These twentieth century theories are not supported by homosexual
men who feel their sexual orientation to be innate, deeply embedded
in their personality, and not determined by external factors.
It is significant that most homosexual men neither seek nor
desire therapeutic reorientation.
Twenty-first century
In the age of Primal Health Research, we are learning that
most personality traits and states of health are to a great
extent determined during fetal life. It is time to cease contrasting
genetic and environmental factors. We now understand that the
expression of our genes is influenced by early – particularly
prenatal and perinatal - environmental factors. Our study of
the genesis of sexual orientation provides an exemplary opportunity
to realize that we are entering a new phase in our understanding
of human development. On one hand, the role of genetic factors
in sexual orientation is well researched. On the other hand
there is accumulating of data confirming that the sexualisation
of the brain is to a great extent determined during fetal life.
Today, by combining anatomical data, the genetic perspective,
the results of animal experiments and the results of physiological
and epidemiological studies, we can draw valuable conclusions.
Anatomical data.
There are many reasons to wonder if the anatomical structure
of the hypothalamus is the same among heterosexual and homosexual
men. The hypothalamus is an archaic brain structure that develops
early in life and is involved in the regulation of the typically
male sexual behaviour.
Simon LeVay, a neuroscientist at the Salk Institute in San Diego,
set out to answer this intriguing question by examining the
hypothalamus of 41 subjects – 19 homosexual men who had
died of complications of AIDS, 16 heterosexual men, and six
heterosexual women. A characteristic feature of the brains of
gay men is the small size of one hypothalamic nucleus, INAH
3, which LeVay found to be the same size as in women and only
half the size found in heterosexual men.(5) INAH 3, he concludes,
is dismorphic not with gender, but with sexual orientation.
It is noticeable that six of the heterosexual men had died of
AIDS but nevertheless had a large INAH 3. Statistical analysis
indicated that the probability of the result’s being attributed
to chance was about one in 1000. The brains of lesbians might
yield interesting results but are seldom available for research,
since this group is at low risk of AIDS.
Let us recall that, as early as the 1960s, Gunter Dorner, from
East Berlin, had already conducted animal experiments in order
to demonstrate the importance of the hypothalamus in sexual
behaviour.(6,7,8) Dorner’s conclusions were reinforced
by Gorski and colleagues who found that, in rats, the size of
the ‘sexually dismorphic nucleus’ of the hypothalamus
is established very early in life and influences later sexual
behaviour.(9) Subsequently, the same team of researchers showed
that two nuclei of the hypothalamus, INAH 2 and 3, were twice
as large in men as in women.(10)
Gorski and colleagues have reported another feature in brains
that is related to sexual orientation. The anterior commissure,
a bundle of fibres running across the midline of the brain,
is larger in women and gay men than in heterosexual men.
The genetic component
Now we understand that the expression of our genes is influenced
by early environmental factors; we can therefore accept that
even a genetic factor that reduces reproductive success can
transmit itself in a population. The two main research tools
to evaluate the genetic component are twin and family studies
and DNA linkage analysis.(11)
Twin and family tree studies are based on the principle that
genetically-influenced traits run in families. The first modern
study of patterns of homosexuality within families was published
in 1985 by Richard Pillard and James Weinrich of Boston University.
Since then, many other systematic studies of twins and siblings
of gay men and lesbians have confirmed the initial results.
The first pooled data for men showed that about 57% of identical
twins, 24 % of fraternal twins and 13% of brothers of gay men
are also gay. For women, approximately 50% of identical twins,
16% of fraternal twins and 13% of sisters of lesbians are also
lesbian. Michael Bailey of Northwestern University estimates
that the overall heritability of sexual orientation is about
53% for men and 52% for women. One of the latest evaluations,
based on the sexual orientation in a US national sample of twin
and non-twin sibling pairs, confirmed that resemblance for sexual
orientation was greater in the identical twins than in the fraternal
twins and that sexual orientation is ‘substantially influenced
by genetic factors.(12)
Family trees of male sexual orientation show that the rates
of homosexuality in maternally-related males are far above the
incidence of 2% in the average population, while the rates in
paternal relatives are close to those of the average population.
This finding raised the possibility of X chromosome involvement
. Males have two sex chromosomes – Y inherited from the
father and an X from the mother. Thus, a trait inherited through
the mother’s side logically might be influenced by a gene
on one of her X chromosomes. This hypothesis is the basis of
the X chromosome DNA analyses by Hamer and his colleagues It
appeared that one small area at the tip of the X chromosome
– Xq28 – was shared by a large percentage of gay
brothers.(13)
The results of such DNA analyses focusing on the X chromosome
can help interpret a study among an Italian population, in which
the mothers of gay men produced an average of 2.7 babies, compared
to 2.3 for the other mothers.(14) It seems that maternally-
inherited factors favouring male homosexuality also promote
female fecundity. This might explain why a genetic factor that
reduces reproductive success remains in the population.
Pre- and perinatal environment
The turning point in our understanding of the effect of environmental
factors in sexual orientation was generated by the work of Gunter
Dorner. Probably because Dorner was based in East Berlin his
pioneering research in the 1970s was not widely noticed.(15,16,17)
Before Dorner there had been unsuccessful attempts to compare
the hormonal profiles of adults expressing different sexual
orientations. Dorner’s studies revealed the importance
of the critical period when the sexual differentiation of the
brain happens. ]While this critical period may vary slightly
from one species to another, it is always around the time of
birth.
Dorner started with animal experiments. Male rats were castrated
on the first day of life and were injected with male hormones
when adults . These male rats expressed a complete inversion
of sexual behaviour. In other words, being deprived of testosterone
during the critical fetal period of sexual determination produced
homosexual behaviour in their adult lives.
What we know now about the hormonal profile of homosexuals
fits perfectly with the hypothesis of a transitory lack of testosterone
during the critical period. Homosexuals usually have the same
level of total testosterone as heterosexuals, but their level
of ‘free testosterone’ (testosterone that is not
combined with other chemicals) is lower. The levels of pituitary
hormones which control testicular functions are relatively high
and so are the levels of oestrogens . It is important to realize
is that if this hormonal profile were to be artificially reproduced
in an adult, it would not give rise to homosexual behaviour.
When a fetus is faced with a lack of testosterone at the end
of pregnancy it compensates for this by increasing secretions
of pituitary hormones. At the same time as the fetus tries to
increase the level of male hormones by a feedback mechanism,
it increases in parallel the level of oestrogens. In fact, oestrogens
increase the binding capacity of sexual hormones with proteins
and lower the level of free testosterone.
This raises the question of how and why some fetuses lack male
hormones at the end of pregnancy. The answer could be that certain
stressful situations at this time might trigger a high level
of activity in the mother’s adrenal glands. The adrenal
glands release male hormones the action of which is different
from testosterone, but similar enough to compete with testosterone
in the fetal brain to lower the amount of free testosterone
. Furthermore, a complementary question is raised: Can prenatal
stress play a causal role in human male homosexuality?
The multidisciplinary approach of Dorner’s team provides
answers to this question. They found a significantly-increased
proportion of homosexual males in the former East Germany among
the population born between 1941 and 1947 (with a maximum relative
frequency in 1944-1945).(18) In a further study by Dorner’s
team, one hundred bi- or homosexual men were asked about the
occurrence of maternal stressful events during their prenatal
life. A significantly-increased incidence of prenatal stressful
situations was found in bisexual and, particularly, in homosexual
men.(19)
Since the 1980s, several experimental studies have supported
the interpretations suggested by Dorner. They have confirmed
in particular that female rats in stressful situations during
pregnancy are more likely to have male offspring who exhibit
in adulthood female receptivity postures (lordosis) in the presence
of other males, than those that are not stressed. Some of these
experimental studies suggested that alcohol consumption may
also modulate the fetal testosterone surge.(20) The effects
of stressful situations, but not of alcohol consumption, were
confirmed by another study involving 7500 human beings and their
mothers. This study was the first to suggest that nicotine has
masculinizing/defeminizing effects on the sexual orientation
of female offspring.(21)
At a time when our focus is on the critical period of sexualisation
of the brain and when we understand how artificial is the separation
of the different components of the ‘Primal adaptive system’
(nervous system, endocrine system and immune system), we can
easily offer interpretations of relevant recently- published
data. According to a Canadian study involving 302 homosexual
men and an equal number of heterosexual men, the presence of
older brothers was linked to an increased probability of homosexuality
in the later-born males, while having older sisters neither
enhance nor counteract this effect.(22) The most plausible interpretation
takes into account that male fetuses are more antigenic to the
mother than female fetuses and thus more likely to provoke maternal
immune reactions. This reaction strengthens after each pregnancy
with a male fetus. The connection between the mother’s
immune reaction and the child’s future sexual orientation
is perhaps some effect of the maternal antibodies on sexual
differentiation of the brain. It is noteworthy that male-specific
Y-linked H-Y antigen(23), which is considered the basis for
the greater antigenicity of male fetuses(24), appears to be
well-represented on the surfaces of brain cells.(25)
The future
The recent accumulation of data regarding the genesis of sexual
orientation has opened several avenues for research:
- Sexual orientation of genetically female subjects:
Compared with the sexual orientation of genetically male subjects,
the sexual orientation of female subjects has not been widely
studied, although Dorner had demonstrated that a complete inversion
of sexual behaviour occurs in female rats following androgen
excess during sexual differentiation of the brain.17
Perkins looked at the morphology of lesbians.(26) He reported
that lesbians had narrower hips and more muscular builds than
non-lesbian women. Within lesbian relationships, those who played
the more dominant role were taller (a statistically highly significant
difference). The dominant women also had broader shoulders and
narrower hips than did lesbians who played passive or intermediate
roles, although only the shoulder measurements were statistically
significant. These differences are consistent with one developmental
process affecting masculinity in both build and personality.
A powerful piece of evidence for lesbianism being related to
prenatal masculinisation comes from a comparison of the auditory
systems of heterosexuals and homosexuals: click-evoked otoacoustic
emissions of lesbians resembled the male pattern rather than
the female pattern.(27)
- The way babies are born
The Primal Health Research Data Bank contains a certain number
of studies accessed via the key-words ‘sexual orientation’
and ‘homosexuality’
It is striking that none of these studies looked at possible
perinatal factors. Since the sexualisation of the brain of mammals
in general is influenced by pre- and perinatal factors, we might
also wonder if the ratio of hetero- to homosexuals is related
to the way babies are born. Is this ratio the same among those
born by the vaginal route without any drug or intervention,
compared with those born after labour induction, or after elective
caesarean, or after a cesarean during labour, or by a premature
birth followed by some weeks spent in an intensive care unit?
- Intrauterine pollution
Since the late 1990s we could gather a sufficient amount of
data to realise that a major threat to the health of the unborn
generations is intrauterine pollution by fat-soluble man-made
molecules. The extensive list of such pollutants includes PCBs,
dioxins and Bisphenol-A that accumulate over the years in our
adipose tissues. Many of these synthetic chemicals are considered
hormonal disruptors. More precisely they mimic oestrogens. This
is how we currently explain the increasing rates of disorders
of the male genital tract (undescended testicles, hypospadias,
testicular cancers, decrease in average sperm counts, etc).
It is plausible that this modern form of pollution can also
influence the sexualisation of the brain. Relevant results of
preliminary animal experiments have already been published.(28)
Interestingly Dorner is already looking at milk pollution (a
marker of intrauterine pollution).(29)
If our centre were rich enough to bestow a ‘Primal Health
Research Award’ I guess that Gunter Dorner would be the
likely candidate .
Michel Odent
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