,
level of childhood molestation, etc. Fortunately, a U.S. university maintains an index of such twins
who were raised separately since birth.
Schizophrenia occurs in about 1% of the adult population. In previous
decades, the disorder was believed to be caused by incompetent parenting.
Studies of identical twins overturned this belief. They showed that if one twin
was schizophrenic, the chances of the other twin having the disorder is 65%.
This shows that schizophrenia has a very strong genetic component. 1
Decades ago, autism was also blamed on the parents. Identical twin
studies turned up similar results: if one twin was autistic, there was a 68%
chance that the other was also autistic. 1 Similarly the
cause of homosexuality has been attributed to lack of bonding between a child
and the same-sex parent. This belief has been abandoned by almost all mental
health professionals. However, many religious conservatives still promote this
principle. Studies of identical twins have shown that if one twin is gay, the
other has about a 55% probability of also being gay. Again, there is a very strong genetic
component at work.
The interview of perhaps 2,000 males, all of whom had separately raised
The identification of about 100 gays.
Tracking down the other twin in each case, to determine their sexual
Not a trivial task!
Two possibilities are:
There is no genetic basis for sexual orientation. That is, a
statement by Parents and Friends of Ex-gays is correct: "To date, all
information and studies involving genetics have proven homosexuality to be
environmental, not genetic."
2Then one would expect close to 5% of the second twins would
also be gay.
A homosexual orientation is totally caused by genes. One would then
expect that between 0 and 100% of the other twin would also be gay, depending
upon a property of the gene called its "penetrance"
-- a type of power or effectiveness.
This type of test has been performed by various groups of researchers. The
first such study found that 100% of the second twins were also gay. But this
study was based on a very small sample size; the results it turned out to be a
statistical fluke. Subsequent, larger, tests all reported that somewhat in
excess of 50% of the second twins are also gay. This indicates that genes play a
very significant role in determining sexual orientation. however, the number is
not 100%. This indicates that environment plays a role in determining sexual
orientation. It might be an event in the womb, or in early childhood. Other
studies, explained below, show that a person's eventual sexual orientation is
determined before they reach school age.
One problem with this and similar studies is that the researchers can ask
what a person's orientation is, but not necessarily get a valid answer; a gay
individual may be so firmly "in the closet" that they will not admit even
to a stranger what their actual orientation is. Also, most studies classify
subjects as either heterosexual or homosexual. A bisexual person might be
involved with persons of the opposite gender and consider themselves to be
heterosexual; their twin may also be bisexual but be involved with members of
the same gender, and identify themselves as homosexual. Another problem is that
identical twins share the same environment (their mother's womb) for the 9
months prior to birth. Thus, if there is a environmental factor which determines
sexual orientation, it might work on the fetus before birth.
This series of
replicated studies proves beyond any doubt that at least male homosexuality is
largely determined before birth. As the American Psychological Association's statement on homosexuality in
1994-JUL asserts: "Research suggests that the homosexual orientation is in
place very early in the life cycle, possibly even before birth..."
In 2003-OCT-2, researchers at University of East London and at King's College announced the results of a study which differentiates between
homosexuals and heterosexuals. They studied groups of homosexual and
heterosexual men and women and found significant differences in an involuntary
response to being startled with bursts of a loud noise. This is formally called "prepulse
inhibition" or (PPI). Subjects were exposed to a low level noise, followed
by an intense noise. Researchers measured the strength of the involuntary
eye-blink responses. These data were then compared to similar strength
measurements taken after exposure to a loud noise without the preceding low
level noise. The lower the response, the stronger the level of inhibition.
The researchers stated that:
reaction of the lesbian test subjects was closer to that which would be expected
among straight men. And, gay men reacted closer that of women, although to a
They found that the average PPI was:
40% for heterosexual men.
32% for gay men.
13% for heterosexual women
33% for lesbians.
Startle responses is known to be an involuntary response rather than learned
reaction. It controlled by the limbic system, a region of the brain that also
One of the researchers, Qazi Rahman, said:
response is pre-conscious and cannot be learned...This is very strong evidence
that sexual orientation may be 'hard-wired' in this region" of the brain."
The researchers claimed that:
"... this study offers the first independent
evidence of a non-learned neurological basis for sexual orientation."
"These findings may well affect the way we as a society deal
with sexuality and the issues surrounding sexual orientation." 1 to
Kevin Schattenkirk, "Being gay - A choice? Debunking some popular views
on homosexuality," The Daily University of Washington, 1999, at: http://archives.thedaily.washington.edu/ (No longer available online)