Human Sexual Orientation: "The Black Box"

from A Separate Creation: The Search for the Biological Origins of Sexual Orientation by Chandler Burr

A "black box" is a biological mystery. It s what biologists call any human trait they've observed, studied, and measured on the "outside" of human beings but whose "inside" origins-- which genes formed it, which hormones made it that way-- remain mysterious. Each aspect of the trait that biologists establish (these are pretty standard: prevalence data (how many people in a random population have it), twin data, correlation data (do blacks or Asians have it more than whites, for example), pathology or not, and so on) is a clue pointing to the solving of the mystery.

We have a long list of black boxes human traits-- and each of these traits has a "clinical profile," the sum total of clues we have from looking at the outside, clinical level. They are mysteries waiting to be biologically solved on the inside, the mystery of what creates them: eye color, height, cystic fibrosis, cancer, intelligence, Tay Sachs, baldness, athletic ability, resistance to some viruses and susceptibility to others, skin tone and muscle mass and allergies and sexual orientation. Some traits can be defined simply by looking at the person, like hair color or height. Some cannot, like cancer or blood type (A, B, or O). Some human traits are behavioral, like manual dexterity, sexual orientation, hand-eye coordination, and schizophrenia, and some are not, like blood type (A, B, or O), race, or the hardness of tooth enamel. Some are disease traits: hemophilia, schizophrenia, cancer, color blindness. Some are politically and religiously charged: skin color, sex. Some traits are not politically and religiously charged: the hardness of the enamel on your teeth, which is controlled by a single gene, whose location is known, and whose functioning we understand.

Here s a mystery for you (which should be pretty easy to figure out). One particular black box interests us here. It has been the object of decades of empirical observation, and researchers have compiled a pretty complete "clinical profile" for it. We know what the trait is; what we don t know is what creates it. This is What We Know, the clinical profile:

  1. The trait is referred to by biologists as a "stable bimorphism, expressed behaviorally."

  2. Its exists in the form of two basic internal, invisible orientations, over 90% of the population accounting for the majority orientation and under 10% (one reliable study puts the figure at 7.89%) for the minority orientation, although there is still debate about the exact percentages.

  3. Only a very small number of people are truly equally oriented both ways.

  4. Evidence from art history suggests the incidence of the two different orientations has been constant for five millenia.

  5. A person's orientation cannot be identified simply by looking at him or her; those with the minority orientation are just as diverse in appearance, race, religion, and all other characteristics as those with the majority orientation.

  6. Since the trait itself is internal and invisible, the only way to identify an orientation in someone else is by observing in them the behavior or reflex that express it. However--

  7. The trait itself is not a "behavior." It is the neurological orientation expressed, at times, behaviorally. A person with the minority orientation can engage, usually due to coercion or social pressure, in behavior that seems to express the majority orientation-- several decades ago, those with the minority orientation were frequently forced to behave as if they had the majority orientation-- but internally the orientation remains the same. As social pressures have lifted, the minority orientation has become more commonly and openly expressed in society.

  8. Neither orientation is a disease or mental illness. Neither is pathological.

  9. Neither orientation is chosen.

  10. Signs of one's orientation are detectable very early in children, often, researchers have established, by age two or three, and one's orientation has probably been defined at the latest by age two, and quite possibly before birth.

These first intriguing observations began to catch the attention of researchers. The trait looked biological in origin. The data was indicating that the trait had a genetic source:

  1. Adoption studies show that the orientation of adopted children is unrelated to the orientation of their parents, demonstrating that the trait is not environmentally rooted.

  2. Twin studies show that pairs of identical (monozygotic) twins, with their identical genes, have a higher-than-average chance of sharing the same orientation compared to pairs of randomly selected individuals; the average (or "background") rate of the trait in any given population is just under 8%, while the twin rate is just over 12%, over 30% higher.

  3. The incidence of the minority orientation is strikingly higher in the male population-- about 27% higher-- than it is in the female population, a piece of information that gives indications to the biological conditions creating the trait.

  4. Like the trait eye color, familial studies show no direct parent-offspring correlation for the two versions of the trait, but the minority orientation clearly "runs in families," handed down from parent to child in a loose but genetically characteristic pattern.

  5. This pattern shows a "maternal effect," a classic telltale of a genetically-loaded trait. The minority orientation, when it is expressed in men, appears to be passed down through the mother.

That s the clinical profile for this trait, this black box.

Excerpted from A Separate Creation: The Search for the Biological Origins of Sexual Orientation by Chandler Burr

What is the trait?....