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The relationship between oxytocin and human sexual response is unclear. At least two non-controlled studies have found increases in plasma oxytocin at orgasm – in both men and women. Plasma oxytocin levels are notably increased around the time of self stimulated orgasm and are still higher than baseline when measured 5 minutes after self arousal. The authors of one of these studies speculated that oxytocin's effects on muscle contractibility may facilitate sperm and egg transport. In a study that measured oxytocin serum levels in women before and after sexual stimulation the author suggests that oxytocin serves an important role in sexual arousal. This study found that genital tract stimulation resulted in increased oxytocin immediately after orgasm. Another study that reports increases of oxytocin during sexual arousal states that it could be in response to nipple/areola, genital, and/or genital tract stimulation as confirmed in other mammals. Murphy et al. (1987), studying men, found that oxytocin levels were raised throughout sexual arousal and there was no acute increase at orgasm. A more recent study of men found an increase in plasma oxytocin immediately after orgasm, but only in a portion of their sample that did not reach statistical significance. The authors noted that these changes "may simply reflect contractile properties on reproductive tissue."
- It is also important to note that more studies have been done to examine sexual arousal in women compared to men. Women experience longer orgasms compared to men and have a more complex reproductive endocrine system with clearly identified cycles such as, menstruation, lactation, menopause, and pregnancy. This allows more opportunities to measure and examine the hormones related to sexual arousal.
- Oxytocin evokes feelings of contentment, reductions in anxiety, and feelings of calmness and security around the mate. In order to reach full orgasm, it is necessary that brain regions associated with behavioral control, fear and anxiety are deactivated; which allows individuals to let go of fear and anxiety during sexual arousal. Many studies have already shown a correlation of oxytocin with human bonding, increases in trust, and decreases in fear. One study confirmed that there was a positive correlation between oxytocin plasma levels and an anxiety scale measuring the adult romantic attachment. This suggests that oxytocin may be important for the inhibition of brain regions that are associated with behavioral control, fear, and anxiety, thus allowing orgasm to occur.
- Sexual arousal. Oxytocin injected into the cerebrospinal fluid causes spontaneous erections in rats, reflecting actions in the hypothalamus and spinal cord. Centrally administrated oxytocin receptor antagonists can prevent non contact erections, which is a measure of sexual arousal. Studies using oxytocin antagonists in female rats provide data that oxytocin increases lordosis, indicating an increase in sexual receptivity.
- Increasing trust and reducing fear. In a risky investment game, experimental subjects given nasally administered oxytocin displayed "the highest level of trust" twice as often as the control group. Subjects who were told that they were interacting with a computer showed no such reaction, leading to the conclusion that oxytocin was not merely affecting risk-aversion. Nasally administered oxytocin has also been reported to reduce fear, possibly by inhibiting the amygdala (which is thought to be responsible for fear responses). Some researchers have argued that oxytocin has a general enhancing effect on all social emotions since intranasal administration of oxytocin also increases envy and schadenfreude.
- Affecting generosity by increasing empathy during perspective taking. In a neuroeconomics experiment, intranasal oxytocin increased generosity in the Ultimatum Game by 80% but has no effect in the Dictator Game that measures altruism. Perspective-taking is not required in the Dictator Game, but the researchers in this experiment explicitly induced perspective-taking in the Ultimatum Game by not identifying to participants which role they would be in.[28