There is considerable evidence that whatever the biological foundations to introversion and extraversion are, they have a genetic basis. The various studies of Sheldon in which the trunk index remained constant are a good indication of this. And they can be complemented by Susan Faber's Identical Twins Reared Apart. What better test of the stability of somatotypes could there be than to look at how different these identical twins have become by living in different environments? There has been any amount of publicity about the seemingly uncanny coincidences between identical twins brought together after many years of separation. Their wives might have the same first name, or their dogs, or they like the same kind of foods, or meet for the first time wearing the same kind of clothing and so forth. But it is their identical physical appearance that is often the key to their finding each other. Just how close are they physically? They are within 2/3 of an inch in height, and they usually agree in weight within 10 pounds, with women more labile than men due, perhaps, to their higher endomorphy. These twins shared the same shape ears, eyes, nose, distribution of hair and pigmentation, and the "shape of the skull varied by little more than the width of a pencil lead". (p. 69) They are so identical in electroencephalogram patterns that these patterns have sometimes been used to determine whether they are identical twins or not. In one case a neurologist studying a pair of twins saved himself time by using only one report. The twins appear alike in electrocardiogram patterns and blood pressure, and they are highly concordant in eye characteristics. In fact, we so expect these twins to look alike that it takes us a moment to reflect on the fact that their similarities are a confirmation of Sheldon's somatotype work. Some other evidence for a genetic basis to somatotype has been summarized by Rees (1968), and there is growing evidence that temperament, too, has a substantial genetic foundation, as can be seen in the work of Buss and Plomin (1975) and Thomas and Chess (1977), and is implied by Walker's studies of the temperament of young children that we already saw, and more recent studies, for example, Kagan and his coworkers' "Biological Bases of Childhood Shyness".

Introversion and extraversion has also been the subject of innumerable studies that have indicated a large genetic component. In 1956 Eysenck, for example, in a study of identical and paternal twins, found that identical twins resembled each other more closely than fraternal twins in extraversion and introversion. He summarized the evidence for the heritability of personality in "The Biological Basis of Personality" (1967). Scarr (1969), studying the results of ten twin studies including Eysenck's and her own found moderate to high genetic contributions to social introversion -extraversion, and Horn, Plomin and Rosenman (1976) found that talking to strangers, which is a good indicator of introversion - extraversion, was the strongest of all the traits in which identical twins were more concordant than fraternal twins.

We could cite many more studies on the genetic foundation to somatotype, temperament and introversion and extraversion, but it is more interesting to ask about the genetic mechanism by which these traits are transmitted, and here virtually nothing is known. Therefore the stage is set for another and final one of our speculative flights of fancy with the hope that this one, as well, will help us begin to think of psychological types and somatotypes in terms of biochemistry and genetics.

My wife and I have over our years of studying types noticed a pattern of introversion and extraversion in families. Put in its simplest form, the daughters seem to follow the attitude, i.e. the introversion or extraversion, of the father, while the sons follow the attitude of the mother. For example, in one family the father was extraverted and the mother introverted, and the two sons introverted and the two daughters extraverted. In another the father was extraverted, the mother introverted, the four daughters extraverted and the two sons introverted. And in still another the mother was extraverted, the father introverted, and the daughter introverted. This brings to mind the old saying, "Like father like daughter, like mother like son." By now we have observed this pattern so many times we are sure something substantial is there, and have gone on to ask ourselves just what it implies.

At the very least it means that even though somatotypes, temperament and psychological types are polygenic, that is, controlled by many genes, there is probably a substantial X-linked factor. The X chromosome is a very interesting one in which more than 120 genes have already been found, but whose secrets have not been fully deciphered. Mary Lyon, for example, discovered that in women one of their two X chromosomes is randomly inactivated so they do not receive a double dose of the various X genes. But recent research shows that this inactivation is not complete.

Once we became sensitized to the X-linked nature of introversion and extraversion, we began to see various pieces of this genetic puzzle. Morgan Worthy, for instance, had suggested various reasons for an X-linked factor in eye color reactivity. They included sex differences in reactivity itself, the fact that one of the genes for eye color is on the X chromosome, and that some endocrine functions are X-linked, and here he cites work showing that biological clock functions in fruitflies are influenced by a gene on the X chromosome.

In Eye Color, Sex and Children's Behavior Gary and Glover, following Worthy, describe a somewhat informal but fascinating study in which university graduate students stopped every fifth pedestrian on the street and asked them questions like, "I enjoy the company of people a. always, b. sometimes, c. never.11 or "I enjoy parties a. always, b. sometimes, c. rarely." And along with the question they tried to determine by asking and observing what color eyes these people had. In answer to the first question, among the dark-eyed people 130 responded always, 5 sometimes and I never, while among the light-eyed people 14 said always, 88 sometimes and 8 never. For the second question it was dark eyes 63 always, 39 sometimes and 34 rarely, and light eyes 16 always, 41 sometimes, and 53 rarely.

Winge (1921) found a tendency for the eye color of the child to take after that of the parent of the opposite sex. Brues (1946) found some evidence for the occurrence of dominant or partially dominant eye color genes on the X chromosome. We can speculate that both introversion and extraversion, or put in another way, the biochemical mechanism underlying introversion and extraversion and eye color, could be X-linked dominant characteristics. This would explain the patterns that Gary and Glover found.

Another piece of this puzzle can be found in the evidence. connecting manic-depressive (bi-polar) disease with X-linked dominant transmission. (Cadoret and Winokur, 1976; Winokur and Tanna, 1969). Sheldon described manic-depressives as failing in the mechanisms of inhibition. They lack ectomorphy-ectotonia, or put in another way, they could be called deficient in introversion. It is highly suggestive, then, that a disease that appears like a failure of the introversion - extraversion mechanism should be associated with the X-linked dominant transmission.

If introversion and extraversion are X-linked dominant characteristics, then we would expect the somatotypes of the mother and son, or father and daughter, to agree to the extent that both would fall within the territories we have delineated for introversion and extraversion on the somatotype chart. If we look at the major cases that Sheldon described in the Varieties of Temperament, and determine the psychological attitude of the men and their mothers simply on the basis of somatotype, they agree in all 6 cases. A further comparison of the siblings of the subjects also shows agreement between child and the parent of the opposite sex with one exception. However, in most cases it appears that the parents share the same attitudinal type, so that the agreement of the children with this attitude is not particularly striking. If we remember the description of Kretschmer's parents we would say that his mother was an extravert and his father was an introvert, and Kretschmer, himself, following his mother, was an extravert.

Parnell (1958) in a study of 45 families, found a strong tendency for the child's somatotype to fall on or near a line drawn between the somatotypes of the parents when plotted on the basic somatotype chart. If we take this line principle as an expression of the polygenic nature of somatotypes, then we would expect it would also, in some fashion, express the X-linked nature of introversion and extraversion within this polygenic framework. If we examine the 45 cases and tentatively determine the introversion and extraversion of the child in relationship to that of the parent of the opposite sex by plotting the somatotypes on our chart that shows both somatotypes and psychological type, we find that out of the 45, 42 are in fairly close agreement and can be interpreted in terms of the child and the parent of the opposite sex sharing either introversion or extraversion. The cases that show a notable departure are cases 11, 19 and 20. Parnell found a number of cases that departed from his line principle. One of the cases that he pointed out as departing from it was case 11. Cases 19 and 20 also depart from the line principle.

Parnell (1959) reported on another study where men entering Birmingham University (Great Britain) were somatotyped, and then chose the somatotype of their fathers and mothers from a series of photographs. He then plotted the somatotypes of the parents for the men who had chosen their own somatotype correctly. The results are in fairly close agreement to our expectations that the somatotypes of the sons will fall in the introverted territories if the somatotype of the mother falls in the introverted territories, and vice versa for extraversion. For example, in a distribution of sons who chose 4-6-2 as the father's somatotype, and 2.5-2-6 as the mother's, 8 out of 11 fell in introverted territories. In a distribution of sons who estimated their father as 3-4-4 and their mother as 3-3-4, 9 out of 12 fell within the introverted territories. There are limitations to these kinds of comparisons. First there are obvious limitations springing from how the somatotypes of the parents were determined. Secondly, both father and mother were somatotyped, as is reasonable, on the same chart, but there is a strong possibility that the relationship between somatotype and psychological type is distributed differently in each case.

Brues (1950) in an article entitled, "Linkage of Body Build with Sex, Eye Color and Freckling" concluded that X-linked factors may be involved in the genetic foundation of body build. She felt it was fortuitous to have hit on a morphological index closely related "to a simple developmental reality". This index turned out to be our old friend height over the cube root of weight, which Sheldon used in his final objective method as an indication of ectomorphy. Tanner and his colleagues later wrote:

"We have investigated Brues' data on 83 families, and find no evidence for sex-linkage in genes controlling stature, but fairly conclusive evidence in the case of linearity of build as measured by height over the cube root of weight." ("The Genetics of Human Morphological Characters", p. 194)

Richard Stafford gave a spatial visualization test to 104 fathers and mothers and their 58 sons and 70 daughters, and found the highest correlations between father and daughter, and mother and son, which he considered compatible with an X-linked recessive pattern of inheritance. Stafford's work was reviewed by David Garron who compared it with the results of studies of women with Turner's syndrome in which a defect involving the X chromosomes is associated with a lack of spatial abilities, though it was not clear why an X-recessive trait would show as a defect in the absence of one of the X chromosomes.

In a recent Swedish study of risk factors for stroke in middle-aged men, Welin and his coworkers found three factors: increased blood pressure, abdominal obesity, and a maternal history of stroke. Could these men have inherited a more stroke-prone somatotype and psychological type from their mothers? Here, again, we see the difficulty in sorting out the disease itself and the underlying typological patterns.

If there are, indeed, X-linked eye color, linearity and spatial visualization traits, could they be signs of our deeper introversion - extraversion biological axis? This brings us to the possible mode of transmission of this X-linked introversion and extraversion. Our observations seem to indicate that this relation-ship between father and daughter, and mother and son, is rather consistent. To be absolutely sure of this is difficult because of the diagnostic problem of psychological types which, in this case, is exacerbated by the differences between men and women, and the fact that siblings can take on different typological roles in the family. The older child, for example, can appear more extraverted, and a younger more introverted. But let's look at the possible modes of transmission. In X-linked recessive traits almost all the effected people are males, and females transmit the trait to one-half of their sons. This diverges too much from our observations. In X-linked dominant behavior, while males transmit the trait to all of the daughters, heterozygous females transmit it to one half of all their children and homozygous females transmit it to all their children. This is closer, but what would be ideal would be a mode of transmission in which the paternal X was always decisive for the attitude of the daughter, and both maternal Xs carried the gene for her dominant attitude so that this attitude was always expressed in the son.

Recent research in methylation imprinting suggests that "specific genes must be inherited from either the mother or father" (Marx, "A parent's sex may affect gene expression"), and this might shed light on this strange pattern of inheritance if it, indeed, exists.

The location for this putative introversion - extraversion gene is best sought on the terminal portion of the short arm of the X chromosome (Xpter). This segment of the chromosome contains the genes for the Xg blood group system, monoclonal antibody, steroid sulfatase and chondrodysplasia punctata. There is also evidence that links ocular albinism to the Xg blood group, though it is not on the Xpter itself.

The Xg(a) (McKusick 31470) acts like an X-linked dominant. The X chromosome, if not the Xg, has been linked to manic-depressive disease (Baronet al. 1987) and there is evidence that it is not inactivated. Monoclonal antibody (31347), an antibody to human leukemia T cells, also escapes inactivation and along with the Xg is linked to the Y chromosome. Steroid sulfatase deficiency (30810) or ichthyosis, a scaling of the skin, appears to avoid inactivation or lyonization as well. In some cases it is connected to deep corneal opacities. People with ichthyosis have increased plasma cholesterol sulfate levels which predominate in the low-density lipoprotein fraction of the plasma. Associated corneal problems have been connected to a reduction of cholesterol in the stratum corneum. Chondrodysplasia punctata (30295) may be an X-linked dominant. Patients can exhibit "hypoplasia of the distal phalanges" and "widespread atrophic and pigmentary lesions of the skin in a linear or whorled patterns" (McKusick 1986, p1335), and occasionally short stature.

This terminal of the short arm of the X chromosome exhibits pairing with corresponding loci on the Y chromosome. Among the genes on the Y chromosome is a test is-determining factor, which may be the same as the H-Y antigen which might be regulatory for a structural locus on the short arm of the X. There is also a gene relating to stature on the Y chromosome, and Tanner and his coworkers have argued that genes determining slower maturation must be on the Y. (McKusick 1986, plxxv-lxxvi; Tanner et al. "Genes on the Y chromosome influencing rate of maturation in man").

Is it possible that in the descriptions of these X and Y terminals we are catching a fleeting glimpse of our biological introversion-extraversion axis, which embrace manic-depressive disease, cholesterol levels, eye and skin pigmentation, stature and maturation rate?

Goodenough and his colleagues in "A Survey of X Chromosome Linkage with Field Dependence and Spatial Visualization" examined 67 families with three sons, and found a possible link between the Xg and the portable Rod and Frame Test, and the Embedded Figure Test. Both tests are measures of field dependence which is the degree a person is effected by the surroundings of a particular task to be accomplished, and probably are fairly good indicators of certain sorts of introversion and extraversion. These results bring to mind similar studies on form and color perception. Worthy found evidence that light-eyed people were better at form perception than color perception, and dark-eyed people were the reverse. Studies of Kretschmer's cyclothymia and schizothymia found pyknics (Sheldon's endomorphic mesomorphs) did better at color than form perception, while the schizothymes were the opposite. (Eysenck, "Cyclothymia and Schizothymia as a Dimension of Personality").

The short stature of Turner's syndrome patients appears connected to a deletion on the short arm of the X chromosome. Can this be related to their visualizing disabilities and thus to a lack of ectomorphy? If the defect were on the non-inactivated Xpter it would help explain why a defect on one X chromosome could manifest itself despite the lyonization of most of the chromosome.

The Xq, or long arm, seems to have its own area of lack of inactivation and a defect in it can lead to problems of ovarian function. As Daniel Federman nicely puts it:

"The ovary is the most precisely doomed structure in the human body: it carries in its makeup the destruction of its own seeds. When the fetal ovaries differentiate, they contain about 7 million oocytes. Through an unknown process, these oocytes begin immediately to disappear, by birth there are about 3 million left, by the menarche about 400,000, and the menopause occurs when under 10,000 remain. Menses never occur in the patient with 45,X Turner's syndrome, but not because she is born without oocytes. Rather, it is that after appearing, her oocytes disappear at an accelerated rate and are gone by the age of two, i.e., the menopause occurs before the menarche. Thus, something on the normal female's second X chromosome slows the disappearance of ovarian follicles and allows menstrual function and fertility to occur." ("Mapping the X-Chromosome , p. 162)

This can be viewed as a startling contrast to the slow sexual maturation of the ectomorph, not to mention our friend the tuatara lizard with the well developed pineal eye from the last chapter. The "tuatara reproduce only once every four or five years. It takes two or three years for the female to manufacture eggs. After mating, she holds the eggs in her oviduct for another seven months before laying them. 12 to 16 months later, tuatara babies finally hatch." (Discover, April '89, p. 14)

The X chromosome has any number of other interesting genes that may hold the potential to add to the picture of the biological introversion - extraversion axis. These include: retinitis pigmentosa (31260) with visual defects and lumps of pigment, reduced flicker sensitivity and sometimes a rare blue-yellow color defect. (Worthy comments that light-eyed people are more receptive to blue light.); ocular albinism (30050) with the eyes and skin showing macromelanosomes and "stripe-like areas of retinal hypopigmentation in carrier"; and the anhidrotic ectodermal dysplasia (30510) we saw in the previous chapter.

It is possible that the lack of inactivation of this Xpter and its links with the Y chromosome play a role in the strange inheritance pattern of introversion and extraversion. And what could be the reason for such a pattern? It would tend to overcome some of the physical and psychological differences between the members of the family, for each parent could more easily recognize her or himself in the child of the opposite sex, and this would tend to bind the family closer together. This kind of inheritance would also shed an interesting light on discussions of the Oedipus and Electra complexes.

But the matter might go deeper than this and be a way of safeguarding our basic patterns of adaptation to life. Jung suggested that introversion and extraversion themselves had their roots in two basic ways of procreating. One was a broadcasting of many offspring with the hopes that a few would survive, while the second was the birth of a few offspring raised in highly protective conditions. In short, introversion and extraversion would represent two kinds of evolutionary adaptation, two viable ways of surviving in this world. And this kind of inheritance would guarantee that neither one of these patterns would die out , and as a result mankind would maintain the plasticity and adaptability, which is one of its most valuable attributes. This would be a confirmation on the psychological plane of Ohio’s law of the evolutionary conservation of the X chromosome of mammals.

After the typological odyssey of the preceding chapters it is time for general impressions and conclusions.

Jung's psychological types should remain firmly rooted in the process of individuation. A split between types taken interpersonally, and types taken interpersonally, impoverishes them both.

Type diagnosis is a critical issue, but its solution lies principally, not in the direction of the reformulation of psychological types, or the creation of more refined type tests, but in the realization that we are dealing with very complex natural phenomena and we have to school ourselves to see them correctly. To abdicate, working at this kind of carefully cultivated seeing that both Jung and Sheldon possessed in favor of a type test is certainly easier, but in the long run will do us a disservice, for it will not be as accurate or as far seeing.

Cultivating seeing is the foundation of a genuine observational method in psychology which ought to interact in lively dialogue with the experimental method. Again, any split will impoverish them both.

Sheldon's somatotypes and temperament types represent one of the finest achievements of psychology. The most promising way to save them from oblivion is to integrate them with Jung's psychological types. Then somatotypes will gain by being seen in relationship to the whole psyche and its dynamics, and psychological types will begin to come to grips with their biological foundations. The most weighty evidence for a relationship between somatotypes temperament types and psychological types is not in material of the sort collected in Chapter 8, but will be based on our own ability to look at individuals from both points of view and see these relationships for ourselves. Exactly which somatotypes go with which psychological types is an issue for the future to refine. But there are no psyches walking around without bodies, and there are no psychological types without their highly distinctive body and temperament types. Any lingering Cartesian dualism should give way to the realization that, as Jung said, "Somewhere the psyche is living body, and the living body is animated matter."

The future of typology holds out the promise that an integrated typology of body - temperament and psychological types will be completed by a biochemical typology. If this promise is fulfilled, we will see that our particular type is rooted in the very chemistry of our bodies and brains, and stretches to the innermost recesses of our psyches. Such a growing awareness seems inevitable, but will we find the wisdom to use this knowledge not to reinforce our prejudice, but enrich our human community? Jung, by describing how each of us is in a certain way all the types and must strive to become all of them, holds out to us the promise of an understanding of human differences that will become the foundation for tolerance and compassion.

Over the years many people helped my wife and me in our pursuit of knowledge about typology, whether by their writings, or in response to our questions and letters, or by being such interesting types themselves! I would like to mention just some of those whose contributions stay warmly in my memories.

John Beebe and Wayne Detloff, Jungian analytical and typological experts, for encouragement and fascinating conversations. Jess Groesbeck, also a Jungian analyst and type expert, for his thought provoking letters. Humphrey Osmond, who bridges the worlds of Sheldon, Jung and biochemical typology with his well developed intuition, for delightful letters and his reminiscences about carrying the Atlas of Men to Jung. Joan Albert for the hospitality of the C.G. Jung Library in San Francisco.