Several articles in news media have recently reported on the new “definitive” findings that penis size really does matter. An article published in the National Academy of Sciences claims to have found evidence that females prefer males with larger flaccid penises and that these preferences expressed by cave women are responsible for the abnormally large penis-to-body ratio of the human male among primates.

…or does it? As a general rule I am skeptical of any research that extrapolates modern human behavior onto prehistoric ancestors in an attempt to justify them using evolution. The greatest difficulty is one which anthropologists appreciate keenly, and that is of viewing the evidence without the lens of our own cultural upbringing. As Americans, we have certain values built into our psyche that, try as we might to deny them, influence our every day thinking. For one, Americans are pretty obsessed with size, and bigger is better. The Japanese, on the other hand, value details over gross quantity. For another, Americans have inherited certain beliefs from Victorian England regarding the inherent moral, sexual and physical differences between men and women. While our official position is that men and women are the “same,” except for perhaps body size and genital arrangement, our perception of human behavior rests in a large part on the structure of female frailty and male ruggedness.

In addition to contemporary differences in human behavior and preferences, we are also compelled to view any evidence in its appropriate historical context. We have the difficulty that human preferences have not been stable over the few thousand years for which we have historical records, and in addition to that we have the larger problem of inferring from our historical records what the millions of years of human evolutionary history may have held. The first fossils of homo erectus are claimed to be 1.89 million years ago, while neanderthals did not appear until approximately 200,000 years ago, and disappeared by 30,000 years BC1. So before we claim that a mere sample of a few hundred humans today, representative of less than one tenth of today’s cultures, and known to be only one of many realizations of historical values, can represent all of humanity on a fundamental, evolutionary, existential level, we should perhaps eat a small slice of statistical humble pie.

With a healthy serving of skepticism and a slice of humility for dessert, let us look at what the actual research says about penis size and human evolution.

Abstract

Compelling evidence from many animal taxa indicates that male genitalia are often under postcopulatory sexual selection for characteristics that increase a male’s relative fertilization success. There could, however, also be direct precopulatory female mate choice based on male genital traits. Before clothing, the nonretractable human penis would have been conspicuous to potential mates. This observation has generated suggestions that human penis size partly evolved because of female choice. Here we show, based upon female assessment of digitally projected life-size, computer-generated images, that penis size interacts with body shape and height to determine male sexual attractiveness. Positive linear selection was detected for penis size, but the marginal increase in attractiveness eventually declined with greater penis size (i.e., quadratic selection). Penis size had a stronger effect on attractiveness in taller men than in shorter men. There was a similar increase in the positive effect of penis size on attractiveness with a more masculine body shape (i.e., greater shoulder-to-hip ratio). Surprisingly, larger penis size and greater height had almost equivalent positive effects on male attractiveness. Our results support the hypothesis that female mate choice could have driven the evolution of larger penises in humans. More broadly, our results show that precopulatory sexual selection can play a role in the evolution of genital traits.

Let’s understand what the scientists who carried out this research were trying to do. To begin with, many animals have penises that are in some ways equipped to increase the likelihood that the female they just mated with actually gets pregnant. Some people have suggested that the flared head on the human penis, as compared to the smooth head on a dog’s penis, is designed to scoop out the semen of other males, replacing it with whoever’s penis is in that particular vagina most recently. This “postcopulatory” characteristic affects reproduction regardless of who mates with whom. “Precopulatory” characteristics are ones which would increase the likelihood of a male being able to mate at all.

Some features of the human penis, when viewed in contrast to the penises of other mammals, make more sense when viewed as part of the attraction and wooing part of reproduction than in the actual act of coitus. Motivated by this observation, the researchers designed an experiment  to answer the question of whether or not the modern human penis shape could have been influenced by how females decide who to have sex with.

In any statististical study it is necessary to choose which variables we will measure and which we will ignore. These researchers chose three variables: height, shoulder-to-hip ratio, and flaccid penis size2. Many other factors could play into male attractiveness and female choice, but out of necessity, only a few could be measured in the experiment. The restriction to these three variables may or may not have been benign. With such a small source of variation, it is possible that the women participating in the study were able to perceive what the underlying question was. As a result, they could have hyper focused on penis size, or either of the other two variables, artificially increasing the effect within the experimental setting. Psychologists have long been aware of the ability of framing, or suggesting the goal of the experiment to the participants, to bias the statistical outcomes, and have developed methods like double-blind testing in order to reduce it. With such a simplified, stylized experiment, one must wonder whether or not this effect was properly controlled for.

Without access to the original article (sorry, I don’t have a subscription), I cannot speak to the actual significance levels of the findings. Most news articles reporting on this research don’t mention it at all. They only talk about the relationship between penis size and attractiveness. In any statistical test there is always the possibility that a relationship is found when there is no underlying cause. This is called “spurious correlation” and is simply a feature of random variables: flip a coin enough times and you’ll get a string of 20 heads in a row. This doesn’t mean that heads is suddenly more likely than tails, it’s just a thing that happens. We already have reason to doubt the strength of these findings based on the experimental design, but we can still analyze the findings, temporarily suspending our disbelief.

The first reported relationship is that penis size has a quadratic effect on attractiveness. This means that to a small extent, bigger is better, but eventually it’s not really that much better. To those men and women who are just itching for evidence that a bigger penis is always better, remember that there are downsides to a large penis such as women who plainly refuse to be penetrated by it. So what this article is saying is that as you go from smaller to larger, you consistently appear more attractive, but you may not be improving your chances for better or more satisfying sex. Also consider that twelve-inch flaccid penises were not featured in this experiment, so it could easily be the case that there is a maximally attractive flaccid penis length and that beyond that bigger is definitely worse.

Next, the article reports that having a larger penis was more important for taller men than for shorter men. Another way to report this finding is by saying that proportional penis size matters. It isn’t as flashy as saying that size really does matter, but flashy is not what makes the news. The next two claims state that a more masculine body was associated with greater penis size effect, and that height and penis size had approximately the same size of effect on attractiveness. So it seems to be the case that a large flaccid penis is more of an added bonus than it is a deciding factor. A short man with a pear-shaped body would be out of luck, even if he had a huge wang. As a final note, the article concludes that its findings support the hypothesis of female mate choice as a factor in evolutionary traits — not that bigger is better.

So what can we take away from this research? Certainly it is the case that modern women consider penis dimensions as a feature in male attractiveness. It is also the case that women consider height and other body proportions. We might just conclude that women consider physical features of men that are considered to be attractive as attractive, but not all to the same degree. It’s a pretty watered down conclusion from the overblown “Neanderthal women mated with men with big penises and so size does matter (and see? we knew you small pricked losers were, well, losers)” that the news has been reporting. But the goal of the research was not to say that bigger penises are better. It was motivated by the modern western notion that bigger is better, but it asked an entirely different question. In the end it could not even conclude that penis size was a deciding factor, merely that it enhanced what was already considered generally attractive.

  1. Source for claim on evolution timeline
  2. The abstract uses the word “size” but many articles refer only to length, ignoring girth.
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