The following represents the fortuitous collision of a personal pet peeve of mine with an ongoing discussion among a very limited circle of friends and family. For most, the information within will seem a little like reinventing the wheel or, perhaps more damningly, like toppling a strawman. For those of this mindset, let me just encourage you to ignore this or, at the very least, trust that it is necessary (if not to vindicate a theory that by no means needs my meager defense than at least as a means of personal, public catharsis).
It is tragic to me that certain outmoded theories of personality still exist, particularly those binary understandings of personality that want to lump large segments of society into one category or another. I have in mind particularly left-brain/right-brain (from here on, "LB-RB") theories of personality, though my calumny can be just as easily applied to other once serious psychological theories that now only persist in the bloated lexicon of pop psychology (e.g. type-A/type-B personalities). Rather than mounting an exhaustive critique of LB-RB theory, I think a more constructive approach might be to offer a positive alternative. Thus, I give you the Five Factor Model, a theory which attempt to measure personality along five personality continua: openness, conscientiousness, extraversion, agreeableness, and neuroticism. Here are the reasons why I think the Five Factor Model is superior to other models, especially the LB-RB theory:
The Five Factor Model is grounded in serious, ongoing scientific research. Notably, so many of the theories which dominate in pop psychology are no longer current in the psychological or neuroscientific communities. LB-RB, for example, was a product of research in the 1960s on split brain patients, and its formulation was tied social theories about breaking away from "oppressive" traditional (linear, left-brain) ways of thinking. Similarly, type-A/type-B theory was part of medical research going on the in 1950s which has also since been exceeded. The Five Factor Model, in contrast, is the subject of ongoing psychological and scientific debate to determine its strengths and its limits. As one example, consider ongoing attempts to translate the theory into languages and culture other than English to test its universal applicability.
The Five Factor Model allows for a balance between heredity and environment. There is no denying that there is some genetics involved in personality. One need only look at children who grew up estranged from a parent but who nevertheless express many of his or her characteristic personality traits (i.e. sons of divorced parents who grow up just like their father's anyway). The LB-RB model, however, doesn't allow for much of anything beyond heredity to play a role. By claiming to tie personality directly to the anatomical make up of one's brain, it makes personality something static and intrinsic. In reality, our personalities are shaped by our experiences as much as our genetics; who you are is more than your DNA. Testing of the five traits that make up the Five Factor Model shows that each trait is influenced roughly equally by heredity and environment, reflecting a more balanced approach to the nature-nurture debate than you receive in LB-RB theories.
The Five Factor Model is first and foremost descriptive, not predictive. There is a temptation when we classify anything, but particularly something as abstract as personality, to take that classification and apply it like a hard-and-fast rule. This temptation is particularly pronounced with LB-RB theory because it so neatly claims to predict modes of thinking and therefore types of behavior. "Person X will never be an artist. He's too left-brained." That sort of thinking limits not only the ability of people to explore and excel and grow, but it limits the thinkers ability to interact authentically with people who are so surgically sorted into artificial categories. Proponents debate the degree to which the Five Factor Model can be used to predict behavior, but all agree that the five traits it offers are a useful tool in describing and understanding behaviors. It is more open to the question "why do you think you did that" than the prediction that, inescapably, "you will do that."
Finally, the Five Factor Model gives a more complex and thus more believable picture of human personalty. The biggest flaw in most forms of binary thinking is precisely that they are binary. As with Hegel's view of history and Zoroaster's view of deity, collapsing a complex reality into two broad categories is almost always erroneous. LB-RB theory tries to simplify reality and convince us that there are essentially only two kind of people. It is a tantalizing prospect, but our experience teaches us that there are millions of kinds of people. The Five Factor Model harmonizes better with our experience of people because it makes personality a combination of any number of five variables, each of which a given person can possess to varying degrees on a continuum. For example, why do we have two supremely organized women, both friendly toward people, thirsty for new ideas, and generally unfazed by problems in the world, one of whom becomes a big-time defense attorney and the other a mild-mannered suburban librarian. The LB-RB model has no answer for why two people who manifest the same kind of thinking would have radically different life paths. The Five Factor Model recognizes that people can be almost identical in numerous personality categories (four of the five in this example) and still have very different personalities and different paths in life based on small differences in certain personality features (in this case, degree of extraversion). That is only one rough analogy, but the point is simple: personality is a complex reality that requires a complex understanding that binary modes of thinking simply cannot satisfy.
Obviously, any attempt to quantify personality and to put people into categories (even five factored categories) is going to suffer from the same pitfall of oversimplifying and artificially labeling realities which we fundamentally do not understanding. Just because no theory is perfect, however, does not mean that all theories were created equal. It is time for serious-minded people to cast aside antiquated modes of thinking and convenient tools for ordering their world and advance into the messier but more useful methods presently endorsed by the psychological and scientific communities. Anyone interested in the Five Factor Model might consider a wonderful little book, very accessible in its writing style, written by a professor at the University of Newcastle and published by Oxford University: Personality: What Makes You the Way You Are?