Handedness and Brain Lateralization
(and who cares?)
WHAT IS BRAIN LATERALIZATION?
The human brain is a paired organ; it is composed of two halves (called cerebral hemispheres) that look pretty much alike.
The term brain lateralization refers to the fact that the two halves of the human brain are not exactly alike. Each hemisphere has functional specializations: some function whose neural mechanisms are localized primarily in one half of the brain.
In humans, the most obvious functional specialization is speech and language abilities. In the mid-1800s, Paul Broca (a French neurosurgeon) identified a particular area of the left hemisphere that plays a primary role in speech production. Shortly afterwards, a German neurologist, Carl Wernicke, identified another part of the left hemisphere primarily concerned with language comprehension.
Most humans (but not all) have left hemisphere specialization for language abilities. The only direct tests for speech lateralization are too invasive to use on healthy people, so most of what we know in this area comes from clinical reports of people with brain injuries or diseases. Based on these data, and on indirect measures, we estimate that between 70% to 95% of humans have a left-hemisphere language specialization. That means that some unknown percentage of humans (maybe 5% to 30%) have anomalous patterns of specialization. These might include: (a) having a right-hemisphere language specialization or (b) having little lateralized specialization. The more one knows about the neurological mechanisms underlying language abilities, the more complicated these issues become. For instance, some language functions (like prosody-- the emotive content of speech) is specialized in the right hemisphere of people with left-hemisphere language specializations. The bottom line is that, despite overly-simplistic descriptions of left-brain / right-brain stuff one finds in introductory textbooks and the public press, there is still a great deal about brain lateralization that we simply do not yet understand.
  WHAT IS HANDEDNESS?
«Handedness» is a vague term, and can mean many things to many people. Most people in our society define handedness as the hand you use for writing. Within the scientific community, the vagueness of this term has led to much debate. Researchers define handedness based on different theoretical assumptions. For instance, some define handedness as (a) the hand that performs faster or more precisely on manual tests, while others define it as (b) the hand that one prefers to use, regardless of performance. Some think that there are two types of handedness: (a) either left or right, or (b) either right or non-right, while others think there should be three categories (to include ambidexterity). Some think there are two different kinds of ambidexterity. Some think that handedness should not be lumped into 2 or 3 or 5 categories, but rather measured along a scale of a continuum. These are just examples of a few of the differing criteria for handedness! My work attempts to resolve some of these issues.
WHAT DOES HANDEDNESS HAVE TO DO
  WITH BRAIN LATERALIZATION?
The same chap that identified a region of the brain specialized for language Paul Broca (Paul Broca) also suggested that a person's handedness was opposite from the specialized hemisphere (so a right-handed person probably has a left-hemispheric language specialization). But the kick is: this is not a mirror correlation (that is, a majority of left-handers also seem to have a left-hemispheric brain specialization for language abilities). Tricky business, eh? For over 150 years, many researchers have been trying to figure out this robust-but-imperfect correlation between handedness and brain lateralization. We are still trying.
WHO CARES?
The primary historical reason that the hand-brain link was considered important and became a generally-accepted methodology, was because for nearly a century it was the only hint that a neurosurgeon had prior to surgery which hemisphere was specialized for language. Clinicians used handedness as a marker for brain lateralization until the Wada (sodium amytal) Test was introduced in the 1960s.
This association between hand and brain captured the imaginations of researchers because it would be so useful (so easy, so non-invasive, so cheap) to study patterns of brain asymmetries by using a person's handedness as a marker for brain lateralization (direct methods involve neurosurgery, invasive drug testing, or expensive imaging techniques). I have argued, however, that many fundamental problems exist with this methodology, and advocate going back to the drawing board to work out some of these basic problems, rather than continue to embrace 19th-century methodology.
Better understanding how handedness relates to brain function is relevant to many people, among them: academic researchers, medical clinicians, neurological patients, educators and left-handers . Clarifying the relationship between handedness and functional brain specializations, and learning more about the developmental and neurobiological mechanisms that underlie these relationships, may help us better understand a wide range of seemingly unrelated issues such as dyslexia, stuttering, human variation, comparative brain research, developmental neurobiology of the brain, and the origins of human language.