Evolutionary Psychology Insights Regarding Human Sexuality

Russell Eisenman, Ph.D.
University of Texas-Pan American
Department of Psychology
Edinburg, TX 78541-2999
E-mail: eisenman@panam.edu

Evolutionary psychology has presented important insights about many areas, with human sexuality being one of the areas of important insights. Some contributions of evolutionary psychology to six areas are discussed here: Sexual Intercourse vs. Masturbation, Preference by Aging Males for Younger and Younger Females, Ovulation and Female Sexuality, Differences in Male and Female Jealousy, Waist to Hip Ratio, and Childhood Deprivation and Earlier Pregnancy. Also, the importance of realizing that many behaviors are unconsciously motivated is discussed.
Keywords: evolutionary psychology, human sexuality, males, females, masturbation, jealousy, pregnancy.

Evolutionary psychology has been especially helpful in the area of human sexuality, where our understanding lags behind the importance of an area that affects us all. Not only would the human race not survive but for heterosexual attraction and heterosexual intercourse, but also, there are many important aspects of human sexuality, including both biology and male-female relationships, that are poorly understood. There are six areas that I will discuss, in which evolutionary psychology provides extremely important insights, and in which other fields seem unable to explain well what is occurring. Other fields have some insights, but nothing like that provided by evolutionary psychology. The six areas that seem especially important to me and in which evolutionary psychology provides excellent insights are the preference of both males and females for sexual intercourse over masturbation; the preference for males, as they age, for younger female partners; female sexual desire during ovulation; differences in male vs. female jealousy; waist-to-hip ratio; and, childhood deprivation leading to earlier pregnancy. I have discussed some of these briefly before (Eisenman, 2001a) but will discuss them here in greater detail, and add some new studies.

Sexual Intercourse vs. Masturbation
At first glace, it might seem obvious that people prefer sexual intercourse to masturbation. For one thing, our culture often puts down masturbation, and makes it seem like some undesirable thing only engaged in by people who are disturbed, or who are deficient because they cannot obtain a partner. However, the reality is much more complex. Famous sex researchers Masters and Johnson (1966) found that masturbation provides more physiological intensity (presumably enjoyable intensity) for both males and females than does sexual intercourse. So, even with cultural taboos, why would people not prefer masturbation to intercourse? Even when things are tabooed, people usually figure out what causes pleasure, and seek it out.
Evolutionary psychology provides the answer. While there could be other answers from other fields, I have never seen any that explain well the preference for intercourse over masturbation. The evolutionary psychology answer is that sexual intercourse can send our genes into future generations, while masturbation cannot. Since a major desire of humans, according to evolutionary psychology, is to transmit our genes into future generations, we will engage in behavior that promises to do so. Even behaviors that are unlikely to do so, such as using birth control during intercourse, contain the sexual intercourse behavior that does send out genes into future generations. So, sexual intercourse is a powerful, natural desire of humans and animals, regardless of whether or not birth control is also employed.

Preference by Aging Males for Younger and Younger Females
The second area, preference by aging males for younger and younger females, also invokes the gene-spreading explanation. Without this explanation, there is no way to understand why males, the older they get, prefer younger and younger female sex partners. Much of this may be reserved for sexual fantasy, but as Glenn Wilson (G. Wilson, 1992) points out, in sexual fantasy we often see what the person really desires. In real life, it is usually only the very wealthy or powerful who are actually able to live out their sexual fantasies.
As the man ages, he fears that his genes will not be as good. Just as men in general desire youth, attractiveness, and health in their female partners, so that their genes will be good ones and will be spread into future generations, so too the aging man seeks this. But, he seeks it even more so than usual, as he fears, perhaps unconsciously, that his genes are not as good now as they once were. To make up for this, his partner must have excellent genetic potential. The way to achieve this is to balance his aging with her youth.
Years ago, a tennis partner of mine, about 60 years old or so, told me “ I am attracted to young girls. I mean, really young girls. I could get in trouble from this attraction.” He seemed perplexed by his attraction to young females. So was I. Through all these years I could never figure out why this would be true for him, and for other men I have discussed this with. Now, evolutionary psychology has come along and provided a theoretical structure of male and female desires, which explains why aging men would desire young, female partners.

Ovulation and Female Sexuality
The third area is female sexual desire during ovulation. As I got interested in this area, I started looking at introductory psychology textbooks to see what they say about ovulation, since it plays such an important part in female sexuality. However, I found that most of the ones I examined do not even mention ovulation. If it is covered at all, it is just briefly mentioned as part of how conception occurs. But, ovulation has powerful effects on female sexual behavior. For example, I have found that women often “forget” to use birth control during ovulation (Eisenman, 2003). I believe this is an unconscious motivation to get pregnant, and thus continue the human race, and to send her genes into future generations. Grammer (1996) found that the closer women in bars were to ovulation, the skimpier the clothing they wore. This would fit again with my explanation about unconscious desire to become pregnant and spread genes to future generations. It is of interest to note that the skimpy dress is a sexual signal sent to men, that indicates the woman’s receptiveness to sexual intercourse. She may not even realize she is sending this signal, so, again, it is an unconscious signal. I think the concept of the unconscious is important, but many in evolutionary psychology and other fields avoid it, perhaps because of Freud’s usage in his psychoanalytic theory, which they reject. However, if one does not like the word “unconscious” one could use “unaware” or some other term which indicates that the person does not know why they do something.
Human females can be receptive to sexual intercourse at all phases of their menstrual cycle, but desire increases some at ovulation (Alcock, 1989). Further, when women have sexual affairs with someone other than the husband or boyfriend, the affair often occurs during ovulation, the woman and her partner typically use no birth control, and the partner chosen by the woman has some quality that the husband/boyfriend lacks (Baker & Bellis, 1993; Bellis & Baker, 1990). For example, if the husband is attractive but not intelligent, the woman is likely to have an affair with an intelligent man. Not using birth control increases the likelihood of pregnancy, and thus increases the quality of her gene pool, since she now combines her genes with that of an intelligent man, whereas previously she combined her genes with that of an attractive man. Thus, ovulation would seem to have powerful effects on the woman and on her sexual behavior. It needs to be given much more consideration that it has typically received. And, it is evolutionary psychology findings that make us appreciate the importance of ovulation, beyond simply being an important part of conception.

Differences in Male and Female Jealousy
For many years, until getting into evolutionary psychology, I have been perplexed by something. On almost any measure of sexual behavior or fantasy, the male seems more interested in sex than the female. Males engage in more different kinds of sexual behaviors than women, are more receptive to sex with someone they do not know, have more varied sexual fantasies than women, etc (G. Wilson, 1992). Men seem much more permissive, much more liberal, in sex than do women. Yet, why is it, then, that women are often much more accepting of their mate having a sexual affair? I have observed this repeatedly, both in people I know, and in famous people. The woman will tolerate her boyfriend or husband having sex with others, but the male will often seem to “go crazy” when his mate does that, and may engage in violence. It does not seem to fit the pattern of the more accepting male.
Here is the insight from evolutionary psychology, which explains this otherwise unexplainable phenomenon. David Buss (1989,1999) studied human sexuality in 37 different cultures. He found stable male vs. female sex differences across the 37 cultures. Males wanted partners with youth, attractiveness and health, while females wanted partners with money, status and power. Men want many partners, while women are more interested in a partner who will care for her and her children. Each sex is seeking the kind of partner who will best help them spread their genes into future generations. When it comes to jealousy, there are also sex differences. Men are threatened by any sexual intercourse their partner engages in with others, since this could result in his raising a child who will not spread his genes into future generations. He might not know this is not his offspring, and thus invests resources into a child whose genes are not his, and the future generations gene payoff for the man (with regard to this child) is zero. For the woman, the threat is when the man has emotional investment in the other partner. Sexual intercourse per se is not that big a threat. The real threat is that he will stop caring for the wife (or current girlfriend) and start caring for the new partner(s). So, jealousy often springs from different sources in men and women.

Waist to Hip Ratio
Just as Buss (1989) found that his evolutionary psychology findings on sex differences applied in different cultures, so too does the work of Singh on wait-to-hip ratio hold up in different cultures (Singh, 1993a, 1993b, 1995; Singh & Luis, 1995). Singh has shown that there is a preference among men for a certain waist-to-hip ratio in females. A 70% waist-to-hip ratio indicates, apparently, health and fertility in the woman, and is the male ideal. This holds up across different cultures, suggesting, like the Buss (1989) sex differences in mate preferences, that it is a universal reality. Singh has pointed out that even women who look very different may have similar wait-to-hip ratios. For example, he says that the famous actress Marilyn Monroe and the current skinny model Kate Moss both have the ideal waist-to-hip ratio. Even though they look quite different, their having the ideal waist-to-hip ratio would help explain their appeal, and why they have become stars when other attractive women have not. There is also a female preferred waist-to-hip ratio for men, of 80-95% (Singh, 1995).

Research by Wedekind and Furi (1997) indicated that smell may be a major part of female attraction toward men. Smelling t-shirts, women tended to prefer the odor that, unknown to them, showed that the male wearer possessed the MHC gene opposite from them. If they had a child with such a man it would give them a child with immune system advantages, as the child could have the different genetic MHC qualities from both parents. The women participants often remarked that the smell was like that of their husband/boyfriend. So, they had apparently chosen their boyfriend/husband on the basis of a smell which indicated an opposite MHC gene, and thus immune system advantage for their children, who would inherit different protective genes—similar and dissimilar MHC–from the mother and father.

Childhood Deprivation and Earlier Pregnancy
Evolutionary psychology provides insights into how childhood deprivation leads to earlier menarche (the age of first menstrual period) and thus—in many cases—earlier pregnancy. The findings are summarized by Palmer and Palmer (2002). At first glance, one would think the variables would have nothing to do with one another. Why should the age when a girl begins her menstrual cycle and becomes able to become pregnant have anything to do with how she is treated at home? It seems a quantitative strategy is invoked when a child is treated badly, either by abuse, having a less than loving parent, or even having a stepfather (who might be considered more inclined to mistreat or sexually abuse the female child than would a biological father). A quantitative strategy is used when the person will have many children, to make up for the fact that many will face tough odds and not survive. If things were better, a qualitative strategy could be employed, having few children and investing much time and resources in them. But, if resources are not good—e. g., if there is an abusive parent—somehow the female child comes to menarche earlier and is inclined to produce many children.
Many have heard of young female children in poverty households having repeated pregnancies. It all seemed explainable by learning. But, the evolutionary psychology theory here says it is not all due to learning, but is a strategy that seeks to ensure survival of children, viz., the offspring of the young girl who becomes pregnant. In turn, this ensures survival of her genes into future generations. Of course, the young girl who becomes pregnant does not realize she is employing a quantitative strategy, etc. But, her body changes to allow her to become pregnant earlier than would otherwise be the case if everything in her family were all right (Belsky, Steinberg, & Draper, 1991).

Some Data
Some recent data from a study by Dantzker and Eisenman (2005) show sex differences between males and females at a Hispanic-serving university in the United States, where the enrollment is about 88% Mexican-American. For a related study, see Eisenman and Dantzker (2006).
From three college courses a total of 126 surveys were distributed and returned. There were 56 males and 70 females in the sample. For ethnicity, 114 identified themselves as Hispanic, 8 as non-Hispanic, and 4 did not answer this question. Thus, the sample was overwhelmingly Hispanic. The 126 surveys were coded, entered into an SPSS database, and analyzed, using nonparametric Chi Square to assess the gender differences.

To examine the relationship by gender for each statement, a nonparametric Chi Square was used, degrees of freedom=4 (df=rows-1 x columns-1), using SPSS. A 2×5 Chi Square was employed (2 genders: male or female x the 5 items on the 1-to-5 scale). Comparing the mean scores for each statement by gender, a statistically significant difference for 19 of the 38 statements was found.
Among the 19 statistically significant differences of the means by gender, 8 found males to be more agreeable to the statement than females. These statements included the idea that:

  • Premarital sex is acceptable for males
  • Oral sex before marriage is acceptable
  • Males should have sexual experience prior to marriage
  • Forcing a person to have sex if spouse is OK
  • Marriage should not stop person from having sex with other people
  • "Topless clubs" are acceptable places for adult entertainment
  • Condoms interfere with the pleasures of sex
  • Prostitution should be legalized in all states>
Females agreed with 11 statements more than males, to a statistically significant extent. These ideas included:
  • When growing up, parents told me premarital intercourse is unacceptable
  • When growing up, parents told me any type of sexual behavior before marriage is unacceptable
  • Sex should only occur with a person you love
  • Lust and love are two different emotions
  • If I were to have sex, I would always practice safe sex
  • Clubs that promote all nude dancing should not be allowed to exist
  • Magazines such as Playboy are demeaning to women
  • Magazines like Playboy are pornographic and should not be published
  • Pornographic material causes males to become sexually aggressive
  • Masturbation is wrong
  • I would be jealous if my partner had sexual intercourse with someone else
These results, in general, are consistent with evolutionary psychology theory regarding sex differences between males and females.

Evolutionary psychology has provided a radical new theory, which has had a tremendous effect on psychology and other fields (Barkow, Cosmides, & Tooby, 1992; Buss, 1999; Palmer & Palmer, 2002). The theory helps bring back a proper focus on such neglected areas as biology, genetics, and the brain. Too often, psychology and other fields overemphasized learning as the sole or primary explanation. The theory is especially powerful in the area of human sexuality, where it explains a great deal that other theories cannot explain, or explain in a less satisfactory manner. That is why, in part, the theory has been so persuasive to so many.

The Unconscious
However, evolutionary psychologists have underemphasized something very important. In my view, many or most of the findings refer to things that are unconscious: the person does not consciously know it. For example, if you were to ask people why they did something, they would not necessarily say “To have children” or “To spread my genes into future generations” even though these may be the underlying motivations. Thus, we need to use the concept of “the unconscious” more often, since much of what the theory deals with is not part of conscious awareness.

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