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Sexual arousal (also sexual excitement) is the arousal of sexual desire, during or in anticipation of sexual activity. Things that precipitate human sexual arousal are called erotic stimuli, or colloquially known as turn-ons. There are mental stimuli and physical stimuli such as hormones which can cause a person to become sexually aroused.

Sexual arousal may not lead to any actual sexual activity, beyond a mental arousal and the physiological changes that accompany it. Given sufficient stimulation, sexual arousal in humans will typically end in an orgasm; but arousal may be pursued for its own sake, even in the absence of an orgasm. It is thought that women may get more aroused at a certain time during their menstrual cycle, either before, after or during their period, however, there are studies that do not support this theory.

When men are aroused they typically get an erection. When women are aroused, they typically get vaginal lubrication (wetness).

Terminology
There are several informalities, terms and phrases to describe sexual arousal including horny, turned on, randy, steamy, and lustful.

Erotic stimuli
Depending on the situation, a person can be sexually aroused by a variety of factors, both physical and mental. A person may be sexually aroused by another person or by particular aspects of that person, or by a non-human object. The physical stimulation of an erogenous zone or acts of foreplay can result in arousal, especially if it is accompanied with the anticipation of imminent sexual activity. Sexual arousal may be assisted by a romantic setting, music or other soothing situation. The potential stimuli for sexual arousal vary from person to person, and from one time to another, as does the level of arousal.

Stimuli can be classified according to the sense involved: somatosensory (touch), visual, and olfactory (scent). Auditory stimuli are also possible, though they are generally considered secondary in role to the other three. Erotic stimuli which can result in sexual arousal can include conversation, reading, films or images or a smell or setting, any of which can generate erotic thoughts and memories in a person. Given the right context, these may lead to the person desiring physical contact, including kissing, cuddling, and petting of an erogenous zone. This may in turn make the person desire direct sexual stimulation of those parts of their body which would normally be out of bounds, such as breasts, nipples, buttocks and/or genitals, and to sexual activity. The erotic stimuli may originate from a source unrelated to the object of subsequent sexual interest. For example, many people may find nudity, erotica or pornography sexually arousing, which may generate a general sexual interest which is satisfied with sexual activity. When sexual arousal is achieved by or dependent on the use of objects, it is referred to as sexual fetishism, or in some instances a paraphilia.

There is a common belief that women need more time to achieve arousal. However, recent scientific research has shown that there is no considerable difference for the time men and women require to become fully aroused. Scientists from McGill University Health Centre in Montreal, Canada used the method of thermal imaging to record baseline temperature change in genital area to define the time necessary for sexual arousal. Researchers studied the time required for an individual to reach the peak of sexual arousal while watching sexually explicit movies or pictures and came to the conclusion that on average women and men took almost the same time for sexual arousal — around 10 minutes. The time needed for foreplay is very individualistic and varies from one time to the next depending on many circumstances.

Unlike many other animals, humans do not have a mating season, and both sexes are potentially capable of sexual arousal throughout the year.

Sexual arousal disorders
Sexual arousal for most people is a positive experience and an aspect of their sexuality, and is often sought. A person can normally control how they will respond to arousal. They will normally know what things or situations are potentially stimulating, and may at their leisure decide to either create or avoid these situations. Similarly, a person's sexual partner will normally also know his or her partner's erotic stimuli and turn-offs. Some people feel embarrassed by sexual arousal and some are sexually inhibited. Some people do not feel aroused on every occasion that they are exposed to erotic stimuli, nor act in a sexual way on every arousal. A person can take an active part in a sexual activity without sexual arousal. These situations are considered normal, but depend on the maturity, age, culture and other factors influencing the person.

However, when a person fails to be aroused in a situation that would normally produce arousal and the lack of arousal is persistent, it may be due to a sexual arousal disorder or hypoactive sexual desire disorder. There are many reasons why a person fails to be aroused, including a mental disorder, such as depression, drug use, or a medical or physical condition. The lack of sexual arousal may be due to a general lack of sexual desire or due to a lack of sexual desire for the current partner. A person may always have had no or low sexual desire or the lack of desire may have been acquired during the person's life. There are also complex philosophical and psychological issues surrounding sexuality. Attitudes towards life, death, childbirth, one's parents, friends, family, contemporary society, the human race in general, and particularly one's place in the world play a substantive role in determining how a person will respond in any given sexual situation.

On the other hand, a person may be hypersexual, which is a desire to engage in sexual activities considered abnormally high in relation to normal development or culture, or suffering from a persistent genital arousal disorder, which is a spontaneous, persistent, and uncontrollable arousal, and the physiological changes associated with arousal.

Physiological changes
Sexual arousal causes different physical changes, most significantly in the sex organs (genital organs). Sexual arousal for a man is usually indicated by the swelling and erection of the penis when blood fills the corpus cavernosum. This is usually the most prominent and reliable sign of sexual arousal in males. In a woman, sexual arousal leads to increased blood flow to the clitoris and vulva, as well as vaginal transudation - the seeping of moisture through the vaginal walls which serves as lubrication.

Female physiological changes
The beginnings of sexual arousal in a woman's body is usually marked by vaginal lubrication (wetness), swelling and engorgement of the external genitals, and internal enlargement of the vagina. There have been studies to find the degree of correlation between these physiological responses and the woman's subjective sensation of being sexually aroused: the findings usually are that in some cases there is a high correlation, while in others, it is surprisingly low.

Further stimulation can lead to further vaginal wetness and further engorgement and swelling of the clitoris and the labia, along with increased redness or darkening of the skin in these areas. Further changes to the internal organs also occur including to the internal shape of the vagina and to the position of the uterus within the pelvis. Other changes include an increase in heart rate as well as in blood pressure, feeling hot and flushed and perhaps experiencing tremors. A sex flush may extend over the chest and upper body.

If sexual stimulation continues, then sexual arousal may peak into orgasm. After orgasm, some women do not want any further stimulation and the sexual arousal quickly dissipates. Instructions have been published for keeping the sexual excitement going and moving from one orgasm into further stimulation and maintaining or regaining a state of sexual arousal that can lead to second and subsequent orgasms. Some women have experienced such multiple orgasms quite spontaneously.

While young women may become sexually aroused quite easily, and reach orgasm relatively quickly with the right stimulation in the right circumstances, there are physiological and psychological changes to women's sexual arousal and responses as they age. Older women produce less vaginal lubrication and studies have investigated changes to degrees of satisfaction, frequency of sexual activity, to desire, sexual thoughts and fantasies, sexual arousal, beliefs about and attitudes to sex, pain, and the ability to reach orgasm in women in their 40s and after menopause. Other factors have also been studied including socio-demographic variables, health, psychological variables, partner variables such as their partner's health or sexual problems, and lifestyle variables. It appears that these other factors often have a greater impact on women's sexual functioning than their menopausal status. It is therefore seen as important always to understand the "context of women's lives" when studying their sexuality.

Reduced estrogen levels may be associated with increased vaginal dryness and less clitoral erection when aroused, but are not directly related to other aspects of sexual interest or arousal. In older women, decreased pelvic muscle tone may mean that it takes longer for arousal to lead to orgasm, may diminish the intensity of orgasms, and then cause more rapid resolution. The uterus typically contracts during orgasm, and with advancing age, those contractions may actually become painful.

Male physiological changes
It is normal to correlate the erection of the penis with male sexual arousal. Physical or psychological stimulation, or both, leads to vasodilation and the increased blood flow engorges the three spongy areas that run along the length of the penis (the two corpora cavernosa and the corpus spongiosum). The penis grows enlarged and firm, the skin of the scrotum is pulled tighter, and the testes are pulled up against the body. However the relationship between erection and arousal is not one-to-one. After their mid-forties, some men report that they do not always have an erection when they are sexually aroused. Equally, a male erection can occur during sleep (nocturnal penile tumescence) without conscious sexual arousal or due to mechanical stimulation (e.g. rubbing against the bed sheet) alone. A young man — or one with a strong sexual drive — may experience enough sexual arousal for an erection to result from a passing thought, or just the sight of a passerby. Once erect, his penis may gain enough stimulation from contact with the inside of his clothing to maintain and encourage it for some time.

As sexual arousal and stimulation continues, it is likely that the glans or head of the erect penis will swell wider and, as the genitals become further engorged with blood, their colour deepens and the testicles can grow up to 50% larger. As the testicles continue to rise, a feeling of warmth may develop around them and the perineum. With further sexual stimulation, the heart rate increases, blood pressure rises and breathing becomes quicker. The increase in blood flow in the genital and other regions may lead to a sex flush sometimes, in some men.

As sexual stimulation continues, the muscles of the pelvic floor, the ductus deferens (between the testicles and the prostate), the seminal vesicles and the prostate gland itself may begin to contract in a way that forces sperm and semen into the urethra inside the penis. This is the onset of orgasm and it is likely, once this has started, that the man will continue to ejaculate and orgasm fully, with or without further stimulation.

Equally, if sexual stimulation stops before orgasm, the physical effects of the stimulation, including the vasocongestion, will subside in a short time. Repeated or prolonged stimulation without orgasm and ejaculation can lead to discomfort in the testes that is sometimes called 'blue balls'.

After orgasm and ejaculation, men usually experience a refractory period characterised by loss of erection, a subsidence in any sex flush, less interest in sex, and a feeling of relaxation that can be attributed to the neurohormones oxytocin and prolactin. The intensity and duration of the refractory period can be very short in a highly aroused young man in what he sees as a highly arousing situation, perhaps without even a noticeable loss of erection. It can be as long as a few hours or days in mid-life and older men.

Male
The relationship between sexual desire and arousal in men is complex, with a wide range of factors increasing or decreasing sexual arousal. Physiological responses, such as heart rate, blood pressure, and erection, are often discordant with self-reported subjective perceptions of arousal. This inconsistency suggests that psychological, or cognitive aspects, also have a strong effect on sexual arousal. The cognitive aspects of sexual arousal in men are not completely known, but it does involve the appraisal and evaluation of the stimulus, categorization of the stimulus as sexual, and an affective response. Research suggests that cognitive factors, such as sexual motivation, perceived gender role expectations, and sexual attitudes, contribute to sex differences observed in subjective sexual arousal. Specifically, while watching visual stimuli, men are more influenced by the sex of an actor portrayed in the stimulus, and men typically prefer a stimulus that allows objectification of the actor and projection of themselves into the scenario. For more information on cognitions and sexual arousal visit the |American Psychological Association: The science of sexual arousal. There are reported differences in brain activation to sexual stimuli, with men showing higher levels of amygdala and hypothalamic responses than women. This suggests the amygdala plays a critical role in the processing of sexually arousing visual stimuli in men.

Human sexual response cycle
During the late 1950s and early 1960s, William H. Masters and Virginia E. Johnson conducted many important studies into human sexuality. In 1966, they released Human Sexual Response, detailing four stages of physiological changes in humans during sexual stimulation: excitement, plateau, orgasm, and resolution.

Singer's model of sexual arousal
Singer presents a model of the process of sexual arousal, in which he conceptualized human sexual response to be composed of three independent but generally sequential components. The first stage, aesthetic response, is an emotional reaction to noticing an attractive face or figure. This emotional reaction produces an increase in attention toward the object of attraction, typically involving head and eye movements toward the attractive object. The second stage, approach response, progresses from the first and involves bodily movements towards the object. The final genital response stage recognizes that with both attention and closer proximity, physical reactions result in genital tumescence. Singer also notes that there is an array of other autonomic responses, but acknowledges that the research literature suggests that the genital response is the most reliable and convenient to measure in males.

Male
One of the responses involved with sexual behaviour in males is penile erection; therefore, the volume (or circumference) change during penile erection is a convenient measure of sexual arousal, which was first developed by Kurt Freund. This measurement of blood flow to the male genitals is known as penile plethysmography. This is commonly measured using a strain gauge, which is comprised of a simple, mercury strain gauge encompassed in a ring of rubber. The ring surrounds the penis, but does not constrict or cause discomfort. The measure is found to be a reliable and valid measurement of male arousal. More recently, thermography has been developed to measure the physiological measurements of sexual arousal. Studies have found temperature change specific to the genitals during sexual arousal, which supports the validity of this measure.

Category-Specificity
Category-specify is a sexual-arousal pattern that is more commonly found amongst men than women. Category-specificity refers to a person showing sexual arousal to the categories of people they prefer to have sex with. Sexual arousal studies involving category-specify look at genital responses, or physiological changes, as well as subjective responses, or what people report their arousal levels to be. Heterosexual men experience much higher genital and subjective arousal to women than to men. This pattern in reversed for homosexual men. Studies have found that women have a non-specific genital response pattern of sexual arousal, meaning their genital responses are only modestly related to their preferred category. Women subjective responses are category-specific, because they typically report their highest level of arousal to their preferred stimulus, however, the reported difference in levels of arousal is typically much smaller than men’s.

Hormones and sexual arousal
Several hormones affect sexual arousal, including testosterone, cortisol, and estradiol, however, the specific roles of these hormones are not clear. Testosterone is the most commonly studied hormone involved with sexuality. Testosterone plays a key role in sexual arousal in males, with strong effects on central arousal mechanisms. The connection between testosterone and sexual arousal is more complex with females. Research has found testosterone levels increase as a result of sexual cognitions in females that do not use hormonal contraception. Also, women who participate in polyandrous relationships have higher levels of testosterone, however, it is unclear whether higher levels of testosterone cause increased arousal and in turn multiple partners or whether sexual activity with multiple partners increases testosterone as an effect. Inconsistent study results point to the idea that while testosterone may play a role in the sexuality of some women, its effects can be obscured by the co-existence of psychological or affective factors in others.



Sexual arousal in animals
While human sexuality is well understood, scientists do not completely grasp how other animals relate sexually. However, current research studies suggest that many animals, like humans, enjoy sexual relations that are not limited to reproduction. Dolphins and bonobos, for example, are both well known to use sex as a "social tool to strengthen and maintain bonds." Ethologists have long documented the exchanges of sex to promote group cohesion in social animals. Cementing social bondage is one of the most prominent theorized selective advantages of group selection theory. Experts in the evolution of sex such as John Maynard Smith advocate for the idea that the exchange of sexual favors helps congeal and localize the assortment of alleles in isolated population and therefore is potentially a very strong force in evolution. Maynard Smith also has written extensively on the "seminal fluid swapping theory" logistic application of the assortment of alleles as a more accurate synthetic depiction of the Hardy Weinberg Equilibrium in cases of severely interbreeding populations.

Evolutionary models
The effect of sexual response is thought to be a plastic positive reinforcement behavior modifier associated with the Baldwin Effect. The end result of these sorts of things can be very novel structures such as the Pseudo-penis of the female spotted hyena. The display of secondary sex characteristics in humans such as a penis-like enlarged clitoris in females during arousal and gynecomastia in males are thought to have once been objects of mate selection in human evolution because of the persistence of the phenomenon of these features invoking sexual arousal for potential mates in cross-cultural studies. A dramatic example of this is the high rates of secondary sex characteristic dimorphism in some Southeast Asia human populations.