As I noted in my previous post, Rare and Different, a transgender identity is rare, frequently obvious from an early age to someone with that identity (although interpreting what transgender identity individuals feel is often far less obvious, especially to those around them), and probably hard-wired, if not from the time of conception from very early on in life.
Transgendered people are not the only people who share neurological traits which are rare, obvious and probably hard-wired to a significant extent.
Non-transgendered homosexuals, schitzophrenics and bipolar individuals, for example, also fit this pattern, and neuroscientists, psychiatrists and other behavioral scientists have studied all of these populations rather carefully, considering their rarity, because the distinctiveness of these populations has made studying them feasible far in advance of any biological hypotheses about the basis of these conditions. Indeed, there is an entire discipline, abnormal psychology, devoted to understanding those who are rare and different (many for reasons that current scientific studies suggest strong are hard-wired, and others for reasons that are ill understood or believed to be primarily cultural or sociological).
While the populations mentioned above are individually quite distinct from the rather ill-defined common sensical notion of "normal" and are relatively well defined, an enduring and core discussion in the field of psychiatry is how one draws the line between "normal" and "abnormal", and what normative content those distinctions should carry. In 2008, it is conventional and fashionable to consider transgender identities and homosexuality to be merely distinctive, while considering schitzophrenia and bipolar disorder to be illnesses.
People of good will can agree that all four of these traits are relatively rare and distinctive, that all four have strong biological components, and that all four tend to be fairly stable elements of an individual's brain, social functioning and identity throughout life. While schitzophrenia and bipolar disorder are both currently classified as "illnesses," both conditions are widely viewed as incurable but amenable to current and improved future treatment regimes as they are better understood.
The trend in addressing transgender identity, while not considered an illness per se, is to use medical treatments to "cure" not a person's distinctive mental characteristics relative to their apparent gender identity at birth, but to "cure" their physical manifestations of their apparent gender identity so that their bodies can be reconciled to a greater degree with their relatively immutable minds.
Essentially the entire respectible medical and behavioral health establishment has concluded that the appropriate response of their disciplines to homosexuality is help people who are homosexual find comfortable ways of living lives consistent with their sexual orientation, rather than engaging in any kind of medical or psychological intervention addressed at homosexuality itself.
General Boundary Issues
While these broad points of agreement exist, however, there are also difficult boundary issues excacerbated by the fact that our understanding of the biological, psychological, cultural and sociological basis of the diversity we observe in the world is ill understood.
For example, there are many neurodistinctive traits for which it is unclear if a trait is an all or nothing affair, a matter of degree in which notable populations are simply extremes of a continuous spectrum of some trait, or a combination of both.
For example, general intelligence is a trait that is widely studied and it is widely agreed by behavioral scientists that it represents a measurable trait that varies significantly in degree within the general population. Yet, we also know for a fact that some people end up deficient in that trait not out of general variation among people in this trait among people from less bright to very bright. Some people are retarded as a result of very specific genetic or environmental causes. Some of the better known genetic causes are genetically dominant single gene mutations or chromosomal defects. Some of the better known environmental causes are iodine deficencies during pregnancy and childhood lead exposure.
The jury is largely out on the question of whether any forms of high general intelligence or mental abilities have causes distinct from ordinary variation within the general population. The most notable exceptions to that rule seem to be the rather strong association between extremely high creativity and bipolar disorder on one hand, and some very rare savant phenomena on the other (one of Denver's daily's for example, reported earlier this year on the small community of people who possess the ability to recall almost everything that happened on a specific date in the past, the grain of truth behind science fiction ideas like the mentats who serve as human computers in the Dune novels by Frank Herbert). There are rather intangible qualitative differences that many educators who specialize in programs targeted at the highly gifted and talented tend to report compared, for example, to older students who have made similar academic progress. But, those observations aren't terribly systematic and I am aware of no research that has pinned down any basis other than practicality, intution and social comfort for distinguishing between those who are highly gifted and talented and those who are merely very bright.
In the area of homosexuality, there is growing partial consensus that for men, sexual orientation is more like an either/or question rather than the kind of continuum that Kinsey, et al, considered early on. There may indeed by a gray area, but those for whom there may be a gray area appear to be a small minority of all men, and of all men who are not unequivocally hetrosexual, rather than comprising a large share of the total homosexual male population. Indications at this time are that it appears that a large share of men who at any given moment identify as bisexual end up later on developing an identity either as a heterosexual man or a homosexual man. In contrast, female homosexuality, while reasonably similar to male homosexuality in prevalence and self-perception, seems to be a less stark either/or affair and seems less dramatically driven by sexual attraction alone.
Boundary Issues In Diagnosing Mental Illnesses
Boundary questions in the area of conditions predominantly considered to be mental illnesses are far more severe. On one hand, heritability studies and clinical experience have shown that even conditions as distinctive as schitzophrenia and autism tend to manifest in a range of extreme to less extreme ways.
The DSM-IV, the current diagnostic bible of the psychiatric profession, currently approaching its fifth revision, has currently reached the uncomfortable compromise position in a large share of all cases that a mental condition exists for clinical purposes only if it is proving to be a problem for the person who has it at the moment. From a practical perspective this is useful and probably appropriate when one considers that the DSM-IV is often used as a gatekeeper to mental health treatment. One gets mental health treatment only if one's condition can be diagnosed clinically with some condition described there, and the people whose lives are being screwed up by a condition are clearly the ones who should be first in line to receive this treatment. But, from a scientific and philosophical perspective, this arbitrary cutoff is vexing, as it has no strong biological basis.
For example, from a health care establishment perspective, it may make sense to diagnosis a 34 year old man who is trying to support a family with young children with an accounting career as suffering from attention deficit disorder, even if the symptoms are relatively mild as ADHD cases go, because those symptoms are wrecking havoc on his professional life, while not providing access to mental health treatment to the very same man if he is a bohemian visual artist with few material obligations whose primary repurcussions from his ADHD are that he drives gallery owners nuts by pulling all nighters before new installations and is perrenially late or a no show at coffee dates with colleagues which he has trouble recalling at the appropriate moments.
But, if one is trying to determine the biological basis of ADHD (if any), or determine what kind of hereditary basis the condition has, or measure the effectiveness of a new ADHD treatment, the DSM-IV definition which diagnosis one person, but not another, who has precisely the same underlying condition, because they have coped with their conditions differently, is maddening.
One issue that these boundary issues and non-biologically based diagnostic definitions leaves open is the extent of the overall prevalence of people who have mental (or other) traits which are rare and different when considered in isolation. What percentage of us have some biologically meaningful degree of a trait which is considered a mental illness when it is present to a degree that impairs the functioning of an individual who has not learned how to cope with it? 5%? 15%? 30%?
According to the Second Edition of the Mental Health Disorders Sourcebook, edited by Karen Bellenir (2000): "During any one-year period, up to 50 million Americans - more than 22 percent - suffer from a clearly diagnosible mental disorder involving a degree of incapacity that interferes with employment, attendance at school or daily life." Of course, the percentage of people who suffer from a clearly diagnosible mental disorder at some point is life is a fortiori higher. Also, a fortiori, more Americans suffer from subclinical expressions of mental disorders that have the same biological basis as those that are clearly diagnosible in any given year.
The Mental Health Disorders Sourcebook goes on to notes that:
20 percent of the ailments for which Americans seek a doctor's care are related to anxiety disorders, such as panic attacks . . .
Some 8 million to 14 million Americans suffer from depression each year. As many as one in five Americans will suffer at least one episode of major depression during their lifetimes.
About 12 million children under the age of 18 suffer from mental disorders such as autism, depression and hyperactivity.
Two million Americans suffer from schitzophrenic disorders and 300,000 new cases occur each year.
15.4 million American adults and 4.6 million adolescents experience serious alcohol related problems, and another 12.5 million suffer from drug abuse or dependence.
Nearly one-fourth of the elderly who are labeled as senile actually suffer from some form of mental illness that can be effectively treated.
Suicide is the third leading cause of death for persons between age 15 and 24.
The Sourcebook then goes on to argue that a very large percentage of these cases are suceptible to effective treatment, even though a very large percentage of cases suceptible to effective treatment are not in fact treated.
While some mental health conditions like a one-time episode of major depression, or one of the not uncommon types of post-traumatic stress syndrome that resolves on its own with time, or a drug addition in someone not particularly prone to addition in general, can be put behind someone once and for all, many mental health conditions are chronic and incurable because they are fundamentally aspects of who someone is, rather than curable illnesses in the conventional sense.
We don't have good data that is easily available to tell us, for example, what percentage of Americans will never suffer from a clearly diagnosible mental disorder, and what percentage of those who will suffer from a clearly diagnosible mental disorder at some point in life have chronic and incurable, but treatable mental disorders, let alone really solid details on the makeup of that population, although data and definitions are making progess. As a result, it is hard to describe with clarity the overall ecology of our society's neurodiversity with much accuracy.
In addition to struggling with the issue of whether particular mental conditions are either/or traits biologically mitigated only by the extent to which one copes well with them, or are fundamentally matters of degree, there is also the question of the extent to which mental conditions are actually distinct from each other.
The Meaning of Co-Mordidity
One of the best predictors of your likelihood of having a particular DSM-IV mental illness is the fact that you have some other DSM-IV mental illness. Co-morbidity is the rule rather than the exception for a wide range of mental illnesses under the current classification regime. But, the meaning of this well established fact is unclear.
Does this mean that there are far fewer mental illnesses than the DSM-IV suggests, and that a large share of all mental illness is really a manifestation of the same underlying phenomena?
The central importance of a fairly modest number of chemicals that serve as neurotransmitters (such as serotonin, dopamine, monamine and norepinephrine) and their related receptor and reuptake systems, and a small number of related hormonal systems (such as the hypothalamic-pituitary-adrenal axis that governs the body's fight or flight system through a cascade of hormones that culminates in production of substances like cortisol by the adrenal glands) to the neurochemistry of mental illnesses that account for a large proportion of diagnosable mental illnesses, particularly the many related to mood, suggests big macrofactors that drive the entire class of chemical imbalance driven mental illnesses (which also happen to be the most treatable because drugs are well suited to treating chemical imbalances).
Also suggestive of the possible closely related causes of many commonly diagnosed mental conditions is the extent to which so many of them seem to involve the limbic system, a part of the brain common to essentially all mammals that plays a large part in instictive responses and human emotions, despite the fact that that the limbic system makes up a fairly modest part of the total volume of the brain. Disfunctions of the amygdala, a small but critical part of the brain associated, among other things with fear, and a part of the large limbic system, seems to be implicated in a great many DSM-IV disorders that impact a great many people.
Another possibility is that there are fewer mental illnesses than the DSM-IV suggests not because of one uber-illness behind all mental illnesses with high co-mordidity, but because the mentally ill have throughout human evolution been more willing to choose their neurodiverse fellow travelers as mates. In this view, neurodiversity has produced a human ecology with some indistinct but real subpopulations high incidences of multiple mental conditions with genetic roots, and other indistinct but real subpopulations with low incidences of almost all mental conditions with a genetic basis. In this view, not only is the apparent co-mordity truly a convergence of multiple separate mental conditions, but furthermore, there may be a number of mental conditions, that are actually multiple distinct co-mordid conditions with independent genetic and/or biological or other causes, that have been classified as a single DSM-IV disorder simply because they are more often co-mordid than are seen in isolation.
For example, perhaps ADHD hyperactive type, is really simply a condition in which an impulse control/hyperactivity condition is so often co-mordid with an entirely separate inattention condition, in diagnosible conditions (subject as noted above to the impact on the individual's life threshold) that the co-mordid syndrome has been diagnosed more often than either pure impulse control deficits and hyperactive conditions without inattention, or as pure inattention without hyperactivity or impulse control problems.
I don't claim to know if that particular example is or is not a scientificly accurate statement about that condition, but simply offer it as an illustration of the concept. It could be that many DSM-IV conditions are simply syndromes in which multiple independent traits manifest in particularly distinctive ways when they are present together. Indeed, Barbara Oakley, in her book Evil Genes, has suggested that emergenic psychological traits, like psychopathy, Machievellian personalities (which may be a key component of psychopathy) and leadership may all have this character, and explores those kinds of inheritence and neurobiological models in some depth.
If this is the case for a particular DSM-IV diagnosis one would expect fairly low heritability from parent to child, for example, despite a high genetic component shown by identical v. fraternal twin comparisons; lots of "background noise" of distinctive traits that don't quite match the emergent individual's traits in family histories; and the existence of many beneficial combinations of the independent source traits that are pathological when combined in the wrong permutations.
In contrast, if heritability data were very consistent across a wide range of genetic relationships, relatively to their degree of commonality of genes, for a DSM-IV condition, and the patterns of inheritance were fairly straightforward for a particular condition, one might expect a DSM-IV condition to be unlikely to maks multiple independent conditions that merely present as a new condition because of the way that they present when they interact.
Evolutionary Biology and Neurodiversity
One of the underlying scientific questions which motivates and animates this analysis is how evolutionary biology could bring about the neurodiverse ecology we see in humanity today, particularly in the case of those who are rare and different. (Incidentally, I should offer a belated acknowledgement of A Mind Apart: Traveler's In a Neurodiverse World by Susanne Antonetta, herself a bipolar individual, as my source for the term neurodiversity. Antonetta and Oakley, in particular, have been particularly seminal in framing my views in this post and a few earlier posts, as has been Armand Marie Leroi through the book Mutants: On Genetic Variety and the Human Body.)
Usually, traits become genetically frequent because they have an adaptive purpose. But, sometimes traits simply get carried along in the kludge that is the human genome, because most of the time they don't do any decisive harm and were lucky enough to end up in the same gene pool as genes that proved highly adaptive.
Also, some astronishingly sophisticated cascades of manifested traits can arise from a quite simple (and hence prone to recur independently) genetic mutations or pre-natal circumstances. As I explored at this blog a couple of years ago, the human genome appears to be heavy on subroutines for creating biochemicals, general tissue types and the like (call them collectively material and submaterial specifications), and relatively light on morophology and big picture design directions (call this the "blue prints" of the body). Thus, we have lots of genes telling the body how to make cells in general, quite a few genes telling the body how to make the specialized tissue types that appear in your eyes, a fair number of genes telling your body how to assemble those tissue types into an organ we call your eyes, and a very slim handful telling your body how many eyes to make and where to put them.
As a result, a very small number of strategically placed mutations can produce astonishingly great apparent change of the type that intelligent design/creationist people like to call "macroevolution" in a final culminating fell swoop, such as an extra set of eyes or ears.
The patterns of distinctiveness we see in human neurodiversity suggests that the most ancient parts of our brain (e.g. those that control respiration) are extremely fine tuned, or at least, result in miscarriages when any of their moving parts go awry, because they are so central to maintaining life itself from moment to moment.
The "middle brain" is less precipitous in punishing failure, and also has a particularly detailed architecture with many finely tuned moving parts, so it is particularly prone to non-immediately life threatening errors, even maladaptive ones that either don't survive and yet recur through new mutations, or remain in the gene pool despite their maladaptive natures at low frequencies, despite their maladaptive natures.
The higher brain functions associated with the cortex and distinct largely to primates, in contrast, don't appear to be nearly so fine tuned and hard wired. We know, from the signicant similarities between most people in how particular parts of the cortex are used for particular purposes, that there is some architectural plan for the cortex in our genes, but we also know, from sources like people who have regained functionality after suffering brain damage to the part of the brain usually responsible for a particular brain function, that this part of the brain is more malleable and less specifically pre-ordained than the evolutionarily older parts of our neuroanatomy. Fewer moving parts in a genetic architecture sense, means fewer kinds of things that can go wrong with it.
Thus, for example, we have far more people who suffer from panic attacks, something linked to the limbic system's fear management system, than we do people who see grossly distorted images of the world because the highly complex visual processing architecture in their cortex has a glitch.
This isn't to say that mental illness and other rare and different forms of neurodiversity are entirely concerned with neurochemistry and the limbic system, but the disproportionate share of problems associated with these parts of the system may have something to do with the number of not instantly fatal things that go can wrong with these aspects of our brains.
This analysis is particularly pertinent to the extent that many mental illnesses have genetic causes, and that their incidence is to a great extent a function of independently recurring mutations in parts of the genome particularly prone to mutation. In this view, co-morditity is a function of the fact that some people have more mutations in the genomes than others. In particular, this analysis is supported by the strong connection between advanced paternal age and many common mental conditions which is presumed to flow from sperm mutations either as a result of accumulated mutagenic compound loads due to environmental exposures over a longer life, or due to simple reduced quality control for sperm inherent in the aging process itself. This solved the problem of why a maladaptive mutation would persist despite the selective effects of evolution, because these mutations arise over and over and over again, only to be weeded out of the gene pool again, and again and again, much like other dominant single gene mutations that have devistating physical effects.
Most of these evolutionary biology explanations, while servicable, and perhaps even correct, however, are not nearly as uplifing as the notion that those who are rare and different may provide reserves of talent to the species that allow the species as a whole to survive in rare but pivotal moments in our evolutionary history past, current and future.
The notion would be that most of the time, the human race doesn't need an Einstein or a Joan of Arc, but that the traits that allow one to be born and rise to prominence are in our genome not by accident, but because, from time to time in our evolutionary history, these rare and unique individuals have allowed our species to survive by allowing it to do what would be impossible for those who are merely ordinary and typical to do, in a Gaia/Deep Ecology kind of way.
Such notions are, of course, damnably hard to prove, but are nevertheless profoundly inspiring, and are at the root of a whole subgenre of science fiction, like the TV series Heroes or the novel Darwin's Radio in which a next big step in human evolution occurs just in time to save the world.