Männliche Geschlechtsidentität und männliches Verhalten: Die Rolle von Sexhormonen in der Gehirnentwicklung

Ein Artikel (PDF) fasst einiges zu der Rolle von Hormonen und ihren Auswirkungen zusammen und geht dabei auch auf gesellschaftliche Faktoren ein.

Auch hier wird das bereits bekannte Modell dargestellt:

Apart from the sexual differentiation of the genitalia, sex hormones in the prenatal environment influence the differentiation of the brain into male or female. Pre- and early neonatal exposure of the brain to sex hormones leads to permanent changes in the nervous system. These effects are referred to as organizational effects. From vertebrate models we learn that the steroid hormone testosterone accounts for the majority of the known sex differences in neural structure and behavior. In lower animals, the presence or absence of testosterone at the time of a critical period of brain sexual differentiation influences the morphology of certain brain nuclei. Like its influence on the development of the genitalia, the presence of testosterone leads to male sexual differ en tiation of the brain and results in male-typical behavior, while a female brain and female-typical behavior are found to be the outcome of the absence of testosterone. When the testes are formed, they begin to produce testosterone and from this moment on there is a sex difference in testosterone concen trations between male and female fetuses. Through its effects on neurogenesis, cell migration, cell death, and the differentiation of neural circuits, testosterone has its effects on neuronal organization.

Also das klassische Modell der Formatierung des Gehirns durch pränatale Hormone.

Einige aus meiner Sicht ganz interessante Stellen aus dem Bericht:

Etwas zur Entwicklung der Geschlechterrollen bei Kindern:

With regard to the concept of gender, children first learn to identify their own and others’ sex (gender labeling). Next, they learn that gender is stable over time (gender stability). Finally, they learn that superficial changes in appearance or activities (a boy does not become a girl overnight if he puts on a wig or plays with Barbie dolls) does not change one’s gender. This is the last stage of gender constancy (gender consistency). This last phase is reached between 5–7 years, but long before that age, children appear to have knowledge about gender stereotypes (for an overview see Ruble et al. 2006).

Das ist erst einmal ein interessanter Überblick, der eben auch die Geschlechterrollen mit einfließen lässt. Es ist auch interessant in Bezug auf die Kinder mit Gendervarinanz und auch in Hinblick auf Steroid-5α-Reduktase-Mangel.

 Zu dem Spielverhalten von Kindern:

At very early ages children become interested in same-sex playmates. Boys like other boys better than girls and spend a fair amount of time in the company of other boys. Changing this peer preference appears to be difficult. Children thus spend an important part of their time in all-male or all-female groups. Boys tend to play in larger groups, play in more public places and with less proximity to adults, and play rougher and with more body contact. Boys fight more and their social interaction is oriented more toward issues of dominance. Girls’ groups are less hierarchically organized and their friendships are more intense. Girls appear to use language to create and maintain relationships, to criticize others in acceptable ways, and to interpret accurately the speech of other girls. In boys, speech is used to attract and maintain an audience, to assert one’s position of dominance, and to assert oneself when others have the floor. So gender segregation has far-reaching consequences for children’s social development and friendships.

Hier zeigen sich also die Unterschiede bereits bei den Kindern. Sie nehmen die Unterschiedlichkeit wahr und spielen deswegen lieber untereinander. Ich hatte dazu bereits einige Artikel:

Zur Aktivierung der vorformatierten Gehirne durch postnatale Hormone:

Later in life, neural circuits and behavioral patterns are activated by changing levels of sex hormones. An example of these so-called activating effects is the stimulation of the already sexually differentiated nervous system by gonadal hormones during puberty. Because steroid-dependent organization of brain and behavior also takes place during adolescence, it has been suggested that these activating effects should also be characterized as organizational effects. The timeframe for organi zational effects may not be limited to prenatal and early neonatal periods, but may also include puberty and adolescence. Steroid dependent organization during puberty implies that certain adult sex-typical behaviors are expressed because pubertal hormones first have organized neural circuits in the developing adolescent brain and that these circuits are subsequently activated by gonadal hormones

Dies wird in dem Artikel noch durch ein Beispiel erläutert.

Weitere Auswirkungen der postnatalen Hormone:

Other sex differences in brain and behavior in adulthood may also be related to the effects of sex hormones. Subcortically, in the hypothalamus, sex differences are observed in the interstitial nuclei of the anterior hypothalamus (INAH-1, INAH-2, INAH-3) and the central portion of the bed nucleus of the stria terminalis, with larger volumes in men than in women. These sex differences in the hypothalamus are thought to underlie sex differences in gender identity, reproduction, and sexual orientation. Gender-related cognitive functioning has been related to size and shape of the corpus callosum. Sex differences have been reported for the corpus callosum, but there is disagreement about the direction of the sex effect and some studies failed to detect such an effect. Men do show more morpho logical asymmetry than women and appear to have a somewhat more lateralized brain with left hemi sphere dominance for language processing and right hemisphere dominance for spatial processing. Regions with developmentally high densities of estrogen and ARs show greater sexual dimorphism. For example, the amygdala has a larger volume in males.

Auch das finde ich sehr interessant. Dort wo die Rezeptoren für Östrogene oder Testosterone dichter sind, zeigen sich auch mehr Unterschiede. Es ist bereits schwer zu erklären, warum unser Gehirn überhaupt diese Rezeptoren im Gehirn vorhanden sein sollen, wenn keine Geschlechtsunterschiede existieren sollen. Nur in einem Modell, wo je nach Stand der Geschlechtshormone unterschiedliche Ausprägungen des Gehirns erfolgen sollen, machen die Rezeptoren überhaupt Sinn.

Zu den Unterschieden in Krankheiten:

Sex differences in the amygdala’s response have been mentioned as factors to explain sex differences in the prevalence of psychiatric disorders. For instance, depression is less common in men than in women and is associated with sex differences in the role of the amygdala in emotional memory. Men and women also differ in the occurrence of other psychiatric disorders. Schizophrenia, attention deficit hyperactivity disorder, and autism primarily hamper men (for an overview see Bao & Swaab 2010), whereas eating- and anxiety disorders are more prevalent in women. Sex ratios for neuro logical disorders differ as well, with Rett syndrome (non-existent in men) and KleineLevin syndrome (non-existent in women) as extremes.

Und weiter zu Personlichkeits unterschieden:

Finally, personality characteristics also show sex differences. In general, physical aggression appears to be higher in men, whereas empathy has been found to be higher in women

Des weiteren geht der Artikel noch ein auf:

  • Diethylstilbestrol (DES) in der Schwangerschaft: a synthetic estrogen that masculinizes and defeminizes brains and behavior in female rodents, have been found to show higher rates of homosexual imagery or homo sexuality than controls, but no masculine gender identity. Effects of exogenous hormones on male behavior and interests are less clear and often conflicting
  • phenobarbital- and phenytoin in der Schwangerschaft: , it was found that the individuals as a group did not differ with respect to gender role behavior, but that higher numbers of prenatally exposed subjects reported current or past gender variant behavior and/or gender dysphoria. Gender dysphoria is the distress resulting from conflicting gender identity and gender of assignment. In a group of 147 subjects, there were also 3 transsexuals. This is a remarkably high rate given the rarity of transsexualism
  • Congenital adrenal hyperplasia (CAH): These women, who are born with more or less virilized external genitalia, are generally treated early in life to normalize hormone levels and often undergo surgery to feminize Maggi_c01.indd 5 Maggi_c01.indd 5 10/15/2011 6:11:27 PM 10/15/2011 6:11:27 PM6 · Male gender identity and masculine behavior their genitalia. Girls with CAH generally show increased male-typical play behavior. Masculine gender role behavior also appears to be common in women with CAH across the lifespan. In women with CAH, a doseresponse correlation has been found; with the more seriously affected “salt-losing” women showing more masculine behavior than the less affected “simple-virilizing” women. In contrast, women with CAH, who were raised as females, mostly have feminine gender identities. However, these women show a less strong female identification, elevated levels of gender discomfort, and even gender dysphoria (∼5%) than non-DSD women.
  • Gender development and 5a-reductase-2 deficiency (5a-RD-2) and 17b- hydroxysteroid dehydrogenase-3 deficiency (17b-HSD-3): Children with 5α-RD-2 have an enzyme defect that prenatally blocks the conversion of testosterone into dihydrotestosterone. Consequently they are born with external genitals that are female in appearance. They are usually raised as girls and seem to have a female gender identity, but, if the condition is not discovered in childhood, these children develop male sex characteristics in puberty: growth of their “clitoris” and scrotum, lowering of the voice, beard growth, masculine muscle development, and masculine body fat distribution. After puberty, many of these youngsters start living as males and develop a sexual attraction toward females
  • Gender development and CAIS/PAIS: Individuals with complete androgen insensitivity syndrome (CAIS), who are raised as girls, are described as very feminine in their gender role behavior, although there may be more variability in their behavior than has long been assumed. They have a female gender identity and in the review by de Vries et al. (2007), none of the women with CAIS reported suffering from gender dysphoria or made a gender transition. But in the partial form of this condition, partial androgen insensitivity syndrome (PAIS), another picture emerges. In female-raised individuals, 11% were gender dysphoric or changed gender (5 of 46). In the male-raised group, this percentage was even higher, where 14% were gender dysphoric or changed gender (5 of 35).
  • Gender development and ablatio penis: Der David Reimers Fall und ähnliche
  • Transsexualität
    •  Post-mortem studies: Postmortem studies into the brain material of transsexuals revealed a sex reversal in volume and neuron number in the central portion of the bed nucleus of the stria terminalis and the interstitial nucleus 3 of the anterior hypothalamus in male-to-female transsexuals (MtFs) and a female-to-male (FtM) transsexual.
    • Luteinizing hormone (LH) regulation: Konnte die vermuteten Ergebnisse nicht erreichen
    • Cognitive studies: In studies examining IQ and verbal/spatial subtests, some samples of individuals with gender identity disorder (GID) show a pattern of cognitive functioning that is not in line with the natal sex. However, the results are too inconsistent to draw definite conclusions.
    • Handedness studies
    • Imaging studies: Magnetic Resonance Imaging (MRI) in untreated MtFs shows that their gray matter volumes are mostly consistent with men (their natal sex), but that the gray matter volume of the putamen was feminized. In addition, cerebral activation patterns in trans sexuals prior to treatment seem to share more features with those of the experienced gender than those of their natal sex. This was observed using Positron Emission Tomography (PET) during the processing of pheromones and using functional MRI while viewing erotic film excerpts. Finally, differences have been found within the cortical network engaged in mental rotation between MtFs (prior to as well as during hormonal treatment) and control males