The male orgasm plays a vital role in human reproduction, given that it typically coincides with ejaculation. Indeed, a man who cannot reach orgasm would likely have a very difficult time ever reproducing. By contrast, the same cannot be said for women--as long as a woman is near the fertile phase of her cycle, she has the potential to become pregnant regardless of whether she orgasms during sex.

This observation has generated a lot of research and debate about why the female orgasm exists. For example, does it have an adaptive purpose we aren't aware of? In other words, do women's orgasms facilitate reproductive success in a non-obvious way? Or is the female orgasm just an evolutionary byproduct that has no impact when it comes to reproduction? Scientists don't agree on the answer, but they've proposed a number of theories. Here's a review of the three most popular ones:

1.) It's a "sperm retention mechanism." Some scientists have argued that the female orgasm may serve as a “sperm retention mechanism," with the idea being that uterine contractions during orgasm may draw sperm further into a woman's reproductive tract, thereby increasing the odds of conception.

As some support for this idea, consider a study in which female participants collected “flowback” after unprotected vaginal intercourse [1]. That is, these women collected the ejaculate that seeped out of the vagina after sex (I know--the things some people do for science). The researchers performed sperm counts on the flowback samples and correlated this with women's reports of orgasm. Ultimately, they found that when women reached orgasm around the same time as their male partners, the flowback contained less sperm than when women didn’t orgasm at all or reached orgasm much sooner than their partners. This suggests that the female orgasm helped in retaining sperm.

Of course, this study has its flaws. For one thing, we don’t know how much sperm was actually released during each ejaculation, so it's difficult to make definitive claims about how much of it was truly retained. Moreover, to the extent that we accept the argument that the female orgasm really does help to retain sperm, then why do so many women not reach orgasm routinely during vaginal sex, and why does it typically take women much longer on average to reach orgasm than men?

2.) It's a way of helping women to identify high-quality mates. An alternative theory is that the female orgasm serves as a feedback mechanism that provides women with information about the reproductive potential and quality of their partners. As some support for this idea, consider a recent study published in the journal Evolutionary Psychology, which found that several male partner traits were linked to college women’s orgasm frequency and intensity, as well as their overall sexual satisfaction [2].

Among the factors correlated with greater frequency of female orgasm were having a partner with higher family income (the authors argue that, at least in college samples, this is a proxy for men’s income potential), having a partner with greater self-confidence, and being more physically attracted to one’s partner. Greater physical attraction to one’s partner was also linked to having more intense orgasms and reports of greater sexual satisfaction.

In addition to being linked with physical attraction, sexual satisfaction was associated with a number of other partner traits, including feeling protected by one’s partner, feeling that one’s partner is a “catch,” and having a partner with broader shoulder width.

This pattern of results is consistent with the idea that women tend to have more orgasms and more satisfying sex with male partners who have stronger “mate value” (i.e., men who have good genes and the resources necessary to provide for any children produced). The authors' argument is that the quality of the sex women are having could potentially be influencing with whom women decide to become romantically involved.

3.) It's a "fantastic bonus." One additional theory is that perhaps there is no adaptive value to the female orgasm and, instead, the pleasure it provides is nothing other than a “fantastic bonus” for women [3]. The “fantastic bonus” theory argues that the female orgasm is just a byproduct of how the tissues in the human body are laid out in early development.

Regardless of one's biological sex, everyone initially looks the same in the womb. During the first two months of development, the genital structures are undifferentiated and have the potential to develop into either penises and scrotums or vulvas. The male and female genitals actually develop out of the very same embryonic tissues and nerve structures, and these structures are laid out in order to ensure orgasm if a male develops, given that the male orgasm is necessary for men to reproduce. Because men’s ability to reach orgasm is so heavily favored by our biology, it could be that the female orgasm is simply a byproduct of this.

Some argue that the female orgasm has a lot in common with the male nipple in terms of its reason for existence. Biology favors nipple development in women so as to make breastfeeding possible. Men’s nipples don’t serve any biological or reproductive function (at least not that we know of); however, they do contain many nerve endings, which makes them a potential “bonus” in the bedroom (indeed, some research has found that most men report nipple stimulation can cause them to become sexually aroused). So, perhaps female orgasms and male nipples are both fantastic bonuses of nature.

Of course, the three theories reviewed here aren't the only ones that exist. For instance, others have argued that the female orgasm serves as a reward or inducement for having sex, whereas others have argued that perhaps it plays a role in pair-bonding. Thus, when it comes to the purpose of the female orgasm, we're far from being able to say anything definitive yet.

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[1] Baker, R.R., & Bellis, M.A. (1993). Human sperm competition: Ejaculate manipulation by females and a function for the female orgasm. Animal Behavior, 46, 887-909.

[2] Gallup Jr, G. G., Ampel, B. C., Wedberg, N., & Pogosjan, A. (2014). Do orgasms give women feedback about mate choice? Evolutionary Psychology, 12(5), 958-978.

[3] Lloyd, E. (2005). The case of female orgasm: Bias in the science of evolution. Cambridge, MA: Harvard University Press.

Image Source: iStockphoto.com

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