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When people talk about sex in today’s digital environment, some of the most prominent topics are the prevalence, ease of accessibility, and variety of online pornography. Our focus in the current chapter is on the identity of online pornography consumers and the actual content that is being consumed. We specifically explore sex differences in online pornography consumption using the lens of evolutionary psychology. This perspective allows us to examine how sex-specific challenges over human evolutionary history have shaped general tendencies in mate preferences. Here, we discuss pornography consumption not only with regards to common sexual interests within and across men and women, but also with regards to individual differences in terms of the diversity of explicit images, video, and literature that is available online.
The discussion of evolutionary theory and feminist ideology has existed for decades and has been obstructed by assumptions, generalizations, misunderstandings, and omissions from both points of view. Terminology, in particular, has had important consequences for comprehension. We note, like Barkow (2006), that there is no term that accurately captures the variety of work of those applying Darwinian theory to the study of human behavior. We apply “evolutionary psychology” here as it is a prevalent term that is used currently, but it also reinforces the goal of focusing on human nature as an outcome of biological evolution (Barkow, 2006). To provide as much clarity and simplicity as possible in this chapter, we will refer to evolutionary scientists as “evolutionists” and feminist scholars as “feminists” as they were in Hrdy (1981/1999).
Our interest in the psychological properties of semen arose as a byproduct of an initial interest in menstrual synchrony. In reviewing that literature we discovered several articles (Trevathan, Burleson, & Gregory, 1993; Weller & Weller, 1998) reporting that lesbians who live together fail to show menstrual synchrony. Since the evidence suggests that menstrual synchrony is mediated by the exchange of subtle olfactory cues among cohabitating women (Preti et al., 1986, Stern & McClintock, 1998) this struck us as peculiar, because lesbians would be expected to be in closer, more intimate contact with one another on a daily basis than other females who live together. What is it about heterosexual females that promotes menstrual synchrony, or conversely what is it about lesbians that prevents menstrual synchrony? It occurred to us that one feature that distinguishes heterosexual women from lesbians is the presence or absence of semen in the female reproductive tract. Lesbians have semen-free sex.
Human semen is a very complicated mixture of many different ingredients. If you extract the sperm from semen, what is left is called seminal plasma. We speculated that there may be chemicals in seminal plasma that, through vaginal absorption, affect female biology and triggers the release of pheromones that function to entrain menstrual cycles among cohabitating women. Some of the components in semen pass through vaginal epithelial tissue, and within an hour or two after intercourse heightened levels of certain seminal chemicals can be detected in the female bloodstream (Benziger & Edelson, 1983).
Due to the asymmetric risk of cuckoldry, assurance of paternity is much less significant than maternal assurance. Females, possessing 100% assurance of their link to their offspring, necessarily have evolved strategies to secure the less sure males as supportive and protective fathers. Hofferth and Anderson (2003) compared all types of family and the paternal investment inherent in each. The least-investing type of father was the stepfather. Daly and Wilson (1982) recorded spontaneous remarks in maternity wards regarding the appearance of newborn children. Mothers and their friends and relatives were more likely to comment on how children resembled their fathers than they were to say the child resembled the mother or any other family member. When fathers displayed any doubt, the mothers were quick to reassure them of the child's resemblance. Regalski and Gaulin (1993) have replicated these findings using Mexican families. These researchers concluded that women and their families attempt to reassure the male of this paternity, thus increasing the likelihood that he will invest in the child.
It is obvious that convincing a male of paternity and securing his investment would almost always be in the best evolutionary interests of females. However, this is hardly in the best interests of males. If ascriptions of resemblance were completely persuasive throughout evolutionary history, males would have been deceived numerous times into investing in a child that was not genetically related to them. Indeed, the incidence of cuckoldry ranges from 5 to 30% (see Baker & Bellis  for review).
Among sexually reproducing species, the penis evolved as an internal fertilization device. But across different species, penises exist in a bewildering array of shapes and sizes (see Eberhard, 1985). Among primates, the human penis is distinctive by virtue of both its size and its enlarged glans and protruding coronal ridge (see Gallup & Burch, 2004). There has been some speculation that the human penis evolved not only as an internal fertilization device, but also as a mechanism for displacing semen left by rival males in the female reproductive tract (e.g. Baker & Bellis, 1995).
In a series of studies designed to simulate sexual intercourse under laboratory conditions using artificial genitals, we found that when latex vaginas contained simulated semen, phalluses that approximated the configuration of the human penis displaced 80% or more of the semen by drawing it away from the cervical end of the vagina (Gallup et al., 2003). Through a series of experimental manipulations, we determined that the coronal ridge may be an important feature of the penis in mediating semen displacement. Thus, as a mechanical means of affecting sperm competition, the human penis may enable successive males to displace foreign semen from the female reproductive tract and substitute their semen for those of their rivals.
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