Pränatales Testosteron und „genderbezogenes Verhalten“

Eine Artikel beleuchtet eines meiner Lieblingsthemen: Pränatales Testosteron und seine Auswirkungen auf das geschlechtliche Verhalten.

Testosterone plays an important role in mammalian brain development. In neural regions with appropriate receptors testosterone, or its metabolites, influences patterns of cell death and survival, neural connectivity and neurochemical characterization. Consequently, testosterone exposure during critical periods of early development produces permanent behavioural changes. In humans, affected behaviours include childhood play behaviour, sexual orientation, core gender identity and other characteristics that show sex differences (i.e. differ on average between males and females). These influences have been demonstrated primarily in individuals who experienced marked prenatal hormone abnormalities and associated ambiguities of genital development (e.g. congenital adrenal hyperplasia). However, there is also evidence that testosterone works within the normal range to make some individuals within each sex more sex-typical than others. The size of testosterone-related influences, and perhaps even their existence, varies from one sex-typed characteristic to another. For instance: prenatal exposure to high levels of testosterone has a substantial influence on sex-typical play behaviour, including sex-typed toy preferences, whereas influences on core gender identify and sexual orientation are less dramatic. In addition: there appears to be little or no influence of prenatal testosterone on mental rotations ability, although mental rotations ability shows a marked sex difference. These findings have implications for basic understanding of the role of testosterone in normative gender development, as well as for the clinical management of individuals with disorders of sex development (formerly called intersex syndromes).

Zu Bedenken ist dabei, dass es hier um pränatales Testosteron geht, nicht um die Wirkung von Testosteron, insbesondere auch postnatalen Testosteron, dass weitere Wirkungen hat.

Aus dem Artikel selbst noch ein paar Auszüge:

Hundreds of studies on species ranging from rodents to non-human primates show that testosterone, and hormones produced from testosterone, play a primary role in neural and behavioural sexual differentiation, just as they do in sexual differentiation of the external genitalia

Also hunderte von Studien, die zeigen, dass geschlechterbezogenes Verhalten bei Menschen und Tieren in einem Zusammenhang mit den Hormonen steht. Hier stellt sich wieder die Frage an gesellschaftliche Erklärungen, warum diese Mechanismen bei Tieren bestehen,  bei Menschen aber nicht, wenn beim Menschen im Prinzip die gleichen Mechanismen vorhanden sind.

Despite these differences in the periods when testosterone manipulations alter brain structure and function, the outcomes of the manipulations are largely consistent across species. In both rodents and non-human primates, exposing developing female animals to high levels of testosterone, for example, by injecting them or their pregnant mothers with the hormone, increases subsequent levels of male-typical behaviour (1). In rats, for instance, a single injection of testosterone on the day of birth produces female animals who show elevated levels of rough-and-tumble play, a behaviour that is normally more common in juvenile males than in juvenile females. Similarly, female rats treated with testosterone neonatally show increased capacity for male-typical sexual behaviour and reduced capacity for female-typical sexual behaviour in adulthood. Removing testosterone from developing male rats (e.g. by neonatal castration) has the opposite effects, reducing male-typical behaviour and increasing female-typical behaviour. Comparable effects, on both juvenile play behaviour and on sexual behaviour in adulthood are seen following prenatal testosterone manipulations in rhesus monkeys

Man findet bei den Säugetieren und bei den Affen die diskutierten Systeme. Wenn man bedenkt, dass Menschenaffen natürlich nicht unsere Intelligenz haben, aber dennoch ein Selbstbewußtsein besitzen und in der Lage sind komplexe Sozialsysteme zu errichten und taktisch zu planen, fragt man sich wiederum, warum all diese Verhalten sich in sehr ähnlicher Form bei Menschen zeigen, aber  bei diesem Kultur sein sollen

As is the case for non-human primates, the critical periods when testosterone is higher in developing males versus females, and thus the periods when testosterone would be hypothesized to influence human neural and behavioural sexual differentiation, appear to be largely prenatal. Although information is limited, there appears to be a dramatic sex difference in testosterone levels in human foetuses from about week 8 to 24 of gestation (5). Another, smaller sex difference in circulating testosterone occurs in the early postnatal period in humans as well as in rhesus monkeys, but studies to date have produced little evidence that it plays a role in sexual differentiation of behaviour (5, 6), and research on the impact of testosterone on human development has focused largely on the prenatal period.

Es bestehen nach bisherigem Forschungsstand verschiedene Zeitfenster für verschiedene Bereiche des Gehirns. Da unser Gehirn nicht konstruiert wird, sonder wächst werden verschiedene Bereiche eben zu verschiedenen Zeiten aufgebaut.

Research on girls and women with CAH has provided some support for the hypothesized influence of testosterone on human behavioural development. Girls with CAH show increased male-typical play behaviour, including increased preferences for toys that are usually chosen by boys, such as vehicles and weapons, increased preferences for boys as playmates and increased interest in rough-and-tumble play. These findings of increased male-typical play behaviour have been seen using varied measures (interviews, standardized questionnaires, analyses of drawings and direct observation of behaviour in playroom settings), and in samples of girls with CAH in several countries (the United States, Canada, The Nether-lands, Sweden, Germany, Japan and the United Kingdom) (2). Like the CAH-related virilization of the external genitalia, CAH-related behavioural virilization is not complete. The toy, activity and playmate preferences of girls with CAH are more male-typical than those of their unaffected female relatives as well as of control girls matched for age and other aspects of demographic background, but they are not as male-typical as those of boys. For instance, in regard to playmates, 80–90% of the playmates of healthy boys and girls of elementary school age are children of the same sex, whereas girls with CAH do not appear to show a preference for playing with one sex over the other. They indicate that about 50% of their favourite playmates are girls and about 50% are boys (9)

As adults, women with CAH are less likely to be exclusively heterosexual than are their unaffected sisters, controls matched for age and demographic background, or women with other disorders that cause different hormone abnormalities or abnormalities of the external genitalia (10–12). The nature of this behavioural change is not necessarily to produce a homosexual orientation. In fact, bisexuality may be a similarly likely outcome. For instance, a study by Hines et al. (11), found that five out of 16 women with CAH reported that they were neither exclusively nor almost exclusively heterosexual. Of these five, one was exclusively homosexual, two were almost exclusively homosexual and two were equally homosexual and heterosexual in their behaviour. In addition, this study provided quantitative data, making it possible to compare the magnitude of the effect of CAH on sexual orientation with the magnitude of the effect on childhood play behaviour (see Fig. 1⇓). For childhood play behaviour, females with CAH were moved about 60% of the distance toward mean male-typical behaviour, whereas for sexual orientation, females with CAH were moved only about 10% of the distance toward mean male-typical orientation.

Hier wird noch einmal deutlich, dass CAH-Mädchen Verhaltensänderungen hin zum männlichen zeigen. Das diese sich auch gerade auf das Spielen auswirkt und diese Mädchen etwas wilder und härter sind ist gut damit zu erklären, dass Spielen dazu dient sich auch das Erwachsenenleben vorzubereiten und die dafür benötigten Fähigkeiten zu erlernen. (vgl. Lebensphasen und Geschlecht). Und das bedeutete in der Steinzeit eben, dass man sich für den intrasexuellen Wettbewerb abhärten musste.

Es folgt die zweite Gruppe: CAIS

Other situations where hormone levels or responses are altered prenatally provide additional support for the conclusion that prenatal levels of testosterone influence human psychosexual development. For instance, genetic males with complete androgen insensitivity syndrome (CAIS), an X-linked disorder in which the cells of XY individuals lack functional androgen receptors, and so cannot respond to the testosterone and other androgenic hormones produced by their gonads, are almost always female-typical not only in physical appearance, but also in terms of behaviour, including core gender identity, sexual orientation and childhood play behaviour (15, 16). This suggests that their functional lack of androgen exposure has produced female-typical psychosexual development, despite the presence of a Y chromosome.

Hieran wird deutlich, dass es sich eher nicht um genetische Unterschiede handelt, da diese ja dann bei den CAIS zu einem männlicheren Verhalten führen müßten (wobei natürlich auch auf dem Y-Chromoson einige Daten gespeichert sind). Vielmehr lässt sich in diesem Bereich gut der hormonelle Anteil erkennnen.

Es werden des weiteren verschiedene Medikamente, die die Mutter genommen hat angeführt:

Similarly, girls whose mothers were prescribed hormones that stimulate androgen receptors prenatally, like girls with CAH, show more male-typical childhood play behaviour and interests.

Zu den Unterschieden in den Fähigkeiten:

Specifically, although men perform better than women on tests of mental rotations ability (that is, the ability to rotate two-or three-dimensional figures in the mind and compare them to other figures), these differences are larger in adults than in children (29, 30). In addition, sex differences in performance on other spatial tasks are smaller than the sex differences in mental rotations performance (29, 30). Indeed, for some tasks, such as those requiring spatial visualization skills, or the ability to take spatial manipulations through several steps, sex differences are virtually non-existent (29). Similarly, sex differences in mathematics performance vary with age and the type of task. Among children, girls perform better on measures of computational ability, although there are no sex differences on computational tasks in adults (31). For mathematical concepts, there are no sex differences in children or adults, however, some standardized measures used to screen for admission to University in the United States (the Scholastic Aptitude Test and the Graduate Record Exam) show a sex difference in favour of males

Einige Fähigkeiten weisen nur geringe Unterschiede auf. Bei einigen anderen fallen diese auch erst auf, wenn man  mit höherem Schwierigkeitsgrad prüft. Zu den verschiedenen Werten hatte ich ja bereits einen Artikel eingestellt. Es ist auch zu bedenken, dass einige Fähigkeiten bei Frauen bei einem relativ (für Frauen) hohem Testosteronspiegel gut ausgeprägt sind, der beste Wert bei Männern aber bei einem für Männer niedrigen Wert erreicht wird (auch wenn dieser noch höher ist als der von Frauen).