I've written about female orgasm on two prior occasions, but in those cases I used the subject more as a vehicle for understanding the opposition to evolutionary explanations rather than discussing orgasm itself. The comments section on a recent Cracked article that concludes female orgasm is a byproduct - not an adaptation - attests to the issues I had discussed. There, we see dozens of comments made by people who's expertise consists of maybe having watched some documentary once they sort of remember. Believe it or not, as this part is shocking, these uninformed people also have very strong opinions about whether female orgasm has an evolved function. The most commonly hypothesized function for female orgasm found in the comments is that it motivates women to have sex, typically followed with a "duh". The two assumptions embedded in that idea are (1) women who orgasm during intercourse engage in more sex than women who do not and (2) having more sex means having more children. If either of those points turn out to be false, that hypothesis wouldn't work.
The first point may be true. According to Lloyd (2005), there is some evidence that suggests women want more sex the more frequently they orgasm. Sure, it's correlational in nature, but we'll not worry about that here. It's the second point that raises some more serious issues. As women can only become pregnant during a specific point of their cycle where an egg is available, having more sex during a non-fertile period will do approximately nothing when it comes to a shot at successful conception. Further, in principle - and many times, in practice - you only need to have sex once to become pregnant; having sex beyond or before that point will not make a woman any more pregnant. The heart of the issue, then, seems to concern proper timing. Having sperm present and ready to do some fertilizing at all points may increase the odds of conception, as neither the man or the woman know the precise moment ovulation will occur. However, at some point there will be diminishing returns on the probably of increasing conception from each additional act of intercourse. It's not a simple formula of "more sex = more babies".
I'm going to get soooo pregnant; you have no idea!
If female orgasm evolved to motivate women to have sex with men, it does so rather inefficiently. When women masturbate, the vast majority do not do so in a manner that simulates penetrative intercourse alone, as penetration rarely provides the proper stimulation. When women do achieve orgasm with intercourse, which is often quite variable, most require additional manual stimulation of the clitoris; orgasm is not generally reached through the sex itself. In terms of providing some crucial motivation then, this accounts seems to take an odd do-it-yourself approach to reinforcement. This also raises the question of why so many women are unable to reach orgasm either frequently or at all from intercourse alone if it's supposed to provide some crucial motivation. Under this functional account, women who did not experience orgasms with intercourse would have been selected against, yet they persist in substantial numbers. In terms of taking home the coveted label of adaptation, this account doesn't fare so well.
There are many additional adaptive accounts of female orgasm, but I'd like to discuss only one other hypothesis here: the upsuck hypothesis. Though the account had been proposed prior to Baker and Bellis (1992), they were the first to attempt to empirically test the suggestion that female orgasm may serve a function manipulating the amount of sperm retained or ejected from copulation. To test this suggestion, Baker and Bellis found some very willing volunteers to first collect semen samples from sex using condoms in order to generate an estimate of sperm count in the ejaculate. After this period, the couples engaged in unprotected sex and collected the flowback - the secretions from the vagina following sex, including fluids from both the male and female. Sperm count was then obtained from the flowback samples to estimate how much sperm had been retained. The samples were finally assessed on scales of taste and presentation*.
Don't worry; taste testing was carried out using a double-blind procedure to avoid bias.
The results showed that female orgasm was unrelated to sperm retention in general. However, female orgasms that occurred from one minute prior to male ejaculation to forty-five minutes following ejaculation were associated with greater estimated sperm retention. Lloyd (2005) critiqued this study on statistical grounds, but I'm not currently in a position to evaluate her claims, so I'll ignore those for now (though I will say I'm always uncomfortable relying on median values without accompanying means). Lloyd also mentions that a later reexamination of the data found that female orgasms occurring one minute to ten minutes following male ejaculation actually did not show that effect of increased sperm retention, which would require the odd pattern of female orgasm having no effect prior to one minute before male ejaculation, then it would increase sperm retention, then decrease retention, then increase retention again. It seems more plausible that there's an issue with the data, rather than that such a peculiar pattern exists.
There is another concern of mine regarding Baker and Bellis's flowback data, though it has not been raised by other authors to my knowledge. Perhaps that is for a good statistical reason that escapes me, so bear in mind this may be a moot concern. Naturally, it's hard to recruit for this kind of research. As a result, Baker and Bellis had a small sample size, but did manage to collect 127 flowback samples across 11 couples. Now Lloyd mentions that, of these 127 samples, 93 came from just one couple. What she does not mention is that this couple also happened to have the second lowest median percentage of sperm retention of all the couples, and they were lower by a substantial margin. In fact, the couple providing most of the data retained only about half of the overall median number of sperm. For reference sake, the only couple to have a lower median retention rate was estimated to have retained a negative number of sperm. If most of the data is coming from an outlier, that would be a big problem.
For example, the average income of these men is roughly ten-million dollars a year.
While these results are suggestive, they beg for replication before full acceptance. Nevertheless, let's take the results of Baker and Bellis at face value. While all women in the sample were estimated to be able to either nearly completely retain or expel all the sperm of an ejaculate, regardless of whether they could orgasm during sex or not, Baker and Bellis suggest that female orgasm may play some role in affecting the sperm retention process. To attempt and complete an adaptive account, it's time to consider two other points: first, it's unclear as to whether the additional sperm retention has any effect on conception rates, either between or within men. It might seem as though additional sperm retention would be useful, but that assumption needs to be demonstrated. Second, female orgasm does not reliably accompany intercourse at all, let alone with a specific timing (up to a minute before hand, but not from one to ten minutes afterwards, but then again after ten minutes, and only during ovulation). As most female orgasms require additional clitoral stimulation on the part of either the man or the woman, this would require ancestral humans to have reliably provided such stimulation, and whether they did so is an open-ended question. Even if female orgasm had this potential function of sperm retention, it does not follow that female orgasm was selected for; that potential function could be a byproduct.
There are certain adaptive hypotheses still to be tested, but what we need is more evidence that's less ambiguous. The case of whether female orgasm is an adaptation or not is still open to debate. At present, I find the evidence favoring the adaptation side of the debate lacking, much to the dismay of many people who determine the social and personal value of a trait on the basis of whether it's an adaptation or a byproduct. They seem to think tentatively labeling female orgasm as a byproduct somehow makes it less valuable and reflects a mean-spirited sexism towards women. On a redundant note, they're still wrong.
*That sentence may or may not be true. I wasn't there.
References: Baker, R.R. & Bellis, M.A. (1993). Human sperm competition: Ejaculate manipulation by females and a function for female orgasm. Animal Behavior, 46, 887-909
Lloyd, E.A. (2005). The case of the female orgasm: Bias in the science of evolution. Massachusetts: Harvard University Press.