Sexual arousal, or sexual excitement, is the arousal of sexual desire, during or in anticipation of sexual activity. Things that precipitate human sexual arousal are colloquially known as turn-ons. There are many potential stimuli, both physical or mental, which can cause a person to become sexually aroused. Sexual arousal usually leads to physiological changes in the aroused person, some of which are pronounced while others are more subtle.

Sexual arousal may not lead to an 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 may be pursued for its own sake, even in the absence of an orgasm.

Human sexual arousal

Depending on a situation, a person can be sexually aroused by a variety of factors, both physical or mental. A person may be sexually aroused by another person or by particular aspects of that person, or by a non-human object. A 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 to sexual arousal vary from person to person, and from one time to another, as does the level of arousal. Stimuli are generally 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 above-stated three. Many people may find nudity, erotica or pornography sexually arousing.^[1]

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. A person can take an active part in a sexual activity without sexual arousal.

Unlike many other animals, human beings of both sexes are potentially capable of sexual arousal throughout the year; therefore, there is no human mating season.

Female sexual arousal

Sexual arousal in women may be caused by erotic thoughts and memories as a result of erotic stimuli including conversation, reading, films or images. These may lead to a woman being receptive to physical contact, including kissing, cuddling, and petting of the erogenous zones; which may in turn make her more open to direct stimulation of her primary or secondary sexual areas (for example, the breasts, nipples, buttocks and/or genitals).

Sexual arousal of a woman is usually accompanied by physiological changes in her body, including vaginal lubrication (wetness), swelling and engorgement of the external genitals, and enlargement of the vagina.^[2] 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.^[3]

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.^[2] Other changes include an increase in heart rate as well as in blood pressure, feeling hot and flushed and perhaps experiencing tremors.^[4] A sex flush may extend over the chest and upper body.

If sexual stimulation continues, then sexual arousal may peak into orgasm. Women's orgasms have been estimated to last, on average, approximately 20 seconds,^[5] and to consist of a series of muscular contractions in the pelvic area that includes the vagina, the uterus and the anus. For some women, on some occasions, these contractions begin soon after the woman reports that the orgasm has started and continue at intervals of about one second with initially increasing, and then reducing, intensity. In some instances, the series of regular contractions is followed by a few additional contractions or shudders at irregular intervals. In other cases, the woman reports having an orgasm, but no pelvic contractions are measured at all.^[6]

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.^[7] 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 the 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.^[8]

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.^[8]

Male sexual arousal

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.^[9] 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.^[10] Equally, a male erection can occur during sleep (nocturnal penile tumescence) or at any time simply due to a full bladder. A younger man can experience enough sexual arousal to create an erection from a passing thought or just the sight of a passer by. Once erect, his penis may gain enough stimulation from contact with the inside of his clothing to maintain and encourage it for some time.^[11]

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 develops around them and the perineum. With further sexual stimulation, the heart rate increases, blood pressure rises and breathing becomes quicker.^[9] The increase in blood flow in the genital and other regions may lead to a sex flush sometimes, in some men.^[12]

As sexual stimulation continues, the muscles of the pelvic floor, the vas 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 a mild discomfort in the testes that is sometimes erroneously called 'blue balls'.

After orgasm and ejaculation, men, like women, 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.^[13] 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.^[9]

Sexual arousal disorders

Some people do not feel aroused on every occasion that they are exposed to erotic stimuli. In some cases they may be experiencing a sexual arousal disorder. This may be characterized as a lack or absence of sexual fantasies and desire for sexual activity in a situation that would normally produce arousal. There are many reasons why a person fails to be aroused, including a mental disorder, such as depression, drug use, or some other medical condition, or some form of malnourishment in the person's diet. 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.

Physiological changes

Sexual arousal causes different physical changes, most significantly in the sex 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 usually leads to vaginal lubrication in anticipation of sexual intercourse.

In females:

In males:

Female sexual arousal. In the left image female genitalia are in normal state. In the right image the female is sexually aroused, the vulva is wet and the labia are slightly engorged. Erection of nipples Vaginal lubrication Vasocongestion of the vaginal walls Tumescence and erection of the clitoris and labia Elevation of the cervix and uterus, and expansion of the back of the vagina Change in shape, color and size of the labia majora and labia minora Pupil dilation

Male sexual arousal. Penile tumescence and erection The veins in the penis may become more prominent Tightening and/or retraction of the foreskin (unless circumcised) often exposing the glans penis Emission of pre-ejaculatory fluid Swelling of the testes Ascension of the testes Tensing and thickening of the scrotum Pupillary response

Models of human sexual response

Human sexual response cycle

During the 1950s and 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.^[14]

Other 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.^[15] 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.^[16] A dramatic example of this is the high rates of secondary sex characteristic dimorphism in some Southeast Asia human populations.^[17]

Singer's model of sexual arousal

Singer^[18] 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.

Duration of sexual intercourse

Intercourse often ends when the man has ejaculated. Thus the woman might not have time to reach orgasm. In addition, many men suffer from premature ejaculation. Conversely, many women require a substantially longer duration of stimulation than men before reaching an orgasm.^[19]

In spite of the common belief that women need more time to get aroused, recent scientific research showed 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.^[20] The time needed for foreplay is very individualistic and varies from one time to the next depending on many circumstances.^[20]

According to a Kinsey study, just under half of men reported a time to ejaculation from intromission of five minutes or less.^{[citation needed]} About a fifth claimed that coitus lasted 10 minutes or longer. Others may have taken over one hour.

A survey^[21] of Canadian and American sex therapists said that the average time for intromission was 7 minutes and that 1 to 2 minutes was too short, 3 to 7 minutes was adequate and 7 to 13 minutes desirable, while 10 to 30 minutes was too long.^[22][23]

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."^[24] 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.