What do the debates over the nature of introversion/extraversion and the debates over the nature of giftedness have in common?
At risk of giving away the punch line: both disputes stem from the use of cluster concepts made up of several continuous traits. It's easy enough to classify someone with all the traits, or none. When a person has only a few of the traits, the arguments begin.
For example, it's easy enough to recognize an extreme extravert by her gregariousness, cheerful disposition, adventurousness and often impulsivity, and tendency to direct her attention outward, speaking and acting rather than keeping her thoughts to herself. Such an extravert will also share the biological characteristics of extraversion--lower sensory processing sensitivity, a tendency to drool less when exposed to intense flavors like lemon juice, a tendency to be a morning lark rather than a night owl, etc. Similarly, one can recognize an extreme introvert by his quietness, even shyness, his tendency towards anxiety and dark moods, his preference to stay at home and keep to himself, and his attention to his inner life over speaking or acting. A researcher would notice his high sensory processing sensitivity, his tendency to drool a lot when exposed to lemon juice, and his night owl constitution.*
But what about those who have some traits but not others? Those who wonder whether they are "outgoing introverts" or "extraverts with sensory processing problems?" What about those who are quiet but not shy or fearful, or those who respond to reward like extraverts but pause to check like introverts? Here you will find heated arguments, whether among personality buffs on message boards or personality researchers in academic journals.
Is the extroversion/introversion dimension about shyness, a preference for the brain's activation vs. inhibition system, sensory processing sensitivity & desire for sensory stimulation, or reward sensitivity? Depends on whom you ask*, and which answer you choose determines who gets classed as a moderate extrovert (e.g., an extrovert with sensory processing problems) and who gets dubbed a moderate introvert (e.g., a sociable introvert). Which label you receive in turn dominates others' expectations for you (e.g., are you likely to have a lot of friends and go to parties often?), and may lead them to treat you differently (e.g., trying to make you be more sociable).
But introversion and extroversion are normal, and common, human variations. When scholars and teachers debate the nature of intellectual giftedness, the stakes are much higher. At issue is what children are capable of accomplishing, what sort of education they need, what sorts of thoughts and feelings they possess, and what their adult lives might look like.
A child who fits almost all the traits on any checklist of gifted traits and every theory of giftedness is instantly recognizable. Rather than pushing flashcards on him, the parent will look on with anxiety, worrying about how to slow down their runaway train of a child before he runs out of material to learn. If verbally gifted, the child teaches herself to speak, read, and write early, and appears surprised that her peers do not do the same. A spatially gifted child will show similarly early mastery of puzzles in the toddler years. A mathematically gifted child will do the same for numbers; a man I know remembers he had a set of beads on his stroller and repeatedly ordering them into different-sized groups in order to see how many were left over, eventually deducing the rule that the remainder could not be greater than the number started with. Some celebrated gifted children teach themselves foreign languages as preschoolers. They will demonstrate creativity, intense energy, powerful emotions, impatience, asynchrony between their development in different areas, a desire to do everything as well as they can, a hunger to learn and explore whatever piques their interest, and often, concern over philosophical and existential issues far beyond their age. Incidentally, they may also show a depth of knowledge and skill in academic, artistic or leadership areas unusual for a child of their age, and may perform well on standardized tests. Extreme gifted children can not only focus for long periods of time, but employs this ability to get good grades. They also typically have an IQ over 130.
What about the child with 130 IQ and the highest grades in his class who writes derivative stories (with seemingly no knowledge of their derivativeness), has an even keel, and in fact, comes off as rather boring to most people, in contrast to the intensity typical of the extreme gifted child? What about the child with all of the emotional, personality, and cognitive traits, but an IQ below 130? What about a child with 160 IQ who has difficulty focusing on anything that does not fascinate him, and has such low working memory that even if he hears your instructions, he won't remember that you asked him to do anything 5 minutes later? What about the gifted child who hates school and spends his days designing games, yearning for nothing more than to be a video game designer? Is giftedness about intellectual potential, achievement, motivation, asynchrony, creativity, the hunger to learn, a collection of social-emotional traits, or some combination thereof? When a child lies in the fuzzy middle, with only some of these traits, are they gifted?
In the case of autism, the same child could be diagnosed as having autism, Asperger's, or PPD-NOS depending on which clinician he sees. The DSM-V seeks to eliminate this variability by treating autism as a spectrum rather than a set of binary categories. Indeed, the traits that define the autism spectrum are ones we all have to some degree. However, treating autism as a spectrum may not eliminate diagnostic variability. Clinicians will still differ on how important IQ and non-social developmental delays, language ability, and sensory-motor deficits are to autism. To some, autism is defined by the most extreme cases--those with global developmental delays and without spoken language. To such clinicians, developmental and language delays are such a core part of the cluster that those without such traits must be moved into the outlying "Asperger's" and "PPD-NOS" categories, while to other clinicians, the core deficit is social and the category of "autism" can embrace those lacking developmental or language delays. I suspect that, post-DSM V, clinicians will still disagree on the importance of developmental and language delays within the cluster of autism traits. Instead of disputing whether a "high-functioning" person has autism, Asperger's, or PPD-NOS, they will likely debate whether he should be diagnosed as on the clinical end of the spectrum at all. Ultimately, by defining disorders as continuous traits, the DSM-V will only exacerbate the problem of the fuzzy middle.
It seems to me that the solution to the dilemma lies in recognizing diagnostic categories as the cluster concepts they are, and diagnosing and interacting with individuals in terms of where they lie on each of the traits within the cluster. The DSM currently hand-waves, saying "not everyone has all these traits." Much more useful would be a topology showing where, say, a putative introvert fits on scales of sociability, approach/avoidance, anxiety, sensory processing sensitivity, etc. This would move clinicians and teachers closer to treating individuals according to their own real strengths and weaknesses instead of as representatives of a monolithic label.
*Excuse any details I may have gotten wrong or oversimplified. My knowledge of personality research is largely secondhand, and much of what I know comes from the following article:
Aron, Elaine & Aron, Adam (1997). Sensory-processing sensitivity & its relation to introversion & emotionality. Journal of Personality & Social Psychology, volume 73, number 2, pp. 345-368
*I am tacitly assuming that for the extreme cases, all these models accurately describe people, at least to some degree. I would like to see psychology as a field move away from battles between theories that attempt to describe "all introverts" or "all gifted people" and towards accounts of whose behavior is best explained by which theory, and why.