The Rare Trait Marker Model

With a research literature replete with suggestions that left-handedness is predominantly associated with negative conditions, some researchers have attempted to provide theoretical models that try to explain why left-handedness may be a marker for pathology.

Clinical researchers would call it a soft sign for pathology, meaning that it is not something that you can directly cut out, weigh, or subject to direct chemical or physical analysis. Handedness is merely an observable behavior that indirectly suggests a problem, rather than a hard sign, which might be actual observation of a damaged section of the nervous system. Hard signs almost always indicate pathology. Thus, a person who tests positive for HIV antibodies almost definitely has been exposed to the disease AIDS. Left-handedness is a soft sign in the sense that an individual who shows it is not definitely pathological but, rather, is more likely to have some pathological condition than a person who does not show this behavioral sign.

The linkage between handedness and these various pathological conditions occurs due to both neurological and statistical considerations. The statistical component has been mathematically explored in the form of the rare trait marker model, which indicates that a number of statistically rare traits are often found to be associated with a range of pathological conditions. Examples of rare traits include animals that are colored differently from the vast majority of their species. For instance, a "blue-marl" collie, a white dog, or an albino human often have major sensory deficits affecting their vision or hearing. These rare traits could include rare palm crease patterns, rare fingerprint patterns, or rare distributions of toe lengths, and even unusual ear shapes are often found to be associated with cognitive or physical deficits. Left-handedness, which affects only about 10% of the population, would also qualify as a rare trait.

According to theoretical considerations, if we start with a trait, such as handedness, that has a common and a rare form, we need only add the possibility that the appearance of the rare versus the common trait can be influenced by pathological factors. If we meet both of these conditions, then because of statistical considerations we have all that is needed to create a clinical soft sign for pathology.

To demonstrate the operation of the rare trait marker model, consider the situation diagrammed in Fig. 4. In this example, we suppose that we are starting with a population in which, if development proceeded naturally, 90% of the people would develop into righthanders and 10% into left-handers. Suppose that there was some disturbance in development—some aberrant condition or some birth risk factor—that caused 10% of each group to deviate from their targeted handedness. The result would be 9% ofthe population (10% of the 90% who would be right-handed)

Figure 4 The rare trait marker theory is demonstrated in this diagram. The circles represent the percentages of normal individuals, whereas the squares represent the percentages of pathologically switched individuals. Notice that in the final group of left-handers, half are pathological, whereas the number of pathological righthanders is less than 2%.

Figure 4 The rare trait marker theory is demonstrated in this diagram. The circles represent the percentages of normal individuals, whereas the squares represent the percentages of pathologically switched individuals. Notice that in the final group of left-handers, half are pathological, whereas the number of pathological righthanders is less than 2%.

"pathologically" shifting from right-handedness to left-handedness, whereas only 1 % of the population (10% of the 10% originally targeted to be left-handed) shifts from left- to right-handedness. The final result is that half of the resulting population of left-handers (9% out of the total of 18% left-handers) are pathological, whereas only 1.2% (1% out of 82%) of the right-handers are pathological. This means that the relative risk of a left-hander being pathological is approximately 41% greater than that of a right-hander being pathological. How well a rare trait predicts pathology depends on its distribution in the population and the likelihood of pathological change. By chance, the approximately 10% incidence of handed-ness falls into the optimal predictive range of values.

The fascinating thing about the rare trait marker theory is that it really does not depend on any specific physiological, injury, or disease mechanisms. It makes no assumptions about the particular vulnerability of various parts of the nervous system. In other words, we do not have to know why or how or even when a particular injury or damage occurred. All we need to know is that there is a common and rare trait, and that some form of pathology (actually any form of pathology) can produce the rare trait.

Although the previous discussion suggests that left-handedness might be a sensitive marker for a number of pathological conditions, there is a flip side that reduces its usefulness in some ways. Given the large number of possible sites for pathology that might cause the observed left-handedness, we also must face the fact that there is little specificity. Left-handedness might be seen as a soft sign indicating increased risk for many problems; however, which specific problems an individual might suffer from, or which neural loci are involved, is not determinable. This means that given an individual about whom all one knows is that he or she is left-handed, one can list the many problems that he or she is at risk for but one cannot determine which of them this particular left-hander is most likely to have which part of the nervous system is most apt to be damaged, or what the original source of the pathology might have been. We can say with some certainty that the more diffuse the pathology that an individual has suffered, the higher the likelihood of non-right-handedness and also the higher the probability that other functions other than handedness will also be disturbed.

Understanding And Treating Autism

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