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Section 9
Issues in Human Sexuality Research

Question 9 | Test | Table of Contents

Contemporary sexual selection theory predicts that the baseline outcome of social evolution is horny, handsome, healthy warriors paired with discreetly discerning damsels. Deviations from this norm must then be explained away using some special argument. But is the theory that makes this prediction correct to begin with? How many exceptions are needed before sexual selection theory is itself seen as suspect?

The time has come to set the glass on the table: to declare that sexual theory is indeed false and to stop shoe-horning one exception after another into a sexual selection framework. We need to face the fact that sexual selection theory is both inaccurate and inadequate. To do otherwise suggests that sexual selection theory is unfalsifiable, not subject to refutation.

The universal claims of sexual selection theory are inaccurate. Males are not universally passionate, nor females universally coy. The social dynamic between males is not universally combat to control females. Diversity among males and among females does not universally fit a hierarchy of genetic quality. Females do not universally select males for their genetic quality. Moreover, sexual selection theory is inadequate to address the diversity in bodies, behaviors, and life histories that actually exists. Darwin didn't bother to explain the exceptions he recognized, and as data on diversity in gender and sex continue to accumulate, sexual selection theory, which addressed only a subset of the facts to begin with, becomes increasingly inadequate.

Let's record, then, the many ways we've seen in which real species depart from the sexual selection norm:
1. Bodies do not conform to a binary model. Gametic dimorphism doesn't imply a binary of body types. The individuals in many species don't make only eggs or sperm for the duration of their lives. In most species, distinct "male" and "female" bodies are undefined or unstable. Sexual selection theory doesn't apply to many species because distinct male and female individuals as envisioned in the theory simply don't exist in those species, a point Darwin recognized.

2. Genders do not conform to a binary model. Gametic dimorphism does not imply a binary of gender roles either. The two sexes, even if located in separate bodies, may each entail more than two genders, defined as distinct morphologies, behavioral roles, and life histories in sexed bodies. Societies with one, two, and three male genders, together with one or two female genders, have been extensively described. However, sexual selection theory is a two-gender theory.

3. Sex roles are reversible. Even when distinct male and female bodies exist, with one gender per sex, the behavioral roles these genders carry out may be the reverse of what sexual selection theory envisions. Pipefish and jacana sperm are tiny and their eggs large, just as in other metazoan species, yet the overall parental investment by the male exceeds that of the female in these species, resulting in a reversed operational sex ratio leading to female-female competition for males and male choice of females. Neither today's extensions to sexual selection theory nor Darwin's original treatment offer any prediction for when this occurs.

4. Sperm are not cheap. According to well-known primatologist Meredith Small, "Non-human primates show us what many single women in America today already know-sometimes it's very hard to get a date. Female rhesus monkeys and baboons often present to males, a clear sign of preference and choice, but males regularly refuse. Lion-tail macaque females, especially subadults, share this rejection. Females of this species initiate almost 70 percent of the copulations but only 59 percent end up in mounts. No one is sure why these males refuse, inasmuch as sperm is supposed to be so cheap, but males often ignore estrous females." Why should males refuse the invitation to sex when sperm are supposedly so cheap, as sexual selection theory requires? Because sleeping together is meaningful in itself. Animal sex is not anonymous. Mating is a public symbol. Animal "gossip" ensures everyone knows who's sleeping with whom. Therefore, mate choice, including male mate choice, manages and publicizes relationships. A male may not want the commitment that accepting a new girlfriend entails.

5. Females do not choose "great genes." Females choose mates for many reasons, but rarely or never to acquire the great genes that a male is supposed to have according to sexual selection theory. Low-ranking males have offspring just as capable as those of high-ranking males. Females select for males who deliver on their promises of parental care and spread the probability of paternity among males to ensure offspring safety. Physical characteristics in a male serve to endow offspring with the bodily markers of a powerful lineage, not to acquire attractiveness; females are buying their offspring membership in the old genes club.

6. Family size is negotiated. Egg and sperm production are not necessarily independent, as sexual selection theory envisions. Males don't have to run around trying to fertilize a fixed number of eggs. Males and females can negotiate to increase the number of eggs a female produces beyond those she would make if she were to raise them by herself. In addition, males need to make sure the eggs they do fertilize are successfully raised-it doesn't matter how much sperm they produce if the quality of parental care is compromised.

7. Social deceit is not demonstrated. The deceit required by sexual selection theory has never been demonstrated. Despite scientists' invention of many categories of social deceit, such as sexual mimicry and egg mimicry, it has never been proved that the mimetic traits are not simply social symbols. Perhaps animals do lie to each other now and then, but biologists have yet to catch them in a lie, so a presumption of honesty is appropriate.

8. Same-sex sexuality is common. Same-sex sexuality is contrary to sexual selection theory, so the existence of homosexuality must be explained away as either an aberration or a deception. Instead, the extensive documentation of same-sex sexuality among vertebrates rules out any further denial of homosexuality and contradicts sexual selection theory.

9. Mating is not primarily for sperm transfer. The purpose of mating, both heterosexual and homosexual, is more often to create and to maintain relationships than to transfer sperm. Sexual selection theory requires that mating be primarily about sperm transfer, whereas the amount of mating that actually takes place is a hundred to a thousand times more frequent than that needed for conception alone.

10. Secondary sex characteristics are not just for heterosexual mating. Sexual selection theory limits the meaningfulness of secondary sex characteristics to heterosexual mating. In species with common homosexual matings, secondary sex characteristics, including genital geometry, are shaped to facilitate all types of mating, including homosexual matings.

The sheer number of difficulties with sexual selection theory precludes plugging all the leaks. An occasional leak might be fixable, but this many leaks make repair impossible. The theory of sexual selection was taking on water long before evidence was found of widespread homosexuality, but homosexuality is the final torpedo.
- Roughgarden, Joan, Evolution's Rainbow: Diversity, Gender, and Sexuality in Nature and People, University of California Press Ltd : London, 2004

Personal Reflection Exercise #4
The preceding section contained information about difficulties with the current theory of sexual selection. Write three case study examples regarding how you might use the content of this section in your practice.

Peer-Reviewed Journal Article References:
Bender, A., Beller, S., & Klauer, K. C. (2018). Gender congruency from a neutral point of view: The roles of gender classes and conceptual connotations. Journal of Experimental Psychology: Learning, Memory, and Cognition, 44(10), 1580–1608.

Crespi, B., & Dinsdale, N. L. (2021). The sexual selection of endometriosis. Evolutionary Behavioral Sciences. 

Gorelik, G. (2021). Domains of female choice in human evolution. Evolutionary Behavioral Sciences. 

Lehmiller, J. J. (2017). Controversial issues in human sexuality research: The state of the science [Editorial]. European Psychologist, 22(1), 1–4.

What are three "holes" in the universal sexual selection theory? To select and enter your answer go to Test.

Section 10
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