It was recently suggested to me that a possible function for female masturbation might be to loosen the cervical mucus. This prompted my asking why the cervical mucus would need loosening in the first place. The response was that cervical mucus can serve as protection against infectious agents and, further, naturally tends to thin around the time when conception is most likely. So there’s a trade-off there between protecting against infection and fertilization, and natural selection already seems to have done the math, so to speak, weighing the relative costs and benefits of thinning versus thickening. The account of female masturbation serving to thin the mucus, then, seems to be less plausible. It would only serve to unbalance the equation between risk of infection and fertilization, and there’s no guarantee that it unbalances it in a productive way. The initial account of female masturbation failed to fully consider whether the “problem” it was trying to solve was actually a problem at all.

The same goes for sexual desire. There is an urge to have sex, and sex typically leds to reproduction. On that front, we’re definitely in agreement. Now, presumably, that pressure/desire/urge to have sex is the result of selective forces: there are probably trade-offs between being focused on sex all the time versus taking care of other tasks, like eating, cultivating social status, and so on. This suggests that selection has already been trying to balance these competing pressures via the creation of cognitive mechanisms that regulate them. Individuals that spent all their time pursuing sex and individuals that spent none of their time pursuing sex probably didn’t do so well, on the whole, relative to those who could better balance these competing demands. Now if masturbation dampens this sexual pressure, it raises two questions: (1) does it do so in an adaptive way and (2) if it does, why hasn’t selection already settled on that calculation itself?

Is that making more sense?

“Balancing sexual pressure” might sound plausible until one wonders why such sexual pressure exists in the first place.

Uhm… to reproduce?

An urge to create offspring doesn’t need to be rational in order to be effective from an evolutionary standpoint.

Or maybe I misunderstood your point?

And women suppressing their own sexuality does make sense from an evolutionary standpoint as long as it serves to increase the level of commitment from the man in exchange for sexual favors. It works if the male sexual urge is strong enough, which it appears to be.

And I have seen the sexually-antagonistic idea. I reviewed a few times in the past. One of the reasons that explanation isn’t good is that it misses the entire developmental story. More goes into determining sexual orientation than just genes, as with any trait. Further, sexually antagonistic selection would need to explain why homosexuality – the orientation – only really seems to exist in humans and rams, as the story could apply equally as well to any sexually reproducing species. Curiously, it doesn’t seem to.

On this point, I would like to note that homosexual behavior isn’t what poses an evolutionary mystery anymore than other, likely nonadaptive behaviors, such as masturbation.

Hmm… but is masturbation really nonadaptive? For sure, it doesn’t lead to babies, so genetically, it is nonproductive.

In men, for example, masturbation seems an obvious ‘safety valve’ to balance ‘sexual pressure’ while negotiating access to female sexuality. Sex without commitment, given that men, in the traditional role play is the one who needs to commit in order to get ‘real sex’.

In women, it seems (c.f. Baumeister & Twenge 2002) that there is a power advantage for women in suppressing female sexuality. Masturbation and lesbian sex (as long as it’s covert so that it doesn’t reveal that the woman is more interested in sex than she claims to be) can serve as alternative outlets. Again, sex without commitment, in terms of agreeing to have children with the man who is given the keys to the castle. I realize that this theory has many flaws, but I’m just throwing it out as a sketch of how the phenomena can be described as adaptive. I’m happy to be demonstrated wrong with the help of good references.

Regarding male homosexuality, there is the Italian theory that the same genes that promote homosexuality in men, seem to increase the number of offspring when present in women.

The findings represent the best explanation yet for the Darwinian paradox presented by homosexuality: it is a genetic dead-end, yet the trait persists generation after generation.

“We have finally solved this paradox,” says Andrea Camperio-Ciani of the University of Padua. “The same factor that influences sexual orientation in males promotes higher fecundity in females.”