The Organizational-Activational Hypothesis states that steroid hormones permanently organize the nervous system during early development, which is reflected in adult male or female typical behaviors.[1] In adulthood, the same steroid hormones activate, modulate, and inhibit these behaviors. [2] This idea was revolutionary when first published in 1959 because no other previous experiment had demonstrated that adult behaviors could be determined hormonally during early development. [3]
The Phoenix et al. study sought to discover whether gonadal hormones given during the prenatal period had organizing effects on guinea pigs’ reproductive behavior[4] It was found that when female controls, gonadectomized (removal of gonads) females, hermaphrodites, and castrated males were injected prenatally with testosterone propionate, the mean number of mounts increased. This increase in male-typical reproductive behavior shows that prenatal androgens have a masculinizing effect. Moreover, the organizing effects of hormones can have permanent effects. Phoenix et al. found that females injected with testosterone propionate while pregnant, instead of neonatally, did not have any effect on lordosis. This demonstrates that when testosterone is given postnatally in females, there may not be lasting effects as compared to prenatally administered testosterone. The data from this study supports the organizational hypothesis that states when androgens are given prenatally there is an organizing effect on sexual behavior, permanently altering normal female mating behavior as adults. [5]
Sexual behavior in rats is organized prenatally and activated with steroids hormones in adulthood. [6] In males high levels of testosterone produced by testes and travel to the brain. Here, testosterone is aromatized to an estradiol and masculinizes and defeminizes the brain. Thus, estradiol is responsible for many male-typical behavior. In females, the ovaries produce large amounts of estrogen during gestation. Rats have alpha-fetoprotein that binds to the estrogen before it can reach the brain. The estrogen is eventually metabolized in the liver. This protein has a low affinity for androgens. Therefore, testosterone can reach the brain without being taken up by alpha-fetoprotein. Due to fact that males have different levels of androgens in the brain, this can lead to organizing effects from androgen exposure with the expression of masculine behaviour.
The organizational-activational theory has three main components.[7]
Finger ratio has been examined in relation to a number of physical traits that show sex differences and evidence suggests it is influenced by the prenatal environment, although there is no direct evidence for the latter.[16] Studies in men have been motivated by two conflicting hypotheses. On the one hand, homosexual men were hypothesized to be exposed to high levels of testosterone in utero, which would be associated with a lower 2D:4D ratio than that found in heterosexual men. On the other hand, homosexual men have been hypothesized to have low prenatal testosterone exposure.
The hormonal control of ovulation is also related to the organizational/activational hypothesis. [17] Both males and females rats exhibit luteinizing hormone (LH) pulses in which LH is released from the anterior pituitary due to the secretion of gonadotropin releasing hormone (GnRH) from the hypothalamus. Females, however, show an increase in LH pulse frequency around ovulation due to the positive feedback mechanism. When estrogen is increased in the blood, the anteroventral periventricular nucleus (AVPV) of the hypothalamus causes the release of GnRH. The GnRH surge brings about a surge in LH and follicle stimulating hormone (FSH). Since females have a cyclic gonadal function, there may be a sexual dimorphism in the gonadal secretion. When female rats are injected with testosterone there is no positive feedback occurring and no LH surge. Moreover, castrated males will exhibit LH surges, similar to female cyclic gonadal behavior.