Brothers and twins

Get any group of people together and let them talk long enough about homosexuality, and I can guarantee you the nature/nurture topic will come up. Many straight people will offer all sorts of pet theories about how people "turned" gay, but almost all gay persons will aver that it is in-born. They believe so because their sexual orientation is not something they have ever chosen, nor something they can unchoose.

This, of course, is oversimplified. A lot of things that are acculturated in us, like feelings of guilt in certain situations, or our sense of ethnic identity, are, except for a handful who can somehow radically unmake themselves, an essential part of us, inerasable till the day we die. In other words, even if our gayness is something we cannot excise from ourselves, it doesn't necessarily mean it must be in-born.

On the other hand, "in-born" or "nature" can mean a lot of things, and the nature/nurture debate also glosses over the differences. Many people associate, fully or in part, "in-born" with the idea of genetic determinism. It's in your genes -- that's why you are what you are. But in fact, embryonic development, or womb conditions, or some hormonal difference in the growing child, or unusual neurological development in our brains... all these are possible too, aren't they, within the meaning of biological nature?

Before my reader expects too much, let me say here and now, that there is, with today's state of knowledge, no definitive answer to the nature/nurture question. Some very interesting research have been done in the last 20 years or so, suggesting genetic markers, differences in brain structure and so forth, but while they are beginning to bring some pieces into the jigsaw puzzle, we still can't see the overall picture. In any case, scientific studies need to be carefully replicated before we can give full credence to the results (but see footnote).

Some researchers have mapped the incidence of homosexuality in families, or between twins, and having read a few of them, I will try to explain my interpretation of them to a lay audience. However, I should caution at the outset, that these studies are far from being the final word on the matter, and are still subject to some dispute even among scientific circles.

The starting point must be an idea of the "background incidence", that is, the percentage of men who are homosexual, in the general population. This itself is far from settled. Findings have ranged from 1 percent to something like 10 percent, or even more, largely because the way homosexuality is defined and the methodology used have great impact on the results (One such study is the subject of the article The Calgary Study) However, most people working in the field are coming to the consensus that the incidence is roughly 5 to 7 percent of adult males, if one uses a moderately stringent definition of homosexuality.

Knowing this "background incidence" is important as a basis for comparison. If you took just any other person randomly, you would have a 5 to 7 percent chance of picking a homosexual person, so if you took a group of people and found that they had a much higher incidence of homosexuality among them, then you could say that some other factor was operating, since pure chance alone would not give you a figure much different from the background incidence.

Dean Hamer et al published a paper in July 1993 showing A Linkage between DNA Markers on the X Chromosome and Male Sexual Orientation. The title alone is a mouthful, and has already worn us out, so we are not going to go into DNA and stuff like that.

However, there was an interesting finding in his study which we can grasp quite easily. He had traced the incidence of homosexuality in families of 76 homosexual men. Among the mass of findings, one stood out: these 76 men had 104 brothers, and of these 104 brothers, 14 were also homosexual. That made it 13.5 percent. That is significantly higher than the background incidence.

Of course, this figure alone does not really tell us very much. True, brothers share some genes, but they also share the same family environment, or the same mother's uterus. Hamer et al went further to study the DNA of these families and came to additional conclusions that there were genetic markers, but we won't go into that, as his paper gets too complicated for people quite innocent of genetic biology, like us.

For now, we'll just bear those two percentages in mind. Background (random) incidence: 5 to 7 percent, but concordance between brothers, 13.5 percent, about twice as high as pure chance would allow.

Studies have also been made of twins. Now, twins are of two kinds: monozygotic twins or identical twins in layman's language, and dizygotic twins, i.e. fraternal twins. Monozygotic twins share identical genetic make-up, that is why they look so much alike. The mother conceived a single embryo, which later split into two individuals. Dizygotic twins originated from separate embryos, each with its own genetic inheritance, except that the mother conceived the two of them at the same time. Genetically, dizygotic twins are equivalent to the common brother relationship, except for being born together.

Studying twins can therefore give us a peep into whether genes are responsible for a particular trait. If the trait being studied is solely caused by a gene, and if one identical twin has it, then his identical brother must invariably have it too, since they share the same genes. But for fraternal twins the trait does not always appear in both together, since they have different genetic profiles.

I have a copy of the paper by Whitam, Diamond and Martin, published in June 1993, entitled Homosexual Orientation in Twins: A Report on 61 Pairs and Three Triplet Sets. Another mouthful of a title. Anyhow, what the researchers did was to collect 61 homosexual persons, and check whether their twins (or triplet siblings in 3 cases) were also homosexual.

Of these 61 pairs of twins, there were 14 pairs of male-male dizygotic (i.e. fraternal) twins. 4 of their 14 twin brothers were also homosexual (28.6 percent), while 10 were heterosexual.

There were 34 pairs of monozygotic (i.e. identical) male twins. 22 of their twin brothers were homosexual (64.7 percent), 10 were heterosexual and 2 were sort-of bisexual and inconclusive.

Other studies have also been made of homosexuality in twins. Bailey and Pillard (1991) in a relatively larger study, found a 52 percent concordance rate for homosexual orientation among 56 male identical twin pairs, and a 22 percent concordance rate for 54 male fraternal twin pairs. Their findings are close to what Whitam, Diamond and Martin found.

In yet another study by King and McDonald (1992), the concordance rate among identical twins was found to be only 25 percent (5 out of 20).

This table summaries the results:

CONCORDANCE RATE OF HOMOSEXUALITY
BETWEEN BROTHERS AND TWINS

Sibling type	Hamer Study	Whitam et al	Bailey & P	King & McD
Common brothers	13.5%	-	-	-
Dizygotic twins	-	28.6%	22.0%	12.5%
Monozygotic twins	-	64.7%	52.0%	25.0%

What conclusions can we draw from these numbers?

1. Brothers have a higher incidence of homosexuality than chance alone.
2. Fraternal (dizygotic) twins seem to have an even higher rate of concordance than common brothers. Genetically, fraternal twins are no closer to each other than common brothers so why should their concordance rate be higher? Since the key difference is that they shared the same pregnancy in the mother, this suggests that there is some factor related to the womb or the mother's hormonal balance during a pregnancy.
3. Identical twins have the highest rate of concordance, strongly suggesting a genetic factor, because they are the only types of brothers who share the same genes. Yet, genes are not the only explanation, because if they were, the concordance rate among identical twins should have been 100 percent.

Together, these findings also seem to rule out the idea that homosexuality is all a matter of upbringing or the environment -- for example, the oft-repeated distant father, dominant mother syndrome. The increase in concordance rate as biological commonalities increase, indicates at least a substantial biological basis for homosexuality.

In other words, it isn't all in the genes, and it isn't all in the environment. Given the complex pattern we see, it is very likely to be a complex mix of causes. And that's about all I can say at this point. We don't have a full answer, though we can begin to lay a few myths to rest.

© Yawning Bread 

Footnotes

1. More commentary on sibling data by C K Ho can be seen in the article Playing fast and loose with science. He was rebutting Christian fundamentalists' misreading of scientific data

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