Sexual motivation and hormones
Sexual motivation is influenced by hormones such as testosterone, estrogen, progesterone, oxytocin, and vasopressin. In most mammalian species, sex hormones control the ability to engage in sexual behaviours. However, sex hormones do not directly regulate the ability to copulate in primates (including humans). Rather, sex hormones in primates are only one influence on the motivation to engage in sexual behaviours.
Measuring sexual motivation
Sexual motivation can be measured using a variety of different techniques. Self-report measures, such as the Sexual Desire Inventory, are commonly used to detect levels of sexual motivation in humans. Self-report techniques such as the bogus pipeline can be used to ensure individuals do not falsify their answers to represent socially desirable results. Sexual motivation can also be implicitly examined through frequency of sexual behaviour, including masturbation.
Testosterone appears to be a major contributing factor to sexual motivation in male primates, including humans. The elimination of testosterone in adulthood has been shown to reduce sexual motivation in both male humans and male primates. Male humans who had their testicular function suppressed with a GnRH anatagonist displayed decreases in sexual desire and masturbation two weeks following the procedure. Research from male rhesus monkeys suggests testosterone functions to increase sexual motivation, thereby motivating males to compete for access to sexual partners. It is postulated that the motivating effects of testosterone in male rhesus monkeys promotes successful sexual competition and may be particularly important motivating tools for low ranking males. It is important to note that elimination of testosterone in primates does not reduce the ability to copulate; rather, it reduces the motivation to copulate.
Testosterone levels in males have been shown to vary according to the ovulating state of females. Males who were exposed to scents of ovulating women recorded a higher testosterone level than males who were exposed to scents of nonovulating women. Being exposed to female ovulating cues may increase testosterone, which in turn may increase males' motivation to engage in, and initiate, sexual behaviour. Ultimately, these higher levels of testosterone may increase the reproductive success of males exposed to female ovulation cues.
The relationship between testosterone and female sexual motivation is somewhat ambiguous. Research suggests androgens, such as testosterone, are not sufficient by themselves to prompt sexual motivation in females. In particular, studies with rhesus macaques have observed testosterone was not significantly associated with variations in level of sexual motivation in females. However, some research with nonhuman primates suggests a role for androgens in female sexual behaviour. Adrenalectomized female rhesus monkeys displayed diminished female sexual receptivity. Later studies revealed this diminished sexual receptivity was specific to the elimination of androgens that can be converted to estrogen.
It is also suggested that levels of testosterone are related to the type of relationship in which one is involved. Men involved in polyamorous relationships display higher levels of testosterone than men involved in either a single partner relationship or single men. Polyamorous women have both higher levels of testosterone and score higher on measures of sexual desire than women who are single or women who are in single-partner relationships.
Estrogen and progesterone
Estrogen and progesterone typically regulate motivation to engage in sexual behaviour for females in mammalian species, though the relationship between hormones and female sexual motivation is not as well understood. In particular, estrogens have been shown to correlate positively with increases in female sexual motivation, and progesterone has been associated with decreases in female sexual motivation. The periovulatory period of the female menstrual cycle is often associated with increased female receptivity and sexual motivation. During this stage in the cycle, estrogens are elevated in the female and progesterone levels are low. At this time, mating can result in female pregnancy.
Females at different stages of their menstrual cycle have been shown to display differences in sexual attraction. Non-pill using heterosexual females who are ovulating (high levels of estrogens) have a preference for the scent of males with low levels of fluctuating asymmetry. Ovulating heterosexual females also display preferences toward masculine faces and report greater sexual attraction to males other than their current partner. From an evolutionary perspective, increases in estrogens during fertile periods in females may direct sexual motivation toward males with preferential genes (the good genes hypothesis).
Following natural or surgically induced menopause, many women experience declines in sexual motivation. Menopause is associated with a rapid decline of estrogen, as well as a steady rate of decline of androgens. The decline of estrogen and androgen levels is believed to account for the lowered levels of sexual desire and motivation in postmenopausal women, although the direct relationship is not well understood.
In her memoir She's Not There: A Life in Two Genders, trans woman Jennifer Finney Boylan wrote that taking estrogen and anti-androgens profoundly diminished her libido, and in trans woman Julia Serano's memoir Whipping Girl: A Transsexual Woman on Sexism and the Scapegoating of Femininity, Serano wrote, in a section of her book she described as limited to hormonal changes that she said are experienced by many trans women she has spoken with, that a sharp decrease in her sex drive was the first thing she noticed when she started taking estrogen and anti-androgens.
Oxytocin and vasopressin
The hormones oxytocin and vasopressin are implicated in regulating both male and female sexual motivation. Oxytocin is released at orgasm and is associated with both sexual pleasure and the formation of emotional bonds. Based on the pleasure model of sexual motivation, the increased sexual pleasure that occurs following oxytocin release may encourage motivation to engage in future sexual activities. Emotional closeness can be an especially strong predictor of sexual motivation in females and insufficient oxytocin release may subsequently diminish sexual arousal and motivation in females.
High levels of vasopressin can lead to decreases in sexual motivation for females. A link between vasopressin release and aggression has been observed in females, which may impair female sexual arousal and sexual motivation by leading to feelings of neglect and hostility toward a sexual partner. In males, vasopressin is involved in the arousal phase. Vasopressin levels have been shown to increase during erectile response in male sexual arousal, and decrease back to baseline following ejaculation. The increase of vasopressin during erectile response may be directly associated with increased motivation to engage in sexual behaviour.
The hormonal influences of sexual motivation are much more clearly understood for nonprimate females. Suppression of estrogen receptors in the ventromedial nucleus of the hypothalamus in female rats has been observed to reduce female proceptivity and receptivity. Proceptivity and receptivity in the female rat are indicators of sexual motivation, thus indicating a direct relationship between estrogen levels and sexual motivation. In addition, female rats receiving doses of estrogen and progesterone were more likely to exert effort at gaining sexual attention from a male rat. The willingness of the female rats to access males was considered a direct measure of the females' levels of sexual motivation.
An increase in vasopressin has been observed in female rats which have just given birth. Vasopressin is associated with aggressive and hostile behaviours, and is postulated to decrease sexual motivation in females. Vasopressin administered in the female rat brain has been observed to result in an immediate decrease in sexual motivation.
Little research has been conducted on the effect of hormones on motivation for same-sex sexual contact. One study observed the relationship between sexual motivation in lesbian and bisexual women and period-related changes in circulating estrogen concentrations. Lesbian women who were at the estrogen peak of their fertile cycle reported increased motivation for sexual contact with women, whereas bisexual women reported only a slight increase in same-sex motivated sexual contact during peak estrogen levels. Interestingly, both lesbian and bisexual women showed decreases in motivation for other-sex sexual contact at peak estrogen levels, with greater changes in the bisexual group than the lesbian group.
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