makes the male male and the female female? Rather than assuming that sex differences in behavior are the result of social and cultural forces, we need to consider to what extent sex differences are determined by biological factors. By this I mean we need to consider to what extent sex differences in body and in behavior arise out of the HAT
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genetic differences between human males and females. Is there something inherent in being a girl or boy, man or woman, which predisposes the individual to behave in certain ways? We know that human behavior, more than that of any other species, is plastic and dependent on experience and learning processes. Thus, any sex-related innate or hereditary factors will necessarily be viewed as predisposing rather than as determining. But what are these innate factors? Anyone can tell whether an infant is male or female - look at the sex organs! But in order to determine how these physical differences, as well as other sex-related physical and behavioral differences, come about, we have to start back at the beginning with the biological building blocks - that is, the hereditary material the chromosomes. A normal human organism has twenty-three pairs of chromosomes in each of the cells of his body. One of the twenty-three pairs is the sex chromosome pair. If the sex chromosome pair consists of two X chromosomes, the individual is female; if the sex chromosome pair consists of an X and a Y chromosome, the individual is male. The egg and sperm each carry half of the normal chromosome complement. The egg normally carries twenty-two chromosomes and an X chromosome; and the sperm carries twenty-two chromosomes and an X or a Y chromosome. With fertilization, the twenty-three chromosomes in the nucleus of the mother’s ovum and the twenty-three chromosomes in the nucleus of the
“Innate Factors in Sex Differences”, by Norma L. McCoy, is preprinted with permission from Beyond Sex Roles, Editor/ Author, Alice G . Sargent, West Publishing Company, 1977, ($9.95) 50 W. Kellogg Blvd., P.O.Box 3526, St. Paul, Minn. 55165.
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father’s sperm join, resulting in an individual with twentythree pairs of chromosomes or forty-six in all. If the ovum is fertilized by an X-bearing sperm, the individual is female and if by a Y-bearing sperm, the individual is male. Chromosomes vary in size; the X chromosome is a fairly large one, while the Y chromosome is a small one and carries very little, if any, genetic information (genes). One result of the fact that men have only this small Y chromosome, instead of a second X chromosome, is that the genes on the X chromosome are unpaired with genes on the Y chromosome. This means that when the male has a defective gene on the X chromosome he will show that defect. This explains the high incidence in males of such conditions as hemophilia ( a bleeder disease) and red-green color blindness, two among many undesirable conditions carried by the X chromosome. In females, just before or just following the implantation of the female embryo in the uterus, one of the X s in each of the several hundred existing cells curls up, becomes what is called a “Barr body” and is thought to be inactive. Which X becomes a “Barr body” is a matter of chance. Thus, while all the cells in a normal man’s body bear exactly the same chromosomes, a woman’s cells differ in that some of them will have a functioning X from the mother and some will have a functioning X from the father. Even if a woman has a defective gene on one of her X chromosomes, chances are good that in approximately half of her cells the other X, bearing a normal gene, will be active and she will not show the defect. Using hemophilia as an example, the male with an X chromosome carrying this defect will be a hemophiliac (bleeder); the woman with one normal X and one X with the defect will have slower blood clotting time but will not suffer from hemophilia. The extent to which her blood clotting time
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will be lowered will depend on the proportion of her cells in which the X with the defect is active. This generally accepted theory of a single-active-X is known as the Lyon Hypothesis, after Mary Lyon, who first suggested it (Stern, 1973). Related in some manner to these sex chromosome differences is the fact that more males than females die at all stages of development from conception through old age. The average life expectancy of females (74.9) is seven-and-one-half years greater than that of males (67.4). Aside from all this, the subject of greatest interest to us is not what the Y chromosome doesn’t do, but what it does do. The latest evidence suggests that it does little more than cause an embryo to develop into a male. In fact, it is known on the basis of individuals who are born with abnormal sex chromosome constitutions that the mammalian Y chromosome is strongly male determining. Given the presence of a Y chromosome, the human embryo will develop as a male even if there are two, three, or four X chromosomes. Conversely, in the absence of a Y, the body of a developing human will appear to be female even if only a single X chromosome is present. This fact has led some (Sherfey, 1972) to the position that the human embryo is innately feminine. However, John Money, a leading authority in this area, insists that progression in each stage of sexual differentiation is from an undifferentiated or bi-potential (either sex) state to a differentiated one (Money and Athanasiou, 1973). This statement is based on what is known about the process of sexual differentiation in the developing embryo and should be self-evident to the reader after becoming familiar with the process of sexual differentiation which follows. Considerable experimentation was required in order to learn about the process of sexual differentiation in human
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beings. Because of the nature of such research, experiments were conducted largely with rabbits, guinea pigs, rats and subhuman primates. Subsequently, much of what has been learned in animal research has proved consistent with data obtained from the study of human beings with various clinical syndromes. At present, the process of human sexual differentiation is thought to occur in the following manner: * After fertilization, the developing organism is normally differentiated according to whether it is either male (XU)or female (XX), genetically. At about the fifth week in gestation, the underlying structure or genital ridge which is destined to differentiate into ovaries or into testes appears. During this period, the embryo also has the beginnings of both male and female ducts. At the sixth week the presence of the Y chromosome, in some manner not yet understood, causes the genital ridge to begin sex development into testes. The testes produce male hormone (testosterone), which causes the male ducts to develop (vas deferens, seminal vesicles, ejaculatory ducts), and a second substance (Mullerian-inhibiting substance) which causes the female ducts to regress. In the absence of testosterone, female ducts develop (uterus, fallopian tubes, upper vagina) and the male ducts regress. The first indication that the underlying structures will develop into ovaries is that by the sixth week they have not begun to develop as testes. As in the case of the gonads (ovaries or testes), the external sex organs of the two sexes also differentiate from identical underlying structures. Until the eighth week of gestation these structures are identical in both males and females. Then, some time between the second and third *After Money & Ehrhardt, 1972.
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month of pregnancy, the presence of male sex hormones causes male external sex organs (urethral tube, scrotum, penis) to begin developing. In the absence of such male hormones these same structures will develop as female external organs (labia minora, labia majora, clitoris). The presence of testosterone in the developing male not only causes the development of male sex organs but acts on a region deep in the brain, the hypothalamus, and directs it to function in a masculine rhythm and style. In the absence of testosterone, the hypothalamus is feminine in cycle and style of functioning. At puberty, this region of the brain will act on the pituitary, a gland attached by a stalk to the hypothalamus, and cause it to act on the ovaries (female) or testes (male) to produce either cyclic hormonal activity resulting in ovulation and menstruation in the female or acyclic hormonal activity resulting in the relatively regular production of sperm in the male. While the presence or absence of hormonal events during gestation causes distinct differences in the male and female brain, it is at puberty that these differences in the brain become fully operative. Puberty is thought to be triggered by the maturation of the hypothalamus, which then acts through the pituitary to bring about sexual maturation. Sex differences in the brain are responsible for male (ejaculation, etc.), as compared with female (menstruation, lactation, etc. ) sexual behavior. The presence of such brain differences suggests the possibility of others, as yet undiscovered, which could be responsible for additional sex differences in behavior. If the cyclic vs. acyclic hormonal activity in women and men, respectively, exerts a general effect upon the nervous system, then one can hypothesize that women are predisposed to certain cyclic biologically based changes in behavior, while
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men undergo neither cyclic changes in blood hormone levels nor resultant changes in behavioral predispositions. Bardwick ( 1974) has summarized the research on this and concludes that there is substantial evidence of hormonally related mood swings i n women. Bardwick speculates that self-assertiveness, self-esteem, and competitiveness are all highest at mid-cycle and that anxiety, tenseness, and depression are highest premenstrually and menstrually. These states are related to hormone levels through a hormonally linked enzyme (monoamine oxidase) which affects brain activity. Considerable correlation between the level of this enzyme and depression has been found; low levels of estrogen are associated with high levels of this enzyme, and high levels of this enzyme a r e associated with depression. As is always true, there is undoubtedly wide individual variation among women. While many sex differences appear at puberty, physical differenccs i n addition to those of the sex organs already exist prior to puberty (Tanner, 1972). Full-term male babies a r e slightly larger in all body dimensions than females, including height and weight, although female infants at birth have a vider pelvic outlet. Females develop faster than males, beginning in the fetal period and continuing throughout the course of development. There are many differences due to this differential growth rate, only one of which is the greater length and breadth of the male’s forearm when compared with whole arm length or whole body length. At all ages, the body composition of girls and boys differs, with girls having more fat and less water than boys, and boys having more muscle tissue than girls. Physical growth in girls is less variable than that of boys, such that the range of growth in a group of girls at a specific age is narrower than that in a group of boys. Thus, the number of teeth in two-
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year-old girls will show less variation than the number of teeth possessed by boys of that age. On the average, girls reach puberty two years earlier than boys. For both sexes, puberty brings about a considerable increase in the length of bones and the thickness of cortex, and girls have a smaller loss of fat. The wider shoulders of the male and the broader hips of the female become striking differences. The “sexy” hip-swinging gait of women can be attributed, at least partly, to the structure of the female pelvis, which is such that for any given length of stride a woman must rotate her pelvis through a greater angle than men do. The proportion of the female pelvis appears to be a compromise between a pelvis suited for walking and one suited for childbearing (Napier, 1967). According to J. M. Tanner (1972), a leading authority on physical growth and development : Before adolescence, boys and girls are similar in strength and shape; after, boys are much stronger, probably due to developing more force per gram of muscle as well as absolutely larger muscles. They also develop larger hearts and lungs relative to their size, a higher systolic blood pressure, a lower resting heart rate, a greater capacity for carrying oxygen in the blood, and a greater power for neutralizing the chemical products of muscular exercise, such as lactic acid [p. 9 11 1. At the end of puberty, each is sexually and reproductively mature and shows all the secondary sex characteristics of the mature male or female. These apparent physical differences between mature men and women may be the result of a long period of evolution in which the male had to hunt, fight, and protect and the female had to bear and nurture young. Perhaps a man who was physically strong and who recovered
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quickly from physical fatigue was more apt to survive and reproduce, while a woman survived and reproduced who bore a child easily. Up to this point we have considered physical differences between male and female human things, both in terms of development and maturity. A more interesting and by far more difficult question is, to what extent do these differences in body and brain relate to, underlie, or predispose to sex differences in behavior? For human beings, one of the most fertile sources of information that we have as to possible biological bases of behaviors is the study of males and females who have developed in abnormal hormonal environments because of unusual genetic or environmental circumstances. Psychologists John Money and Anke Ehrhardt ( 1972) have studied girls who were exposed to male hormones during gestation (androgenized females). Those studies include ten females whose mothers were given hormones during pregnancy in order to prevent abortion, and fifteen females who were exposed to male hormones during gestation due to a genetic defect ( andrenogenital syndrome). These girls ranged in age from four to sixteen years, the majority being in middle childhood. Compared with twenty-five normal (non-androgenized) girls matched with them on the basis of age, I.Q., socio-economic background and race, these psychologists found that the girls exposed to male hormones during gestation differed from the non-androgenized girls in several ways. Twenty out of the twenty-five androgenized girls termed themselves tomboys, a significant difference from the non-androgenized girls. Both groups of androgenized girls showed significantly more athletic energy expenditure and skills, greater preference for male vs. female playmates,
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slacks vs. dresses, and toy cars and guns vs. dolls. They also showed significantly more concern with a career vs. marriage. Of these findings, the activity level and tomboy tendencies are particularly interesting. These are consistent with the results of other experiments where female monkeys were exposed to male hormones in utero. Androgenized female monkeys subsequently resemble normal male monkeys in that they show considerable rough and tumble play - much more than non-androgenized or normal female monkeys display (Goy, 1970). Money and Ehrhardt ( 1972) hypothesize that the masculinization in androgenized girls involves masculinization of part of the brain which mediates “dominance assertion (possibly in association with assertion of exploratory and territorial rights) and manifests itself in competitive energy expenditure” (p. 103). They hypothesize that these androgenized girls differ from normal girls in dominance assertion and, therefore, may find boys more compatible as playmates. The differences they found in toy and dress preferences may develop as a result of the girls’ close companionship with boys rather than reflect sex differences in the brain. Money and Ehrhardt’s findings should be viewed as suggestive rather than conclusive, since genital ambiguity is present in these girls at birth and is known to their parents. This knowledge could well have affected the parents’ treatment of them; that is, parents may have tolerated more masculine activity in these girls than they would have normally; or the knowledge that a daughter was masculinized at birth may have made her parents less tolerant of masculine behavior. When we put aside studies of human clinical syndromes like those of Money and Ehrhardt, we are really left with the
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task of looking at all known sex differences and trying to separate those with a substantial innate basis from those more social in origin. A summary of reported sex differences in behavior is presented in Tables 1 and 2. A more detailed presentation of such differences can be found in Maccoby (1966) and in Maccoby and Jacklin (1974). One approach to the task of determining which of these sex differences is most likely to have a substantial innate basis is to assume that those differences that show up in infancy are less likely to reflect socialization, and, thus, more likely to be innately based. However, the fact that newborn female infants have slightly more mature skeletons and nervous systems than newborn males (Tanner, 1972) makes it necessary in each case to determine whether maturation level alone is responsible for sex differences in infants. It is also possible to reason that puberty is the time that biologically based sex differences will appear. It can be argued that because it is at puberty that both males and females experience high levels of hormones which transform boys and girls into sexually mature men and women, sex differences in behavior appearing at this time are very likely to have a considerable biological component. One of the weaknesses of this argument is that both adolescent males and females encounter intense pressure at this time to conform to adult sex role stereotypes, which could be largely responsible for such sex differences. Another approach is to assume that sex differences which are evident in subhuman primates or cross-culturally in humans must have an innate basis. Although each alone is a substantial rationale, when a sex difference is found both in primates and cross-culturally, the case for an innate basis is a rather strong one.
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Still another tack is to argue that sex differences in behavior which can be related to systematic differences in the behavior of parents or other socializing agents must be social in &gin and, therefore, not innately based. This rationale is weak, since innately based sex differences in human behavior are just as likely to give rise to differential treatment by caretakers as differential treatment by caretakers is to produce sex differences. A final rationale, which is the mirror image of the one just presented, is to suggest that those sex differences in behavior not clearly related to differential socialization of the two sexes must have an innate basis. However, here one cannot rule out the possibility that such. sex differences are somehow indirectly related to differential socialization of the two sexes. Each of these rationales has some merit. In general, the greater the number of rationales that support a biological basis for a specific sex difference in behavior, the more strongly psychologists hold the hypothesis that such a basis exists. The case has been easier to make in some areas than in others. Nurturance behavior in human females and aggressive behavior in human males are two such instances. In most subhuman primates, as well as cross-culturally in humans, females show more nurturance behavior than males, and males show more aggressive behavior than females (see Table 1) (Maccoby and Jacklin, 1974). Rather than viewing these differences as directly related to sex differences in the brain, it is possible that the greater nurturance behavior seen in females comes about because it is the female - not the male - who gestates and lactates. The fact that she can bear an infant and is provided with a means for feeding it causes her to be nurturant. Similarly, it has also been suggested that the higher incidence of aggres-
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summary table of sex differences in social and personality dimenjsions* ** t bable 1.
Age of Subjects fi-12
0-6
Adult
IZ-Adult
Aggression
Boys: strong effect
Boys: strong effect
Boys
Men
Aggression anxiety
Nodata
No d a t a
Girls (one study)
Women
Dependency on others
Mixed: probably none
No good d a t a No good data: girls by self-report
Activity level
Probably none
No good d a t a No good d a t a
Nurturanre h o t very good measure)
Maybe girls
Girls
Girls
Women
Conformity a n d suggestibility
Probably none
Probably none
Girls
Women
Anxiety. parlirularly lest anxiety
NO good data
Girls usually Girls usually
Fear of success
No data
No d a t a
No good d a t a
(not good d a t a ]
No d a t a
Women usually
Women
'Data from Maccoby: The development of sex differences. Stanford Univ. Press, 1966 and Maccoby B Jacklin. unpublished paper. 1972. (Tables 1 and 2.) "From Sccial Issues in Developmentol Psychology by Helen L. Bee: Table 1 [p. 5 ) "Summary Table of Sex Differences in Social and Personabty Dimensions." [Harper h Row, 1074). tThe entries m this table and m Table 2 indicate whether consistent significant differences have been found; the sex named i s the one significantly higher on that behavioral dimension.
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sion in males is only indirectly related to differences in the brain which mediate dominance-assertion (Money and Ehrhardt, 1972) or to differences in bodily structure and available physical energy (Tanner, 1972). Thus, not only is there little agreement as to which sex differences have an innate basis, but even when there is agreement, the exact nature of such a basis is still unclear. Other sex differences commonly thought to have a considerable innate component are verbal fluency and articulation in females and spatial abilities in males (see Table 2 ) . The major argument is that these differences appear consistently and do not seem to be the product of socialization. Further discussion of sex differences and their relationship to biology can be found in Reinisch (1974), Hutt (1972), and Bee (1974). Even if we knew with certainty, many people question whether anything good could come from identifying those sex differences that are largely innate. Isn’t it possible that such knowledge would cause men and women to be restricted to those realms where each has an innately based advantage? Not necessarily so! It might just as well be that as a result of such knowledge we could develop special training procedures to improve performance in the less achieving sex even in those areas of cognitive functioning in which innately based sex differences appear to exist. Thus, no boy or girl, man or woman, would have restricted opportunities simply because of his or her biology. In concluding this discussion, it seems important to consider clinical reports of individuals who have undergone early sex reassignment or have been reared as male when in fact they are genetically female and vice versa. As reported in Money and Ehrhardt (1972)’ it is the sex of assignment and rearing which is the, critical factor in the development of sexual identity - not genetic sex or sex of the genitals.
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Table 2.
s u m m a r y table of sex differences in cognitive functioning* ____-
-
Age of Subjects -___.
Observed difference ~
~ _ _ _ _ ___ _
I2-Ad~lt
6-12
0-6
Adult
~
Verbal abilities articulation
Weak effect: girls
No data
Maybe girls Probably mine Probably none
Weak effect: girls: No d a t a more boys with speech defects Prnbably none Girls Probably none None No d a t a No data
Not relevant
Probably none
Women
Not relevant No good data Not relevant
Girls Maybe girls Mixed; maybe boys Mixed: can't tell Girls Probablv none
Women Maybe women Probably none
Maybe girls Not relevant
Probably none Probably girls
No data None
None
Maybe boys
Boys
Nodata Probablyno dfference Men
None
None
No data
No d a t a
Spatial ability
None
Maybe boys
Boys
Men
Breaking set
No d a t a
Probably boys
Probably boys
Men
Field independence
None
hlaybe boys
Boys
Men
"Analytic" style
Probably none
Boys
Boys
No data
Total 1.Q
None
None
None
None
School grades
Not relevant
Girls
Girls
Not relevant
verbal fluency vocabulary grammatical development in speech grammatical skill. analyzing sentences spelling verbal reasoning reading Mathematical ability counting computation mathematiral reasoning conservation
Girls
Women Women No d a t a
'From Sucrul Issues in Developmental Psychdogy by Helen L. Bee: Table 2 (p. 71 "Summary Table of Sex Differences in Cagnitlve Functionmg." (Harper &Row. 19741.
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A case in point (Money and Ehrhardt, 1972) is that of identical male twins in which one of the twins lost his penis due to a surgical mishap. A circumcision was being performed using electrocautery, and the current, being too strong, burned the entire tissue of the penis, which then died and dropped off. The parents agonized over what to do. When sex reassignment was suggested by a plastic surgeon the parents decided to follow his advice. Medically such reassignment involves both genital reconstruction and hormone replacement therapy; however, it is the social outcome of such reassignment which is of greatest interest. In the case of the male twins, not only did the parents change the name, dress, and hairstyle of the reassigned twin, but they found themselves responding quite differently in their interactions with their “daughter” and their son. Six years following such reassignment the girl is not at all confused about what sex she is and in no way behaves inappropriately. Although genetically identical, the twins have developed quite differently - the one as a boy, and the other as a girl. Money and Ehrhardt (1972) also report several clinical cases where at puberty the body began to mature in a manner inconsistent with the sex of assignment and rearing. In these cases the individuals responded with distress and wished to get rid of the offending organs. Additional case studies that illustrate the importance of socialization and sex of assignment and rearing in the development of sexual identity can be found in Money and Ehrhardt (1972). The clear message here is that even if biologically based sex differences in behavioral predispositions exist, social factors such as the sex which the child is assigned and in which the child is reared can substantially override and obscure them.
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BIBLIOGRAPHY Bardwick, P. M., “The Sex Hormones, the Central Nervous System and Affect Variability in Humans,” in V. Bertle and V. Franks (Eds.), Women in Therapy, New York: Brunner-Mazel, 1974. Bee, H., Social Issues in Developmental Psychology, New York: Harper & Row, 1974. Goy, R. W., “Early Hormonal Influence on the Development of Sexual and Sex-related Behavior,” in G. C. Quarton, T. Melanchuk, and F. 0. Schmitt (Eds.), Neuro-Sciences: A Study Program,. New York: Rockefeller University Press, 1970, pp. 196-207. Hutt, C., “Sex Differences in Human Development,” Human Development, 1972, 15, 153-170. Maccoby, E. E., The Development of Sex Differences, Stanford, Calif. : Stanford University Press, 1966. Maccoby, E. E. and Jacklin, C. N., The Psychology o f Sex Differences, Stanford, Calif. : Stanford University Press, 1974. Money, J. and Athanasiou, R., “Eve First, or Adam?” Contemporary Psychology, 1973, 18, 593-594. Money, J., and Ehrhardt, A. A., Man & Woman, Boy & Girl: The Differentiation and Dimorphism of Gender Identity f r o m Conception to Maturify, Baltimore: Johns Hopkins University Press, 1972. Mussen, P. H., Conger, J. J., and Kagan, J., Child Development and Personality, New York: Harper & Row, 1974. (4th Edition) Napier, J., “The Antiquity of Human Walking,” Scientific American, 1967,2 16 (4),56-66. Reinisch, J. M., “Fetal Hormones, the Brain, and Human Sex Differences; A Heuristic Integrative Review of the Recent Literature, Archives of Sexual Behavior, 1974, 3, 51-90. Sherfey, M. J., The Nature and Evolution o f Female Sexuality, New York: Random House, 1972. Stern, C., Principles of Human Genetics, San Francisco: W. H. Freeman, 1973. Tanner, J. M., “Sequence, Tempo, and Individual Variation in the Growth and Development of Boys and Girls Aged Twelve to Sixteen,” in J. Kagan and R. Coles (Eds.), Twelve to Sixteen: Early Adolescence, New York: Norton, 1972, pp. 907-930.
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genetic differences between human males and females. Is there something inherent in being a girl or boy, man or woman, which predisposes the individual to behave in certain ways? We know that human behavior, more than that of any other species, is plastic and dependent on experience and learning processes. Thus, any sex-related innate or hereditary factors will necessarily be viewed as predisposing rather than as determining. But what are these innate factors? Anyone can tell whether an infant is male or female - look at the sex organs! But in order to determine how these physical differences, as well as other sex-related physical and behavioral differences, come about, we have to start back at the beginning with the biological building blocks - that is, the hereditary material the chromosomes. A normal human organism has twenty-three pairs of chromosomes in each of the cells of his body. One of the twenty-three pairs is the sex chromosome pair. If the sex chromosome pair consists of two X chromosomes, the individual is female; if the sex chromosome pair consists of an X and a Y chromosome, the individual is male. The egg and sperm each carry half of the normal chromosome complement. The egg normally carries twenty-two chromosomes and an X chromosome; and the sperm carries twenty-two chromosomes and an X or a Y chromosome. With fertilization, the twenty-three chromosomes in the nucleus of the mother’s ovum and the twenty-three chromosomes in the nucleus of the
“Innate Factors in Sex Differences”, by Norma L. McCoy, is preprinted with permission from Beyond Sex Roles, Editor/ Author, Alice G . Sargent, West Publishing Company, 1977, ($9.95) 50 W. Kellogg Blvd., P.O.Box 3526, St. Paul, Minn. 55165.
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father’s sperm join, resulting in an individual with twentythree pairs of chromosomes or forty-six in all. If the ovum is fertilized by an X-bearing sperm, the individual is female and if by a Y-bearing sperm, the individual is male. Chromosomes vary in size; the X chromosome is a fairly large one, while the Y chromosome is a small one and carries very little, if any, genetic information (genes). One result of the fact that men have only this small Y chromosome, instead of a second X chromosome, is that the genes on the X chromosome are unpaired with genes on the Y chromosome. This means that when the male has a defective gene on the X chromosome he will show that defect. This explains the high incidence in males of such conditions as hemophilia ( a bleeder disease) and red-green color blindness, two among many undesirable conditions carried by the X chromosome. In females, just before or just following the implantation of the female embryo in the uterus, one of the X s in each of the several hundred existing cells curls up, becomes what is called a “Barr body” and is thought to be inactive. Which X becomes a “Barr body” is a matter of chance. Thus, while all the cells in a normal man’s body bear exactly the same chromosomes, a woman’s cells differ in that some of them will have a functioning X from the mother and some will have a functioning X from the father. Even if a woman has a defective gene on one of her X chromosomes, chances are good that in approximately half of her cells the other X, bearing a normal gene, will be active and she will not show the defect. Using hemophilia as an example, the male with an X chromosome carrying this defect will be a hemophiliac (bleeder); the woman with one normal X and one X with the defect will have slower blood clotting time but will not suffer from hemophilia. The extent to which her blood clotting time
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will be lowered will depend on the proportion of her cells in which the X with the defect is active. This generally accepted theory of a single-active-X is known as the Lyon Hypothesis, after Mary Lyon, who first suggested it (Stern, 1973). Related in some manner to these sex chromosome differences is the fact that more males than females die at all stages of development from conception through old age. The average life expectancy of females (74.9) is seven-and-one-half years greater than that of males (67.4). Aside from all this, the subject of greatest interest to us is not what the Y chromosome doesn’t do, but what it does do. The latest evidence suggests that it does little more than cause an embryo to develop into a male. In fact, it is known on the basis of individuals who are born with abnormal sex chromosome constitutions that the mammalian Y chromosome is strongly male determining. Given the presence of a Y chromosome, the human embryo will develop as a male even if there are two, three, or four X chromosomes. Conversely, in the absence of a Y, the body of a developing human will appear to be female even if only a single X chromosome is present. This fact has led some (Sherfey, 1972) to the position that the human embryo is innately feminine. However, John Money, a leading authority in this area, insists that progression in each stage of sexual differentiation is from an undifferentiated or bi-potential (either sex) state to a differentiated one (Money and Athanasiou, 1973). This statement is based on what is known about the process of sexual differentiation in the developing embryo and should be self-evident to the reader after becoming familiar with the process of sexual differentiation which follows. Considerable experimentation was required in order to learn about the process of sexual differentiation in human
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beings. Because of the nature of such research, experiments were conducted largely with rabbits, guinea pigs, rats and subhuman primates. Subsequently, much of what has been learned in animal research has proved consistent with data obtained from the study of human beings with various clinical syndromes. At present, the process of human sexual differentiation is thought to occur in the following manner: * After fertilization, the developing organism is normally differentiated according to whether it is either male (XU)or female (XX), genetically. At about the fifth week in gestation, the underlying structure or genital ridge which is destined to differentiate into ovaries or into testes appears. During this period, the embryo also has the beginnings of both male and female ducts. At the sixth week the presence of the Y chromosome, in some manner not yet understood, causes the genital ridge to begin sex development into testes. The testes produce male hormone (testosterone), which causes the male ducts to develop (vas deferens, seminal vesicles, ejaculatory ducts), and a second substance (Mullerian-inhibiting substance) which causes the female ducts to regress. In the absence of testosterone, female ducts develop (uterus, fallopian tubes, upper vagina) and the male ducts regress. The first indication that the underlying structures will develop into ovaries is that by the sixth week they have not begun to develop as testes. As in the case of the gonads (ovaries or testes), the external sex organs of the two sexes also differentiate from identical underlying structures. Until the eighth week of gestation these structures are identical in both males and females. Then, some time between the second and third *After Money & Ehrhardt, 1972.
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month of pregnancy, the presence of male sex hormones causes male external sex organs (urethral tube, scrotum, penis) to begin developing. In the absence of such male hormones these same structures will develop as female external organs (labia minora, labia majora, clitoris). The presence of testosterone in the developing male not only causes the development of male sex organs but acts on a region deep in the brain, the hypothalamus, and directs it to function in a masculine rhythm and style. In the absence of testosterone, the hypothalamus is feminine in cycle and style of functioning. At puberty, this region of the brain will act on the pituitary, a gland attached by a stalk to the hypothalamus, and cause it to act on the ovaries (female) or testes (male) to produce either cyclic hormonal activity resulting in ovulation and menstruation in the female or acyclic hormonal activity resulting in the relatively regular production of sperm in the male. While the presence or absence of hormonal events during gestation causes distinct differences in the male and female brain, it is at puberty that these differences in the brain become fully operative. Puberty is thought to be triggered by the maturation of the hypothalamus, which then acts through the pituitary to bring about sexual maturation. Sex differences in the brain are responsible for male (ejaculation, etc.), as compared with female (menstruation, lactation, etc. ) sexual behavior. The presence of such brain differences suggests the possibility of others, as yet undiscovered, which could be responsible for additional sex differences in behavior. If the cyclic vs. acyclic hormonal activity in women and men, respectively, exerts a general effect upon the nervous system, then one can hypothesize that women are predisposed to certain cyclic biologically based changes in behavior, while
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men undergo neither cyclic changes in blood hormone levels nor resultant changes in behavioral predispositions. Bardwick ( 1974) has summarized the research on this and concludes that there is substantial evidence of hormonally related mood swings i n women. Bardwick speculates that self-assertiveness, self-esteem, and competitiveness are all highest at mid-cycle and that anxiety, tenseness, and depression are highest premenstrually and menstrually. These states are related to hormone levels through a hormonally linked enzyme (monoamine oxidase) which affects brain activity. Considerable correlation between the level of this enzyme and depression has been found; low levels of estrogen are associated with high levels of this enzyme, and high levels of this enzyme a r e associated with depression. As is always true, there is undoubtedly wide individual variation among women. While many sex differences appear at puberty, physical differenccs i n addition to those of the sex organs already exist prior to puberty (Tanner, 1972). Full-term male babies a r e slightly larger in all body dimensions than females, including height and weight, although female infants at birth have a vider pelvic outlet. Females develop faster than males, beginning in the fetal period and continuing throughout the course of development. There are many differences due to this differential growth rate, only one of which is the greater length and breadth of the male’s forearm when compared with whole arm length or whole body length. At all ages, the body composition of girls and boys differs, with girls having more fat and less water than boys, and boys having more muscle tissue than girls. Physical growth in girls is less variable than that of boys, such that the range of growth in a group of girls at a specific age is narrower than that in a group of boys. Thus, the number of teeth in two-
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year-old girls will show less variation than the number of teeth possessed by boys of that age. On the average, girls reach puberty two years earlier than boys. For both sexes, puberty brings about a considerable increase in the length of bones and the thickness of cortex, and girls have a smaller loss of fat. The wider shoulders of the male and the broader hips of the female become striking differences. The “sexy” hip-swinging gait of women can be attributed, at least partly, to the structure of the female pelvis, which is such that for any given length of stride a woman must rotate her pelvis through a greater angle than men do. The proportion of the female pelvis appears to be a compromise between a pelvis suited for walking and one suited for childbearing (Napier, 1967). According to J. M. Tanner (1972), a leading authority on physical growth and development : Before adolescence, boys and girls are similar in strength and shape; after, boys are much stronger, probably due to developing more force per gram of muscle as well as absolutely larger muscles. They also develop larger hearts and lungs relative to their size, a higher systolic blood pressure, a lower resting heart rate, a greater capacity for carrying oxygen in the blood, and a greater power for neutralizing the chemical products of muscular exercise, such as lactic acid [p. 9 11 1. At the end of puberty, each is sexually and reproductively mature and shows all the secondary sex characteristics of the mature male or female. These apparent physical differences between mature men and women may be the result of a long period of evolution in which the male had to hunt, fight, and protect and the female had to bear and nurture young. Perhaps a man who was physically strong and who recovered
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quickly from physical fatigue was more apt to survive and reproduce, while a woman survived and reproduced who bore a child easily. Up to this point we have considered physical differences between male and female human things, both in terms of development and maturity. A more interesting and by far more difficult question is, to what extent do these differences in body and brain relate to, underlie, or predispose to sex differences in behavior? For human beings, one of the most fertile sources of information that we have as to possible biological bases of behaviors is the study of males and females who have developed in abnormal hormonal environments because of unusual genetic or environmental circumstances. Psychologists John Money and Anke Ehrhardt ( 1972) have studied girls who were exposed to male hormones during gestation (androgenized females). Those studies include ten females whose mothers were given hormones during pregnancy in order to prevent abortion, and fifteen females who were exposed to male hormones during gestation due to a genetic defect ( andrenogenital syndrome). These girls ranged in age from four to sixteen years, the majority being in middle childhood. Compared with twenty-five normal (non-androgenized) girls matched with them on the basis of age, I.Q., socio-economic background and race, these psychologists found that the girls exposed to male hormones during gestation differed from the non-androgenized girls in several ways. Twenty out of the twenty-five androgenized girls termed themselves tomboys, a significant difference from the non-androgenized girls. Both groups of androgenized girls showed significantly more athletic energy expenditure and skills, greater preference for male vs. female playmates,
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slacks vs. dresses, and toy cars and guns vs. dolls. They also showed significantly more concern with a career vs. marriage. Of these findings, the activity level and tomboy tendencies are particularly interesting. These are consistent with the results of other experiments where female monkeys were exposed to male hormones in utero. Androgenized female monkeys subsequently resemble normal male monkeys in that they show considerable rough and tumble play - much more than non-androgenized or normal female monkeys display (Goy, 1970). Money and Ehrhardt ( 1972) hypothesize that the masculinization in androgenized girls involves masculinization of part of the brain which mediates “dominance assertion (possibly in association with assertion of exploratory and territorial rights) and manifests itself in competitive energy expenditure” (p. 103). They hypothesize that these androgenized girls differ from normal girls in dominance assertion and, therefore, may find boys more compatible as playmates. The differences they found in toy and dress preferences may develop as a result of the girls’ close companionship with boys rather than reflect sex differences in the brain. Money and Ehrhardt’s findings should be viewed as suggestive rather than conclusive, since genital ambiguity is present in these girls at birth and is known to their parents. This knowledge could well have affected the parents’ treatment of them; that is, parents may have tolerated more masculine activity in these girls than they would have normally; or the knowledge that a daughter was masculinized at birth may have made her parents less tolerant of masculine behavior. When we put aside studies of human clinical syndromes like those of Money and Ehrhardt, we are really left with the
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task of looking at all known sex differences and trying to separate those with a substantial innate basis from those more social in origin. A summary of reported sex differences in behavior is presented in Tables 1 and 2. A more detailed presentation of such differences can be found in Maccoby (1966) and in Maccoby and Jacklin (1974). One approach to the task of determining which of these sex differences is most likely to have a substantial innate basis is to assume that those differences that show up in infancy are less likely to reflect socialization, and, thus, more likely to be innately based. However, the fact that newborn female infants have slightly more mature skeletons and nervous systems than newborn males (Tanner, 1972) makes it necessary in each case to determine whether maturation level alone is responsible for sex differences in infants. It is also possible to reason that puberty is the time that biologically based sex differences will appear. It can be argued that because it is at puberty that both males and females experience high levels of hormones which transform boys and girls into sexually mature men and women, sex differences in behavior appearing at this time are very likely to have a considerable biological component. One of the weaknesses of this argument is that both adolescent males and females encounter intense pressure at this time to conform to adult sex role stereotypes, which could be largely responsible for such sex differences. Another approach is to assume that sex differences which are evident in subhuman primates or cross-culturally in humans must have an innate basis. Although each alone is a substantial rationale, when a sex difference is found both in primates and cross-culturally, the case for an innate basis is a rather strong one.
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Still another tack is to argue that sex differences in behavior which can be related to systematic differences in the behavior of parents or other socializing agents must be social in &gin and, therefore, not innately based. This rationale is weak, since innately based sex differences in human behavior are just as likely to give rise to differential treatment by caretakers as differential treatment by caretakers is to produce sex differences. A final rationale, which is the mirror image of the one just presented, is to suggest that those sex differences in behavior not clearly related to differential socialization of the two sexes must have an innate basis. However, here one cannot rule out the possibility that such. sex differences are somehow indirectly related to differential socialization of the two sexes. Each of these rationales has some merit. In general, the greater the number of rationales that support a biological basis for a specific sex difference in behavior, the more strongly psychologists hold the hypothesis that such a basis exists. The case has been easier to make in some areas than in others. Nurturance behavior in human females and aggressive behavior in human males are two such instances. In most subhuman primates, as well as cross-culturally in humans, females show more nurturance behavior than males, and males show more aggressive behavior than females (see Table 1) (Maccoby and Jacklin, 1974). Rather than viewing these differences as directly related to sex differences in the brain, it is possible that the greater nurturance behavior seen in females comes about because it is the female - not the male - who gestates and lactates. The fact that she can bear an infant and is provided with a means for feeding it causes her to be nurturant. Similarly, it has also been suggested that the higher incidence of aggres-
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summary table of sex differences in social and personality dimenjsions* ** t bable 1.
Age of Subjects fi-12
0-6
Adult
IZ-Adult
Aggression
Boys: strong effect
Boys: strong effect
Boys
Men
Aggression anxiety
Nodata
No d a t a
Girls (one study)
Women
Dependency on others
Mixed: probably none
No good d a t a No good data: girls by self-report
Activity level
Probably none
No good d a t a No good d a t a
Nurturanre h o t very good measure)
Maybe girls
Girls
Girls
Women
Conformity a n d suggestibility
Probably none
Probably none
Girls
Women
Anxiety. parlirularly lest anxiety
NO good data
Girls usually Girls usually
Fear of success
No data
No d a t a
No good d a t a
(not good d a t a ]
No d a t a
Women usually
Women
'Data from Maccoby: The development of sex differences. Stanford Univ. Press, 1966 and Maccoby B Jacklin. unpublished paper. 1972. (Tables 1 and 2.) "From Sccial Issues in Developmentol Psychology by Helen L. Bee: Table 1 [p. 5 ) "Summary Table of Sex Differences in Social and Personabty Dimensions." [Harper h Row, 1074). tThe entries m this table and m Table 2 indicate whether consistent significant differences have been found; the sex named i s the one significantly higher on that behavioral dimension.
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sion in males is only indirectly related to differences in the brain which mediate dominance-assertion (Money and Ehrhardt, 1972) or to differences in bodily structure and available physical energy (Tanner, 1972). Thus, not only is there little agreement as to which sex differences have an innate basis, but even when there is agreement, the exact nature of such a basis is still unclear. Other sex differences commonly thought to have a considerable innate component are verbal fluency and articulation in females and spatial abilities in males (see Table 2 ) . The major argument is that these differences appear consistently and do not seem to be the product of socialization. Further discussion of sex differences and their relationship to biology can be found in Reinisch (1974), Hutt (1972), and Bee (1974). Even if we knew with certainty, many people question whether anything good could come from identifying those sex differences that are largely innate. Isn’t it possible that such knowledge would cause men and women to be restricted to those realms where each has an innately based advantage? Not necessarily so! It might just as well be that as a result of such knowledge we could develop special training procedures to improve performance in the less achieving sex even in those areas of cognitive functioning in which innately based sex differences appear to exist. Thus, no boy or girl, man or woman, would have restricted opportunities simply because of his or her biology. In concluding this discussion, it seems important to consider clinical reports of individuals who have undergone early sex reassignment or have been reared as male when in fact they are genetically female and vice versa. As reported in Money and Ehrhardt (1972)’ it is the sex of assignment and rearing which is the, critical factor in the development of sexual identity - not genetic sex or sex of the genitals.
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Table 2.
s u m m a r y table of sex differences in cognitive functioning* ____-
-
Age of Subjects -___.
Observed difference ~
~ _ _ _ _ ___ _
I2-Ad~lt
6-12
0-6
Adult
~
Verbal abilities articulation
Weak effect: girls
No data
Maybe girls Probably mine Probably none
Weak effect: girls: No d a t a more boys with speech defects Prnbably none Girls Probably none None No d a t a No data
Not relevant
Probably none
Women
Not relevant No good data Not relevant
Girls Maybe girls Mixed; maybe boys Mixed: can't tell Girls Probablv none
Women Maybe women Probably none
Maybe girls Not relevant
Probably none Probably girls
No data None
None
Maybe boys
Boys
Nodata Probablyno dfference Men
None
None
No data
No d a t a
Spatial ability
None
Maybe boys
Boys
Men
Breaking set
No d a t a
Probably boys
Probably boys
Men
Field independence
None
hlaybe boys
Boys
Men
"Analytic" style
Probably none
Boys
Boys
No data
Total 1.Q
None
None
None
None
School grades
Not relevant
Girls
Girls
Not relevant
verbal fluency vocabulary grammatical development in speech grammatical skill. analyzing sentences spelling verbal reasoning reading Mathematical ability counting computation mathematiral reasoning conservation
Girls
Women Women No d a t a
'From Sucrul Issues in Developmental Psychdogy by Helen L. Bee: Table 2 (p. 71 "Summary Table of Sex Differences in Cagnitlve Functionmg." (Harper &Row. 19741.
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A case in point (Money and Ehrhardt, 1972) is that of identical male twins in which one of the twins lost his penis due to a surgical mishap. A circumcision was being performed using electrocautery, and the current, being too strong, burned the entire tissue of the penis, which then died and dropped off. The parents agonized over what to do. When sex reassignment was suggested by a plastic surgeon the parents decided to follow his advice. Medically such reassignment involves both genital reconstruction and hormone replacement therapy; however, it is the social outcome of such reassignment which is of greatest interest. In the case of the male twins, not only did the parents change the name, dress, and hairstyle of the reassigned twin, but they found themselves responding quite differently in their interactions with their “daughter” and their son. Six years following such reassignment the girl is not at all confused about what sex she is and in no way behaves inappropriately. Although genetically identical, the twins have developed quite differently - the one as a boy, and the other as a girl. Money and Ehrhardt (1972) also report several clinical cases where at puberty the body began to mature in a manner inconsistent with the sex of assignment and rearing. In these cases the individuals responded with distress and wished to get rid of the offending organs. Additional case studies that illustrate the importance of socialization and sex of assignment and rearing in the development of sexual identity can be found in Money and Ehrhardt (1972). The clear message here is that even if biologically based sex differences in behavioral predispositions exist, social factors such as the sex which the child is assigned and in which the child is reared can substantially override and obscure them.
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BIBLIOGRAPHY Bardwick, P. M., “The Sex Hormones, the Central Nervous System and Affect Variability in Humans,” in V. Bertle and V. Franks (Eds.), Women in Therapy, New York: Brunner-Mazel, 1974. Bee, H., Social Issues in Developmental Psychology, New York: Harper & Row, 1974. Goy, R. W., “Early Hormonal Influence on the Development of Sexual and Sex-related Behavior,” in G. C. Quarton, T. Melanchuk, and F. 0. Schmitt (Eds.), Neuro-Sciences: A Study Program,. New York: Rockefeller University Press, 1970, pp. 196-207. Hutt, C., “Sex Differences in Human Development,” Human Development, 1972, 15, 153-170. Maccoby, E. E., The Development of Sex Differences, Stanford, Calif. : Stanford University Press, 1966. Maccoby, E. E. and Jacklin, C. N., The Psychology o f Sex Differences, Stanford, Calif. : Stanford University Press, 1974. Money, J. and Athanasiou, R., “Eve First, or Adam?” Contemporary Psychology, 1973, 18, 593-594. Money, J., and Ehrhardt, A. A., Man & Woman, Boy & Girl: The Differentiation and Dimorphism of Gender Identity f r o m Conception to Maturify, Baltimore: Johns Hopkins University Press, 1972. Mussen, P. H., Conger, J. J., and Kagan, J., Child Development and Personality, New York: Harper & Row, 1974. (4th Edition) Napier, J., “The Antiquity of Human Walking,” Scientific American, 1967,2 16 (4),56-66. Reinisch, J. M., “Fetal Hormones, the Brain, and Human Sex Differences; A Heuristic Integrative Review of the Recent Literature, Archives of Sexual Behavior, 1974, 3, 51-90. Sherfey, M. J., The Nature and Evolution o f Female Sexuality, New York: Random House, 1972. Stern, C., Principles of Human Genetics, San Francisco: W. H. Freeman, 1973. Tanner, J. M., “Sequence, Tempo, and Individual Variation in the Growth and Development of Boys and Girls Aged Twelve to Sixteen,” in J. Kagan and R. Coles (Eds.), Twelve to Sixteen: Early Adolescence, New York: Norton, 1972, pp. 907-930.
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