Eine gute Zusammenfassung zu dem Thema „Hormone und sexuelle Orientierung bietet der folgende Minireview:
Many people believe that sexual orientation (homosexuality vs. heterosexuality) is determined by education and social constraints. There are, however, a large number of studies indicating that prenatal factors have an important influence on this critical feature of human sexuality. Sexual orientation is a sexually differentiated trait (over 90% of men are attracted to women and vice versa). In animals and men, many sexually differentiated characteristics are organized during early life by sex steroids, and one can wonder whether the same mechanism also affects human sexual orientation. Two types of evidence support this notion. First, multiple sexually differentiated behavioral, physiological, or even morphological traits are significantly different in homosexual and heterosexual populations. Because some of these traits are known to be organized by prenatal steroids, including testosterone, these differences suggest that homosexual subjects were, on average, exposed to atypical endocrine conditions during development. Second, clinical conditions associated with significant endocrine changes during embryonic life often result in an increased incidence of homosexuality. It seems therefore that the prenatal endocrine environment has a significant influence on human sexual orientation but a large fraction of the variance in this behavioral characteristic remains unexplained to date. Genetic differences affecting behavior either in a direct manner or by changing embryonic hormone secretion or action may also be involved. How these biological prenatal factors interact with postnatal social factors to determine life-long sexual orientation remains to be determined.
Quelle: Minireview: Hormones and Human Sexual Orientation (PDF)
Er stellt zunächst verschiedene Modelle von Tieren und deren Erforschung dar, insbesondere Veränderungen in bestimmten Gehirnzentren unter dem Einfluss pränataler Hormone und deren Auswirkungen auf das Verhalten.
Zu der Übertragung auf den Menschen dann das Folgende:
Do these endocrine mechanisms demonstrated in animals have any significance in humans? The answer to this question should be considered in two steps. 1) Do we have any evidence that sex steroids are, in humans like in animals, implicated in the sexual differentiation of morphology (e.g. genital structures) but also of brain (e.g. SDN-POA) and sexual behavior? And 2) are there any data indicating that embryonic sex steroids have, like in animals, organizational effects on sexual orientation in humans? The answer to the first of these questions is clearly yes, and there is probably no need to elaborate on the arguments supporting this conclusion especially in an endocrine journal. To just briefly restate the obvious:
1) Sex steroids (testosterone, estradiol, progesterone) are present in the human plasma in concentrations similar to those observed in other mammals.
2) Receptors for these steroids are present in humans, and their brain distribution is similar and even nearly identical to the general pattern observed in vertebrates.
3) Testosterone action during embryonic life clearly controls the differentiation of male-typical external and internal genital structures.
4) Sex differences in brain structures have been identified, although their control by embryonic steroids is usually not established at this time.
5) Physiological or behavioral differences between men and women are too numerous to be summarized here (22). These differences are complex in nature, and their origin is more difficult to determine than for differences in genital morphology. Learning, education, and expectations of society clearly play an important role in the genesis of behavioral and even sometimes physiological differences. Nevertheless, quite often, these environmental factors build on and amplify smaller, sometimes minor, differences caused by biological factors that were already present at birth. Many physiological and behavioral differences are thus rooted in biology. This is quite obviously the case for many sexually differentiated diseases related to brain function (e.g. anorexia nervosa affects 93 women for every seven men; Gilles de la Tourette syndrome affects 90 men for every 10 women) (see Refs. 22–24 for an extensive list of such differences). How would education or society induce such differences? But many behavioral differences also probably depend to some extent on biological mechanisms often already acting during prenatal life (e.g. increased aggressivity and greater interest in male-typical activities in girls prenatally exposed to high androgen concentration due to congenital adrenal hyperplasia (CAH)
Es gibt also in der Tat viele Hinweise darauf, dass die gleichen Mechanismen, die auch bei den Tieren wirken, auch bei uns noch vorhanden sind. Ein Gedanke, der jedem Evolutionsbiologen auch recht nachvollziehbar erscheint. Gerade so grundlegende Funktionen wie etwa die Geschlechterdifferenzierung ist, gerade wenn auch ansonsten erkennbar viele körperliche Unterschiede zwischen den Geschlechtern bestehen, nicht einfach durch Evolution zu wandeln.
Und schließlich zur sexuellen Orientierung:
Sexual orientation is a sexually differentiated function that might depend, like many other behavioral characteristics, on variations in the early (fetal) exposure to sex steroids (androgens and also possibly their estrogenic metabolites). Exposure to a high concentration of testosterone during a critical phase of development would lead to a male-typical orientation (attraction to women), whereas a lower embryonic exposure to steroids would lead to a female-typical orientation (attraction to men). There would be a critical concentration of testosterone required to masculinize this feature like other aspects of behavior in animals and humans (see Fig. 2). On average, male embryos are exposed to higher concentrations of testosterone than female embryos, but these concentrations vary around a mean value for various reasons (environmental, genetic, etc.). Male subjects at the lower end of this sex-specific distribution could thus acquire a female-typical orientation (and be gay), whereas females at the high end of the concentration curve would acquire a male-typical sexual attraction and be lesbian. Even if they are not attracted by the same specific individuals, females and gay men share an attraction for men, whereas males and lesbians share an attraction for women.
Auch interessant ist ein Hinweis auf eine weitere Möglichkeit zu prüfen, welchem Testosteronstand die jeweilige Person pränatal ausgesetzt war:
Oto-acoustic emissions One set of studies also investigated the physiology of the inner ear and more specifically the small noises produced in the cochlea (presumably) by movements of the tympanic membrane, the so-called oto-acoustic emissions (OAE). OAE are produced either spontaneously or in response to short noises in the environment (e.g. clicks). These OAE are more frequent in women than in men as well as in females compared with males in a variety of animal species. In animals, OAE are masculinized (decrease in frequency) after embryonic treatment of females with androgens. OAE were shown to be significantly less frequent in lesbians compared with heterosexual women, again suggesting that these lesbians were exposed to higher concentrations of androgens than usual during early life. Similar studies assessing other aspects of acoustic physiology (e.g. auditory evoked potentials) that are also sexually differentiated confirmed a masculinization of these traits in lesbians (41). Interestingly, feminization of these features was never observed in gay men, and some studies even reported hypermasculinization of these traits
Klingt auf alle Fälle interessant und sollte man mal näher untersuchen. Das männliche Homosexuelle die Feminisierung nicht zeigen könnte drauf zurückzuführen sein, dass sie über dem „Schwellenwert“ liegen, während der Testosteronspiegel, der für Homosexualität bei Frauen ausreichend ist, eher unter dem liegt, der die Maskulinisierung in diesem Bereich bewirkt
Es werden dann noch unterschiedliche Gehirnstrukturen besprochen, desweiteren:
- 5-reductase deficiency: Ein Wechsel der Geschlechterrollen erfolgt mit der Pubertät unter dem Einfluss der Hormoen
- Congenital adrenal hyperplasia (CAH) : Wesentlich häufiger eine homosexuelle Orientierung, sind pränatal mehr Testosteron ausgesetzt
- Diethylstilbestrol (DES): Personen, deren Mütter während bestimmter Zeiten der Schwangerschaft mit DES behandelt wurden, wurden häufiger homosexuell
- Cloacal exstrophy: Die Personen haben normale Hoden, so dass sie pränatalen Testosteron ausgesetzt sind, aber keinen Penis. Wurde bei der notwendigen Operation eine Geschlechtsumwandlung zu einer Frau durchgeführt, dann haben viele trotzdem später männliches Verhalten und ein sexuelles Interesse an Frauen gezeigt.
Zu den genetischen Komponenten:
Multiple epidemiological studies have demonstrated a correlation between concordance of sexual orientation and genetic relatedness. For example, if a boy is gay, between 20 and 25% of his brothers will share this sexual orientation, compared with 4 – 6% in a control population. Similarly, lesbian women have a greater probability than heterosexual women of having a homosexual sister. Twins studies indicate that this correspondence in sexual orientation probably does not reflect a communality of postnatal experiences (psychosocial factors) but rather genetic similarity. Several studies indeed demonstrated that there is a better agreement of sexual orientation in monozygotic (identical) twins than in dizygotic twins (fraternal twins conceived from different ova and sperm) (65). If a dizygotic gay twin has a brother, there is on average a 15% probability that the brother will also be homosexual, but this probability rises to 65% in monozygotic twins (66). Overall, these studies suggest that in social conditions typical ofWestern societies, 50 – 60% of the variance in sexual orientation in humans has a genetic origin.
Und auch interessant: Die Vererbung scheint eher über die mütterliche Linie zu erfolgen:
Sexual orientation in men tends to be transmitted through the matriarchal lineage: a gay man has a higher probability of having gay men among his ancestors on the maternal side (uncles, cousins), than on the paternal side. This was originally interpreted as a sign of inheritance through gene(s) located on the X chromosome, and one study identified a linkage with markers located in the subtelomeric region of the long arm of the X chromosome, a region called Xq28 (67). This association with Xq28 was replicated in one subsequent study (68) and in another set of data that were not published in a peer-reviewed journal (see Ref. 69) but not in a fourth one (70). A meta-analysis of all these data strongly supports (P 0.0001) the existence of this linkage (71). More recent studies have suggested that the differential heritage through the matriarchal lineage could also be the result of epigenetic modifications of the expression of genes located on several other chromosomes
Für die Gene der Mutter kann es auch vorteilhaft sein (im Sinne des egoistischen Gens) sehr fruchtbare Töchter zu erzeugen, auch wenn dies eine geringere Nachkommenzahl bei den Söhnen bedeutet. Mitochondriale DNA wird nur von der Mutter weitergeben, werden in dieser Veränderungen bewirkt, die im Zusammenspiel mit dem oben genannten Gen besonders fruchtbare Töchter produzieren, dann wird die mitochondriale DNA über diese schneller verbreitet und reichert sich damit im Genpool an. Der mitochondrialen DNA kann der Nachteil der homosexuellen Söhne egal sein, da diese sie eh nicht weiter verbreiten. Deren negative Auswirkungen führen nicht zu einer Selektion der mitochondrialen DNA.
Desweiteren geht er noch auf den Fraternal Birth Order effect ein, also den Umstand, dass Männer mit vielen älteren Brüdern eher homosexuell sind.
Ich finde es gibt eine gute Übersicht und empfehle den Beitrag zu lesen.
Weiteres zur Homosexualität auf diesem Blog:
Alles Evolution 