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 Sexual Motivation & Behavior

A. The Human Sexual Response Cycle

[Masters & Johnson][Human Sexual
                      Response]Masters and Johnson. Up to 1966, we knew very little about what happened to the human body as it engaged in sexual behavior. That was the year that William Masters and Virginia Johson (see photo to the left) published their groundbreaking study, Human Sexual Response. Using a variety of recording and observation techniques with volunteers, Masters & Johnson for the first time detailed the very many changes that go on in the male and female body during the course of sexual intimacy. They proposed a four-stage theory of the "human sexual response cycle" and detailed characteristic physiological changes associated with each stage. These stages in the cycle include (1) excitement, (2) plateau, (3) orgasm, and (4) resolution. Four years later, in 1970, they followed their first book by a second, Human Sexual Inadequacy. In this volume they describe what goes wrong with the normal course of sexual activity in adults and detail treatment approaches in response to these forms of sexual disorder.

[Kaplan's Sexual Desire Disorders][HSK]Helen Singer Kaplan. An alternative to the Masters and Johson model was proposed by the psychologist & psychiatrist, Dr. Helen Singer Kaplan (1929-1995), in her 1979 book, The Disorders of Sexual Desire. She employs a three-stage psychophysical model which includes (1) desire, (2) excitement, and (3) orgasm. Note that Kaplan's first stage, desire, is more fundamentally a psychological motivational state rather than a physical one. Because of its conceptual utility for understanding sexuality more broadly, psychiatry has accepted Kaplan's work and used it (along with Masters and Johnson) in characterizing and classifying the sexual disorders.

Many psychologists, psychiatrists, and sex therapists approach the human response cycle by (1) grouping together the Excitement and Plateau phases of the human sexual response cycle into a unified "arousal" phase and (2) placing Kaplan's Desire phase at the beginning of the cycle as an "appetitive" phase preliminary to sexual activity itself.

Physiological Changes during the Phases of the Human Sexual Response Cycle as uncovered by the work of Masters & Johnson. These are detailed here simply to illustrate that there are clear physiological changes in the body that are found in different stages of the experience of sex.

Excitement & Plateau

  • Vasocongestion (constriction of blood vessels) in sexual organs.
    • In males, erection of the penis while the testes elevate and become engorged with blood.
    • In females, the clitoris becomes larger, the labia majora separate, and the labia minora enlarge (and darken in color)
  • Mytonia (increased muscular tension)
  • Increased heart rate
  • Increased blood pressure
  • Sex flush (on chest and breasts)

Plateau => intensification of Excitement phase. In women the outer 1/3rd of the vagina becomes particularly engorged with blood


Rhythmic muscular contractions of the sexual organs accompanied by experience of strong pleasure

In males, ejaculation of semen accompanies contractions.

Studies of both males and females show nearly identical descriptions of the feelings & experience of orgasm.
In the absence of further sexual stimulation, the body returns to normal. The pace at which this happens is different for different people.

[Human Sexual Response
                  Cycle: Male] [Human Sexual Response
                  Cycle: Female]



Masters & Johnson reported three normal variations among women in the sexual response cycle. (see diagram above)

A. Female experiences orgasm and enters the period of resolution.

B. Female experiences intense levels of sexual stimulation (equivalent to the Plateau stage) without orgasm

C. Female experiences multiple orgasms without entering into a refractory/resolution period.

B. Evolutionary Analyses of Human Sexual Motivation

 Robert Trivers' Parental Investment Theory

Gender Differences: Are there such differences in what people actual do?

Sexual Activity

M(ales) tend to be > F(emales) in respect to

  • Interest in sex
  • Thinking about sex
  • Initiating sexual contact
  • Desire a greater number of partners
  • Have a greater number of partners

David BussMate Preferences

  • Evolutionary Theory: M prefer F with youthfulness (indicator of reproductive potential) & attractiveness (indicator of health)
  • Evolutionary Theory: F prefer M with higher social status, intelligence, education, income & financial resources (all indicators of ability to provide and protect)
Research: By psychologist David Buss (1989) -- survey of 10,000 people from 37 cultures worldwide -- F value social status, ambition, and financial prospects of M while M value youth & physical attractiveness in F

Relationship Jealousy

  •  Evolutionary Theory: Males concern with paternity uncertainty: are they actually the father of the child? Thus, partner infidelity would be particularly threatening to a male.
Research: David Buss (1992) -- US study -- M showed greater distress at sexual infidelty than F while F showed greater distress at emotional infidelity than M.

Critiques of Evolutionary Theory

  • Ignores the impact of cultural and economic forces worldwide which place F at material disadvantage
  • Males are strongly formed by culture to live out gender-role stereotypical behaviors

C. The Mystery of Sexual Orientation

                    Gay FiguresWhat causes homosexuality? This question should be asked with the correlative one: What causes heterosexuality? In other words, before asking the origins of homosexuality, scientists are challenged to explain sexual attraction and orientation more generally.

Sexual Orientation: a preference for emotional and sexual relationships with partners of the same (homosexual), other (heterosexual), or either sex (bisexual).

[Kinsey]Kinsey Sexual Orientation
                    ScaleData suggest that the heterosexual-homosexual distinction is not an either/or proposition, but represents something of a continuum. The groundbreaking study of male sexuality by Alfred Kinsey (on right) in 1948, while not representative of the general population, proposed that sexual orientation be rated on a 7-point scale from exclusively heterosexual (rating of 0) to exclusively homosexual (rating of 6).

For example, Savin-Ritch and his colleagues (2013) studied the sexual arousal patterns of adult males to sexual images of males and females in terms of male genital arousal and pupil dilation. Their data suggested a distinctive continuum of responses consistent with Kinsey's original conception.

Genital arousal to visual stimuli
Male pupil dilation to visual stimuli

LGBT Population of US by States in 2012
                      estimatePrevalence of homosexuality
. How many people in the United States are homosexual? This is a hard question to answer for at least two reasons: (1) definitional difficulties (how do we define homosexuality in light of what was just said about the sexual orientation continuum?) and (2) data collection difficulties (many people are reluctant to respond to sexual orientation questions honestly because of factors such as fear, shame, etc.) However, a variety of surveys and other data from the late 1980s to the 2000s (as well as more recent reports summarized in Bailey et al, 2016) suggest that

  • about 1 to 3% of the adult male US population is now mostly or exclusively homosexual
  • another 1 or 2% of the adult male US population may have strong or complete homosexual arousal patterns but for various reasons do not publicly identify as gay or homosexual (e.g., they may be in a heterosexual marriage, still be closeted, etc.)
  • the percentage of women who are exclusively homosexuals (lesbian) is probably lower than the percentage of males, i.e., 1-2% of women are lesbian.
  • However, a higher percentage of women than men report bisexual feelings, particularly those in close personal relationships.
  • The geographical distribution of adult homosexuals in the United States is quite varied by state (see map). The New England and West Coast states have a far higher homosexual population than the Midwest and Northern Mountain states.
Sexuality prevelances in West

(See further data at the Rainbow site of psychology professor, Gregory Herek, UC Davis or the American Psychological Association's site on Questions about Sexual Orientation. Note that in recent health research studies, investigators are beginning to use the term "men who have sex with men" or MSM rather than the terms "homosexual" or "bisexual" e.g., see Purcell et al., 2012)

1. Environmental Factors associated with Homosexual Orientation

  • No or minimal scientific evidence to support older environmental theories, i.e.,
    • Freudian (psychoanalytic): boy raised by weak, ineffectual father and domineering, overprotective mother
    • Behavioral: child develops preference for same-sex stimulation because of learning from experiences such as male adult seduction
  • Extreme stereotypical "feminine" behavior in younger boys is predictive of later gay orientation (for 75-90% of such boys). However, "feminine" behavioral characteristics are seen in only 15 to 20% of male adult homosexuals (conversely, the behavior of roughly 80 to 85% of gay males is generally indistinguishable from that of most "straight" males). Hence, most adult male homosexuals did not display "feminine" behaviors as children.
  • Most male and female homosexuals report feeling different at an early age, i.e., look back as adults and see in their early childhood feelings and leanings toward same-sex friends before sexuality itself became a conscious issue.

2. Biological Factors associated with Homosexual Orientation

  • Hormonal Difference Theory: NO evidence to support differences in sexual hormone levels in adult homosexuals
  • Genetic Similarity or Relatedness Studies
    • Monozygotic twins are more likely to be concordant for homosexuality (ca. 50% concordance rate) than dizygotic (fraternal) twins (ca. 20%) or adoptive brothers (ca. 11%). This pattern is seen among lesbians as well.
  • Anatomical Difference Studies
    • In the early 1990s, Simon LeVay found differences in a small area of the hypothalamus in the brains of 19 homosexuals and 16 heterosexual men following autopsy. Such differences may be explained by differences in hormonal secretions during critical periods of gestation (pregnancy) when the brain of the fetus is being "wired" for sexual orientation.
    • LeVay's findings have been partially replicated by other researchers more recently.

Bottom Line: Most scientific researchers believe that homosexual and heterosexual orientations are the result of interacting genetic and environmental processes which take place early in the development of the child (particularly during gestation). Thus, there is almost certainly no "gay gene" and there is probably no "gay environment" which causes homosexual orientation.

The latest comprehensive review of non-heterosexuality is Bailey et al (2016) listed below.


Bailey, J. M., Vasey, P. L., Diamond, L. M., Breedlove, S. M., Vilain, E., & Epprecht, M. (2016). Sexual orientation, controversy, and science. Psychological Science in the Public Interest, 17(2), 45-101.

Purcell, D. W., Johnson, C. H., et al. (2012). Estimating the population size of men who have sex with men in the United States to obtain HIV and syphilis rates. Open AIDS Journal, 6, 98-107. doi: 10.2174/1874613601206010098

Savin-Williams, R. C., Rieger, G., & Rosenthal, A. M. (2013) Physiological evidence for a mostly heterosexual orientation among men. Archives of Sexual Behavior, 42, 697-699. doi: 10.1007/s10508-013-0093-1


This page was originally posted on 10/16/03 and last updated on October 19, 2016