An excerpt from Chapter 6 (Formative Years) from Nature via Nurture by Matt Ridley
The 1990s saw a series of studies that revived the idea of homosexuality as
a "biological" rather than a psychological condition, as a destiny rather
than a choice. There were studies showing that future homosexuals had
different personalities in childhood, studies showing that homosexual men
had differences in brain anatomy from heterosexual men, several twin studies
showing that homosexuality was highly heritable in western society, and
anecdotal reports from homosexual men to the effect that they had felt
"different" early in life." On its own none of these studies was
overwhelming. But together, and set against decades of proof that aversion
therapy, "treatment," and prejudice entirely failed to "cure" people of gay
instincts, the studies were emphatically clear. Homosexuality is an early,
probably prenatal, and irreversible preference. Adolescence simply throws
fuel on the fire.
What exactly is homosexuality? It is plainly a whole range of behavioral
characteristics. In some ways gay men seem to be more like women: they are
attracted to men, they may pay more attention to clothes, they are often
more interested in people than, say, football. In other ways, however, they
are more like heterosexual men: they buy pornography and seek casual sex,
for example. (Playgirl's nude centerfolds of men turned out to appeal mainly
to gays, not the intended women.)
People, like all mammals, are naturally female unless masculinized. Female
is the "default sex" (it is the other way around in birds). A single gene,
called SRY, on the Y chromosome starts a cascade of events in the developing
fetus leading to the development of masculine appearance and behavior. If
that gene is absent, a female body results. It is therefore reasonable to
hypothesize that homosexuality in men results from the partial failure of
this prenatal masculinization process in the brain, though not in the body
(see chapter 9).
By far the most reliable discovery about the causes of homosexuality in
recent years is Ray Blanchard's theory of the fraternal birth order. In the
mid-1990s Blanchard measured the number of elder brothers and sisters of gay
men compared with the population average. He found that gay men are more
likely to have elder brothers (but not elder sisters) than either gay women
or heterosexual men. He has since confirmed this in 14 different samples
from many different places. For each extra older brother, a man's
probability of being gay rises by one-third. (This does not mean that men
with many elder brothers are bound to be gay: an increase from, say, 3
percent of the population to 4 percent is an increase of one-third.)
Blanchard calculates that at least one gay man in seven, probably more, can
attribute his sexual orientation to this effect of fraternal birth order. It
is not simply birth order, because having elder sisters has no such effect.
Something about elder brothers must actually be causing homosexuality in
men. Blanchard believes the mechanism is in the womb rather than the family.
One clue lies in the birth weight of baby boys who will later become
homosexual. Normally, a second baby is heavier than a first baby of the same
sex. Boys especially are heavier if they are born after one or more sisters.
But boys born after one brother are only slightly heavier than firstborn
boys, and boys born after two or more brothers are usually smaller than
first-and second-born boys at birth. By analyzing questionnaires given to
gay and straight men and their parents, Blanchard was able to show that
younger brothers who went on to become homosexual were 170 grams lighter at
birth than younger brothers who went on to become heterosexual. He confirmed
the same result-high birth order, low birth weight compared with controls-in
a sample of 250 boys (with an average age of seven) who were showing
sufficient "cross-gender" wishes to have been referred to psychiatrists;
cross-gender behavior in childhood is known to predict later homosexuality.
Like Barker, Blanchard believes that conditions in the womb are marking the
baby for life. In this case, he argues, something about occupying a womb
that has already held other boys occasionally results in reduced birth
weight, a larger placenta (presumably in compensation for the difficulty the
baby experiences in growing), and a greater probability of homosexuality.
That something, he suspects, is a maternal immune reaction. The immune
reaction of the mother, primed by the first male fetuses, grows stronger
with each male pregnancy. If it is mild, it causes only a slight reduction
in birth weight; if strong, it causes a marked reduction in birth weight and
an increased probability of homosexuality.
What could the mother be reacting to? There are several genes expressed only
in males, and some are already known to raise an immune reaction in mothers.
Some are expressed prenatally in the brain. One intriguing new possibility
is a gene called PCDH22, which is on the Y chromosome, is therefore specific
to males, and is probably involved in building the brain. It is the recipe
for a protocadherin (see Chapter 5). Could this be the gene that wires the
bit of the brain that is peculiar to males? A maternal immune reaction may
be sufficient to prevent the wiring of the part of the brain that would
eventually encourage a fascination with female bodies.
Clearly not all homosexuality is caused this way. Some of it may be caused
directly by genes in the homosexual person without the mediation of the
mother's immune reaction. Blanchard's theory may explain why it has proved
so hard to pin down the "gay gene." The main method for finding such a gene
is to compare markers on the chromosomes of homosexual men with those of
their heterosexual brothers. But if many gay men have straight elder
brothers, this method would work poorly. Besides, the key genetic difference
might be on the mother's chromosomes, where it causes the immune reaction.
This might explain why homosexuality looks as though it is inherited through
the female line: genes for a stronger maternal immune reaction could appear
to be "gay genes," even though they may not be expressed in the gay man
himself but only in the mother.
But notice what this does to nature versus nurture. If nurture, in this case
birth order, causes some homosexuality, it does so by causing an immune
reaction, which is a process directly mediated by genes. So is this that
environmental or genetic? It hardly matters, because the absurd distinction
between reversible nurture and inevitable nature has now been well and truly
buried. Nurture in this case looks just as irreversible as nature, perhaps
more so.
Politically, the confusion is even greater. Most homosexuals welcomed the
news in the mid-1990s that their sexual orientation looked "biological."
They wanted it to be a destiny, not a choice, because that would undermine
the argument of homophobes that it was a choice and therefore morally
questionable. How could it be wrong if it was innate? Their reaction is
understandable but dangerous. A greater tendency to violence is also innate
in the human male. That does not make it right. Reasoning that "ought" can
be derived from "is" is called "naturalistic fallacy." To base any moral
position on a natural fact, whether that fact is derived from nature or from
nurture, is asking for trouble. In my morality, and I hope in yours, some
things are bad but natural, like dishonesty and violence; others are good
but less natural, like generosity and fidelity.
