A left hand reaches out, nearly touching a right hand.

Left Hand, Right Hand

Years ago, William Calvin wrote a popular book attributing the evolution of right-handedness to the way it allowed women nursing an infant to still throw stones at a threatening predator. On the other hand, Faurie and Raymond thought it was left-handedness that carried the selective advantage in combat with righties.

The fact is that no one knows where left-handedness comes from, though that hasn’t stopped generations of witless throwbacks from uttering slurs against sinistral folks. Another fact is that a lot of people aren’t exclusive lefties or righties. They write with one hand and throw or eat with another, for example. Geneticists have analyzed patterns of mixed or crossed dominance without much enlightenment.

Left-handedness or right-handedness may have its origin in embryonic development. At any rate, there’s a sex difference that appears in childhood, favoring males with higher incidence. 

Most of what we read about handedness goes no further than correlations. Since no one knows the cause, handedness can’t be said to govern anything in our behavior.

Handedness may be an important clue to something, but for now the scientific world repeats (in unison, please) we just don’t know.

How many ways are there for people to differ—in DNA, early development, and later experience?

A cat demonstrates paw preference.

BIO: Although handedness is most evident in humans, paw preferences can be seen in a number of other mammals, and the terms are useful descriptions of behavioral tendencies.

On the other hand, dominance and lateralization are explanatory rather than descriptive terms. That is, they imply hemispheric specialization in the brain.

The study of cerebral dominance received a boost from surgical efforts to control epilepsy. The practice of split-brain surgery (commissurotomies) started with efforts to prevent the spread of epilepsy within a patient’s brain. Epileptic seizures are triggered by a region in the cerebral cortex called a seizure focus, where neurons fire in synchrony every so often. A seizure consists of waves of synchronized activity among neurons, usually in the cerebral cortex.  A few neurons in the seizure focus that emit activity in bursts may trigger a seizure by recruiting larger numbers of neurons in the synchronized activity.  In larger seizures, high-amplitude EEG waves may be seen to pass across the surface of the cortex in a progression called the Jacksonian march, which is revealed outwardly in the spread of convulsive jerks across the body.

Since one active seizure focus can train other areas of the brain to develop seizure foci, it was logical to try cutting the nerve fibers (or axons) that connected the cerebral cortex in one hemisphere with the corresponding area in the other hemisphere. (These fiber bundles are called commissures, and the largest is the corpus callosum.) This procedure may have been logical, but back in the 1940s, Van Wagenen and Akelaitis (ak-uh-LYE-tis) couldn’t know that the surgery would do more than limit the spread of epilepsy. The corpus callosum looked like a good candidate for surgery. It’s the largest commissure in humans, and the surgeon can see it by spreading apart the cerebral hemispheres. It’s a nearly bloodless procedure, since there is almost no blood supply for the corpus callosum on the midline, where the transection was made.

The surgery created so-called “split-brain” people whose cerebral hemispheres ended up with decreased communication between the left and right halves of the cerebral cortex. The work of Sperry, Bogen, and Gazzaniga has opened new understanding of how functions like language and humor are specialized in one hemisphere more than the other.

In any case, it’s getting more popular to map behavioral differences to brain connections (research here) rather than mapping behavioral functions to supposed centers. The conventional way to interpret the changes in behavior of the three most famous patients in psychology–Phineas Gage, Tan/Leborgne, and H.M.–has been to say that their brain lesions removed islands of function responsible for emotional regulation, speech production, and memory formation.  

Another tack is to investigate the connections that were interrupted, even for Phineas, Tan and H.M. A penetrating wound, stroke, or surgical ablation severs the connections that integrate different brain regions. 

If we think of brain functions as localized to pathways–connections–rather than centers, we can avoid the exaggerations of reductionism, or explaining major behavioral events as no more than what goes on in a few million  neurons. Instead we find behavior in the connectome, though it’s not easy to trace.

PSYCHO: The evolution of lateralized functions in the brain may eventually reveal an interesting example of interaction between our genome and our tool-using phenotypes.

An oddity that accompanies dominance is crossed control of our limbs. The crossed control of movement has fascinated researchers for as long as they have wondered about the crossing of the optic nerves to become the optic tracts. Why should axons cross the midline so mysteriously?

One idea is that the crossed motor control–left hemisphere controlling right-side muscles, and vice versa–is compensation for the left-right reversal of the retinal image, caused by the simple convex lenses of our eyes. This was the explanation taught by British neurobiologist J. Z. Young*, according to former student Lynn Janice Bindman (scroll down) and possibly appropriated without attribution by others. (Maybe it was a case of convergent cognition. I have discovered one or two of my own teaching inventions in lectures delivered by others.)

Knowledge of crossed control is invaluable in assessing brain injury. It may have evolved to support eye-hand coordination.

But this brings up a useful point about evolution: It operates without purpose; it is blind. Mutations in nature are random, as far as anyone knows. So perhaps crossed control is not adaptive. Maybe it’s just a consequence of another change called the somatic twist: a 180-degree rotation  of the body just behind the brain!

Some bioengineers began a discussion of the adaptiveness of design principles that would lead to decussation, or crossing, with the pronouncement that decussation must confer a “significant functional advantage” because of its persistence in evolution. But these are “just-so stories“, as the late Steven Jay Gould labeled them. They promote the notion that things are the way they are because it just seems fitting!

The inventors of just-so stories about evolution have a reply for their critics, of course: “Vas you dere, Charley?

*Always pronounced “Jay Zed Young”, the last letter of the alphabet being zed for our Anglo-Saxon cousins.

SOCIAL: It seems safe to say that 10.6 percent of us are left-handed, even though many of us are not exclusively lefties or righties, but use different hands for writing and throwing. Yet the lopsided majority of right-handers has shaped the world we live and work in.

This is not to say that anyone is “right-brained” or “left-brained”. That would surely be wrong. Even correlations that are often illusory have made a stigma of left-handedness in other times and cultures.

What is normal?

In one sense it means that a behavior meets a criterion, like a normal temperature, body weight, or normal visual acuity. Legally we think of normal behavior as abiding by the law. In child development, normal can mean keeping pace with a chart of milestones. In this sense, abnormality is anything that fails to meet the criterion. This would be a bad way to select behavior for psychotherapy because “normal” refers to a type rather than to a trait dimension. It puts people in a box. If we refer to normal (i.e., age-appropriate) intelligence using this sense, most of us are abnormal!

In another sense, normal means average or modal. If you have normal color vision you match colors the way that most other people do. This is the direction that stereotypes take to stigmatize the behavior of left-handed people. It may be fine for judging the weather but it would be a poor choice for a psychologist.

Finally, normality can refer to a range of behavior. Because of measurement error, no single criterion of normality can be enforced strictly. Normality is always, actually, a range of measurement values.

Normal has to mean “adhering to a norm”, but we have to recognize that each person will vary in displaying a trait, and two people may adhere in different ways as well as in different magnitudes. In most instances psychologists define normality to include most of us. In temperament, intelligence, neuroticism, or introversion-extraversion, most of us are normal. Abnormality is based on the concept of extreme deviance rather than on a stereotype. That is, people who fall in the tails of a frequency distribution of measurements for a trait are treated as abnormal. Thus, an antisocial person is not someone who is never social or always antisocial in extreme degree, but rather someone whose behavior lies several standard deviations from the average (i.e., mean) in repeated observations.

In all of these senses, normality is a construct. It’s not part of nature or part of a person. When a physician describes a knee jerk as normal it’s an observation of physical reality. In contrast, when a psychologist describes someone as having normal creativity it’s an abstraction*. We do not have reflexes for creative behavior or cellular connections for creativity that are fixed in the brain, so normal creativity can be defined as nearly anything as long as it’s defined operationally.

The term may be even more confusing outside psychology. But would you agree that in the case of handedness, both types are normal?

*It’s worth keeping in mind that the diagnostic categories in DSM-V are also constructs!

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