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 Sexual Motivation & Behavior

Note that there is an upper division course, PSY 360 Human Sexuality, that covers this material in far greater depth.

A. The Human Sexual Response Cycle

              & 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.

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 & 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

   • Consider the danger of pregnancy faced by women historically

  Maternal Mortality England Wales

   • What are the differences between the causes of maternal death in the 19th & 20th centuries?
       Note: ("puerperal" means "around the time of birth")
  Causes of Maternal Death

   • Consider the danger of pregnancy faced by women in low-income nations today

Materal Mortality 2015
 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

David BussMate Preferences in Evolutionary Theory

Relationship Jealousy in Evolutionary Theory

Critiques of Evolutionary Theory

   C. The Mystery of Sexual Orientation

Note: The material below reviews research and concepts related to sexual orientation, not gender identity. The issue of gender identity is clearly an increasingly discussed issue in the United States, especially among people of the current college-age generation, but it is one which I will not be looking at.

Famous 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, [which was 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). He challenged sexual orientation as binary (only two

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.

          arousal to visual stimuli
          pupil dilation to visual stimuli

              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

          prevelances in West

(See further data at the Rainbow site of psychology professor, Gregory Herek, UC Davis.  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 or Early Life Factors associated with Homosexual Orientation

2. Biological Factors associated with Homosexual Orientation

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, 57 pages) 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.

Blanchard, R. (2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. Hormones and Behavior, 40, 105–114.

Eastwick, P. W., Eagly, A. H., Glick, P., Johannesen-Schmidt, M. C., ... Volpato, C. (2006). Is traditional gender ideology associated with sex-typed mate preferences? A test in nine nations. Sex Roles, 54: 603-614. doi: 10.1007/s11199-006-9027-x

Loudon, I. (2000). Maternal mortality in the past and its relevance to developing countries today. American Journal of Clinical Nutrition, 72(suppl.), 241S-246S

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

Zenter, M. (2017, Dec. 21). Men want beauty, women want wealth, and other unscientific tosh. AEON Ideas. https://aeon.co/ideas/men-want-beauty-women-want-wealth-and-other-unscientific-tosh

Zentner, M. & Eagly, A. E. (2015) A sociocultural framework for understanding partner preferences of women and men: Integration of concepts and evidence. European Review of Social Psychology, 26(1), 328-373, doi: 10.1080/10463283.2015.1111599


This page was originally posted on 10/16/03 and last updated on October15, 2021