Darwin the Parthian shot

During the preparation of this book it was essential to read widely and to explore a number of fields that impinge directly or indirectly upon the subject of human sexuality and its evolution. These included palaeontology, anthropology, primatology, reproductive biology, genetics, comparative anatomy, sexology, behavioural ecology, and evolutionary psychology. I believe that a better understanding of human sexuality would result from the removal of the barriers which currently tend to divide many of these areas of scientific specialization. Unfortunately, human beings have a strong tendency to form small groups, and to protect their group's interests against those of perceived 'outsiders'. Xenophobia can afflict scientific disciplines, as well as other facets of human culture. In particular, a better dialogue is needed between the comparatively recent and burgeoning field of evolutionary psychology and more traditional disciplines such as anthropology, reproductive physiology, behavioural biology, and sexological research.

Publications within the sphere of evolutionary psychology that deal with sexuality have too often become disconnected from the knowledge base of human behaviour and the physiology of reproduction. This has been detrimental at all levels, including the teaching of university courses, as well as the quality of published research. As examples, here are ten highly questionable findings and views which have been reported and cited during the last two decades:

1. Masturbation, in the human male, serves to optimize sperm numbers, and sperm quality in the ejaculate; this trait evolved because it is advantageous in human sperm competition, and to reduce risks of polyzoospermy (Baker and Bellis 1993a).

2. Female partners of men who are morphologically more symmetrical have more orgasms (Thorn-hill, Gangestad, and Comer 1996). This is adaptive, because their orgasms influence sperm transport.

3. Women's orgasms affect sperm transport and sperm choice in different ways, depending upon preferences for particular males in sperm competition contexts (Baker and Bellis 1993b, 1995).

4. Women exhibit oestrus (Thornhill 2006; Gan-gestad and Thornhill 2008), and nightclub 'lap-dancers', who are in oestrus, are given larger cash rewards by men (Miller et al. 2007).

5. Human semen acts as an anti-anxiolytic in women (Gallup et al. 2002) whether they receive it during vaginal intercourse or oral sex. An additional evolutionary benefit of oral sex may be to reduce risks pre-eclampsia in pregnant women, via chemicals contained in the male's semen (Davis and Gallup 2006).

6. The human penis is morphologically specialized to displace sperm of rival males from the vagina; ergo it is possible that such sperm might then be transferred on the penis, from one woman to another, resulting in fertilization (Gallup and Burch 2006).

7. Men prefer to view pornography which involves interactions between a woman and multiple males; this is because men are more aroused by images indicative of high sperm competition risk (Pound 2002).

8. Partner rape, by men, is more likely to occur if the woman has been unfaithful (or is suspected of being so). This tactic has evolved to reduce possible sperm competition risk (Goetz and Shackelford 2006a; 2006b).

9. Men with larger testes are more likely to engage in extra-pair copulations and sperm competition (Baker 1997).

10. Male genital mutilation (such as circumcision) evolved as a 'signal of sexual obedience'. It reduces the male capacity for extra-pair copulations, by impairing sperm competition. Genital mutilation is 'a hard-to-fake signal of a man's reduced ability to challenge the paternity of older men who are already married' (Wilson 2008).

Such reports, and many others contained in the literature on evolutionary psychology, are deeply troubling in a scientific sense. However, just as it is wise to assess the terrain carefully before embarking upon a course of action, it is also prudent to take care when quitting the field of battle. In ancient times, the Parthian cavalry deceived their enemies by feigning flight, only to turn in their saddles and unleash a salvo of arrows at their pursuers. This tactic was employed with deadly effect by the Parthians during their battles with the Romans. It thus became known as the Parthian shot. The epithet later acquired a literary connotation, denoting the use of a decisive phrase to end a debate. In the current context, it is Darwin who fires a Parthian shot across the centuries, as he cautions against the use of faulty data and flawed scientific reasoning. His views on these matters remain as powerful today as when he wrote them in The Descent of Man and Selection in Relation to Sex:

False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for every one takes a salutary pleasure in proving their falseness: and when this is done, one path towards error is closed and the road to truth is often at the same time opened.

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