1. Sexual and Social Motivation

2. Sex Studies
In the 1960s, gynecologist William Masters and his collaborator Virginia Johnson made headlines by posting research they gathered from studying nearly 700 participants willing to display arousal and orgasm while being studied and recorded. By studying both images and readings from the research, Masters and Johnson were able to determine what they called the sexual response cycle, or the four stages of sexual responding: excitement, plateau, orgasm, and resolution.

4. Sexual Response Cycle
The first phase of the sexual response cycle is the excitement phase. During this phase, men’s and women’s genital areas engorge with blood, the vagina expands and self-lubricates, and her breasts and nipples may enlarge.
The second phase, the plateau phase, excitement peaks as breathing, pulse and blood pressure increase.The penis is fully engorged, and some early fluid - which may include enough semen to cause conception - is released.
The third phase, the orgasm phase, shows a spike in heart rate, and features muscle contractions. A woman’s orgasm positions her uterus to receive sperm released by a man’s orgasm, and also draws the sperm inward.
Finally, the third phase, the resolution phase, has the body systems returning to normal, and both males and females enter a refractory period, where another orgasm will be impossible to achieve.

5. Sexual Dysfunction
Masters and Johnson not only sought to describe the sexual response cycle, but also to understand and treat the inability to complete it. Sexual Dysfunction is a problem that consistently impairs sexual arousal and functioning. For men, this might be erectile dysfunction, which is the inability to achieve or maintain an erection, or premature ejaculation, where orgasm occurs much more quickly than desired.
For women, they may experience pain from sex, or female orgasmic disorder, where they are distressed by infrequent or nonexistent orgasms. A rather large number of women (4 out of 10) reported having some sort of sexual problem.

6. Paraphilia and Sexual Dysfunction
Part of sexual dysfunction is the inability to achieve arousal or sexual functioning. People with paraphilia experience arousal, but often in nonstandard ways. Those with paraphilias are aroused by situations that can be potentially damaging to the self or others, such as exhibitionism, fetishism, or pedophilia. If they are damaging, they are characterized by the American Psychiatric Association as “disordered”.

7. Hormones
Sex hormones have two effects. The first is to direct the development of male and female sex characteristics. The second is to activate sexual behavior, especially in nonhuman animals. In nature, most mammals’ sexual receptivity (being “in heat”) and fertility. This is due to a peak in estrogen, the general term for the female sex hormones, during ovulation. Males do not generally have cycles, and have a constant level of testosterone, the most important of the male sex hormones. Injections of testosterone rarely change sexual behavior in males, although castration, and the removal of the ability to produce testosterone, can decrease sexual desire.

8. Hormone levels
While short term hormonal shifts have little impact on the effect of men and women’s desire, large hormonal shifts over time have greater effects. Once a person enters puberty, their interest in sex and dating greatly increases (in the and 1700s, castratos, boys whose testes were removed pre-puberty, did not experience this). If adult men are castrated, or otherwise lose testosterone through age or medication, the numbers of sexual fantasies and intercourse they experience and desire drop.
Long story short, in humans, having sex hormones (especially testosterone) at all determines if sex is desired; the amount of sex hormones present does not.

9. External Sexual Motivations
Both men and women become aroused when they see, hear, or read erotic material. Like with any stimulus, repeated exposure to sexual material lessens the effect, as people habituate to it. In the 1920s, the hemline of women’s dresses rose to the knee, and this was, for a time, considered slightly erotic. Very few feel this way today.
Too much sexual material can also cause harm. Being exposed to images of women seemingly enjoying rape can give men the wrong impression, and may be more likely to physically assault women. Too much exposure to overly attractive people can devalue the attractiveness of one’s partner.

11. Imagined Stimuli
The brain has often been called our most significant sex organ. Our imagination can influence our sexual arousal and desire. People with spinal cord injuries who no longer have genital sensation can still be sexually aroused. Men and women both can have erotic dreams, and nearly all men, and around half of women, can experience orgasm from a dream (a “wet dream”). Men and women both have sexual fantasies.