"Here is the profile of a trait on which clinical research has been done for decades. It is taken from the published scientific literature. The trait should be rather obvious:

1. This human trait is referred to by biologists as a "stable bimorphism"- it shows up in all human populations as two orientations- expressed behaviorally.
2. The data clinicians have gathered says that around 92% of the population has the majority orientation, 8% has the minority orientation.
3. Evidence from art history suggests the incidence of the two different orientations has been constant for five millennia.
4. The trait has no external physical, bodily signs.  That means you can’t tell a person’s orientation by looking at them. And the minority orientation appears in all races and ethnic groups.
5. Since the trait itself is internal and invisible, the only way to identify an orientation is by observing the behavior or the reflex that expresses it. However-and this is crucial-
6. – because the trait itself is not a "behavior" but an internal, invisible orientation, those with the minority orientation can hide, usually due to coercion or social pressure, by behaving as if they had 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 remained the same and as social pressures have lifted, people with the minority orientation have been able to openly express it.
7. Clinical observation makes it clear that neither orientation of this trait is a disease or mental illness. Neither is pathological in any observable way.
8. Neither orientation is chosen.
9. Signs of one’s orientation are detectable very early in children, often, researchers have established, by age two or three. And one’s orientation probably has been defined at the latest by age two, and quite possibly before birth.

These data indicated that the trait was biological, not social, in origin, so the clinicians systematically asked more questions. And these started revealing the genetic plans that lay underneath the trait:

1. Adoption studies show that the orientation of adopted children is unrelated to the orientation of their parents, demonstrating that the trait is not created by upbringing or society.
2. Twin studies show that pairs of identical 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 rate of this trait in any given population- it’s called the "background rate"-is just under 8%, while the twin rate is just above 12%, more than 50% higher.
3. This trait’s incidence of the minority orientation is strikingly higher in the male population- about 27% higher-than it is in the female population. Many genetic diseases, for reasons we now understand pretty well, are higher in men than women.
4. Like the trait called eye color, the familial studies conducted by scientists show that the minority orientation clearly "runs in families," handed down from parent to child.
5. This pattern shows a "maternal effect," a classic telltale of a genetic trait. The minority orientation, when it is expressed in men, appears to be passed down through the mother.

Put all this data together, and you’ve created the trait profile. The trait just described is, of course, handedness."