Theories on Handedness

Because it develops so early, this brain asymmetry appears to be largely innate. It is possible, however, that environmental factors begin operating even before birth. One hypothesis is that the right hand becomes more skillfull because it has greater freedom to move in the womb. About three-quarters of all fetuses spend the last several weeks of gestation with their right arm facing out—toward the mother's abdominal wall. This arm has more space in which to move than does the left arm, which keeps running into the mother's spine. This may lead to differential growth and wiring of the hand areas on each side of the motor cortex. However, studies to test this prenatal position hypothesis have thus far proven inconclusive.

Another possibility is that the right hand tends to become more skillful because of an innate preference in head orientation. It turns out that most newborns favor turning their head to the right. Because head-turning triggers the asymmetric neck reflex in newborns—a brain-stem response in which both the arm and leg extend on the same side that the head is facing, while the opposite limbs flex (see Figure 6.3)—babies spend a lot more time looking at their right arms than their left, a posture that will preferentially promote hand-eye coordination on the right side. In fact, studies have shown that babies who have just begun reaching tend to favor the same hand as their head orientation, although they don't necessarily preserve this hand preference later. It's not known why babies favor a right-head orientation, but it may have to do with the fact that most parents hold their babies in their left arm, regardless of their own handedness.

Finally, there is one school of thought (though not a very popular one among left-handers) that states that right-handedness is the "normal" or default developmental pathway and that all left-handedness is the result of some kind of pathology. The logic here is that the brain is genetically destined to be right-handed, but that any kind of prenatal or birth-related damage to the left hemisphere is going to switch hand control to the right hemisphere—the left hand. Left-handedness is indeed much more common among people with disorders related to brain damage, such as epilepsy or mental retardation. Some studies have also found that babies experiencing more difficult deliveries are likelier to have altered patterns of hand use. But it seems very unlikely that all left-handers could have experienced some kind of covert left-hemisphere damage that left them cognitively unaffected in other respects. Rather, most researchers believe that there are two categories of left-handed individuals: the majority, whose handedness is largely inherited, and the minority, whose handedness is due to some kind ot pathological event before or during birth.

Three hypotheses for why right-handedness is the dominant trait in humans.