Study 16: Based on studies of identical twins who were separated at birth:
There have been a number of studies of identical twins who were separated at -- or shortly after -- birth
and raised apart from each other. These pairs of twins share identical DNA and yet grow up in totally different environments. Studies of such twins are the standard technique used by scientists when trying to separate the effects of nature (one's genes) from those of nurture (one's environment, culture, religion, and upbringing).
Many studies have shown that if one twin is gay, then the other twin is found to be gay slightly more than half of the time.
If one's homosexual orientation is not at all influenced by genes but is totally determined by the nature of one's upbringing -- e.g. the relationship between child and parent, or by the child being the victim of of sexual molestation, etc. -- then if one twin becomes a gay or lesbian in adulthood, one would expect about 5% of their opposite twins would be gay or lesbian. That is the approximate percentage of gays and lesbians in the general population.
If homosexual orientation is not influenced by one's upbringing, but is totally determined by genes, then many people incorrectly believe that 100% of the other twins would also be gay or lesbian. However, the actual value is somewhere between 0 and 100%. It is determined by the "penetrance" of the allele (variation of the gene). Penetrance is a type of effectiveness factor. If an allele has a penetrance near zero, and might remain dormant; it rarely has any effect on the person's life; a penetrance near 100% is almost certain to activate the allele in their life.
Consider the genes for:
Huntington's Disease: It has two alleles (varieties).
One is a very rare allele that
will inevitably cause the person to fall victim to the disease. The other, much more common, variety prevents the disease. The former allele is 100% penetrant. If you were born with the allele that causes the disease, then you
are absolutely certain to develop the disease later in life.
Type 1 diabetes: The penetrance of the gene which causes
Type 1 (early onset) diabetes is only 30%. If one identical twin has the allele that
causes diabetes, then the other twin will also have the same allele. Both will have a 30%
chance of developing the disorder. Both twins will have the same genetic structure. But it may
or may not be triggered by something in the environment, and later cause diabetes.
Schizophrenia: If one identical twin develops schizophrenia, the other twin has
about a 48% chance of also developing the disorder. 1
Bipolar Affective Disorder: If one twin develops bipolar
affective disorder, (formerly called manic depression) the other twin's chances
are about 60% of having it as well.
Brain neurotransmitter: A New Zealand study examined a gene that
controls the production of an enzyme called monoamine oxidase A (MAOA). One allele is
found in about one third of the male subjects tested. Among these males, 85% of the boys who
were abused during childhood turned to criminal or antisocial behavior as adults.
If the boys were not abused, they would have matured normally. More info.
Multiple Sclerosis: This is a disease for which the cause is
unknown. Perhaps because MS is so mysterious, the genetic factors have
been thoroughly examined:
On the order of 1 person out of every 3,000 North Americans
contracts the disease.
If you have a parent, sibling or child with MS, your chances of
contracting it are slightly increased, but only by a few percent.
If you have a fraternal twin with MS, your chances are increased
from 1 in 3000 to about 1 in 44.
If you have an identical twin with MS, your chances of
contracting it is increased to about 1 in 4.
Further, some identical twins of persons with MS have "...MS
lesions, although the disease had never caused symptoms." 2
Sometimes, as in the case of Huntington's Disease, the presence of an
allele is certain to cause the disease later in life. In most cases, some
genetic combination only makes a person susceptible to a disease or
disorder. It takes some factor in the environmental to trigger the disease
into activity. Without that trigger, the disease never develops.
We do not wish to imply that homosexuality is a disease. We are merely
suggesting that the root cause of many diseases -- and traits like
left-handedness -- are genetic. Most human sexuality researchers who are not
religious conservatives regard homosexual orientation as a genetically caused trait like
One theory that fits the available observations is
that the penetrance of the "gay gene(s)," when they occur, is
approximately 67%. That causes about half of the males with the gene(s) become gay.
Biologist Chandler Burr wrote:
"There could be hundreds of millions of straight men
walking around with this gay allele but who are straight simply because it didn't
In the case of the "gay gene(s)" perhaps about 10% of all
males have the allele that causes homosexuality, but in many cases the allele was not
"triggered," and remains dormant.
The penetrance of the "gay gene" or "gay genes" appears to be about 67%. One would expect
that if one fraternal twin was gay that the other would also be gay about 22% of the time.
This number also agrees with studies of families with fraternal twins.
Nobody knows what triggers the allele. It might occur before birth by some event happening in the womb,
like an abnormal amount or irregular timing of hormones. It might be
some event during early childhood -- before school age. Either way, it is outside the control of the individual
and his family of origin. What is known is that it almost always takes effect before the child
reaches age 5. Child psychologists can interview children at that age and
determine with excellent accuracy who will grow up to be gay or lesbian.
Essentially all conservative Christian authors who have written about homosexuality maintain that sexual orientation is
not determined by one's genes.
Most treat it as an abnormal, unnatural, chosen, and changeable habit or
addiction. They either:
Are unaware of the penetrance factor and assume that if an allele existed that produced a homosexual orientation, it would be activated 100% of the time, or
Are aware of the penetrance factor and assume that if an allele existed that produced a homosexual orientation that it would have to be 100% effective.
Thus, they reason that: since identical twins have the same genetic structure,
then if homosexual orientation were determined by genes, and if one twin is gay, then 100% of the other twins would also be
gay. Since the value found by studies is about 55%, they conclude that homosexual orientation is not genetically caused.
In contrast, many gays and lesbians believe that their orientation is caused by their
genes; they believe that their orientation is normal, natural, unchosen and unchangeable.
With the exception
of one small religious association of therapists, the vast majority of human
sexuality researchers and mental health therapists accept that the root
cause(s) of homosexual orientation are genetic, and that the identical/separated at birth twin studies show that the allele causing homosexual orientation has a penetrance that is much less than 100%.
The data derived from these identical/separated at birth twin studies seems to show that a small minority of individuals -- perhaps
10% -- have a "gay gene" or sequence of "gay genes."
However, the gene is only expressed in slightly more than half of those individuals, as
a result of some unknown trigger factor in the environment. In the remainder, it
remains dormant and the person matures as a heterosexual.