Controversy about evolution[edit | edit source]
It is unfortunate that the human penis has been subject to so many incorrect theories as to how its specific morphology has been influenced by evolution. Most of these incorrect assertions about the evolutionary history of the Human Penis have been influenced by a 1984 paper by Robert L. Smith, having described the human penis as "extraordinary relative to the other hominoids". This view has been further advanced by the likes of Robin Baker and Mark Bellis (The pioneers for the idea that Sperm Competition has played a major role influencing Human Evolution) whom have said that the human penis "is nearly twice as long and over twice as wide as that of the chimpanzee". Alison Jolly has also made similar remarks in her book saying that the "‘peculiarity of humans is that the penis is twice the size for body weight as that of any other primate." In 2000 Geoffrey Miller published a book where he wrote that "adult human males have the longest, thickest and most flexible penises of any living primate". More recent example can be seen in the popular book Sex at Dawn (a book that can only be described as the largest collection of pseudoscientific claims on human sexuality to have ever existed) by Christopher Ryan and Cacilda Jetha describing humans as the "Great Ape with the great penis". It should be interesting to note the both Miller and Ryan are both evolutionary psychologists, not biologists, as well as polyamorists. Typically to account for the apparently grandiose status of the human penis in comparison to other primates, one of the the four following theories are offered as an explanation:
- The Length of the Human Penis has evolved to deliver sperm as closely as possible to the Cervix so as to gain an advantage during sperm competition. (Smith 1984)
- The size and shape of the human penis has evolved to displace rival sperm and thus gain an advantage during sperm competition. (Baker and Bellis 1995; Gallup et al. 2003; Gallup and Burch 2004)
- The size of the penis has evolved to promote female orgasm; as female orgasm influences sperm retention and transport, larger penises will be selected for in order to gain an advantage during sperm competition. (Fisher 1983; Fisher 1992; Small 1995; Baker and Bellis 1993 & 1995; Miller 2000)
- The human penis is more visually accessible than other primate penises and thus has evolved via sexual selection. (Short 1980; Diamond 1997)
Finally we have genetic evidence to show that the evolution of Homo Sapiens involved little to no sperm competition. Comparisons between the Y Chromosome genes involved in sperm production of Bonobos and Chimpanzees shows that bonobos exhibit much lower variation (in these genes), thus indicating that (compared to Chimpanzees) Bonobos have reduced sperm competition. Important to note that Gorillas and Orangutans do not have much variation in the same genes either. Seven regions on the Y Chromosomes involved in the sperm production are identical (remember that Gorillas have a single male/polygynous mating system); this indicates that Humans have evolved out of a polygynous mating system and that, since no significant mutations have affected these regions, they have always been socially monogamous/polygynous. However these regions on the Y Chromosomes in Chimpanzees are not identical to that of Humans and Gorillas. This indicates that promiscuous mating system of Chimpanzees only evolved after they'd split from our lineage; this essentially implies we did not evolved out of a promiscuous mating system and we always 'monogamous'.
On a more humorous note, bats that have promiscuous females have larger testes because of sperm competition but also smaller brains when compared to bats that have faithful females. So not only do Chads with big nutsacks have the immune systems comparable in strength to the French Army (no offense meant) but they also have brains not much larger than their own nutsacks.
Penis size[edit | edit source]
The average human penis is between 13-15cm in length and 10-12cm in circumference when erect (measured with a bonepressed ruler):
Let's see what other primates look like in comparison:
- Pan Troglodyte
- Erect Length 14.4cm (Dixson and Mundy 1994)
- Pan Paniscus
- Stretched length 17cm (Dahl 1988)
- Gorilla Gorilla
- Erect Length 6.5cm (Short 1980)
- Pongo spp.
- Erect Length 8.75cm (Dahl 1988)
As we can see, the human penis is not as magnificent as some would make it out to be. It is similar to the chimpanzee in erect length, only slightly smaller and has a shorter penis that the bonobo (Stretched length and erect length are approximately the same length). It should also be noted that all the above measurements are greater than what was originally claimed by Smith 1984 whom was no doubt the single biggest influence on the view that sperm competition played a major role in human evolution (this is especially true in the field of evolutionary psychology unfortunately). Smith reported measurements of 8cm for the chimpanzee, 4cm for the orang-utan and 3cm for the gorilla. Primatologist Alan Dixson has said in his book about human mating systems that the human penis is not " exceptionally long in relation to those of other primate species, especially when their body sizes are taken into account."
A more likely mechanical explanation for the evolution of the human penis, that does not require the invoking of sperm competition, can be found in a 2008 paper by Edwin Bowman. It posits that the size of the human penis has likely co-evolved with the increasing size and width of the female pelvis, due to the increasingly larger heads of new-borns, in order to have a more satisfactory 'fit'. Regardless of whether or not sperm competition is present, there is always a selection pressure for males to efficiently place sperm so as to maximally increase the odds of fertilization.
The female vagina is between 15-18cm in length when aroused. Let's say a 5cm penis ejaculates a further 4cm upon orgasm; that males sperm still has to travel an additional 6-9cm before even reaching the uterus; this will obviously incur heavy loses amongst his sperm and thus lower the chance of pregnancy. It a man with an average penis with 14cm ejaculated up to 4cm inside the vagina, his sperm do not need to travel very far in order to reach the uterus. Thus there is a selective pressure for penises between the range of 13-15cm. Genital co-evolution is not unique to humans and can be found in mammals outside of primates. Similarly another 2008 paper by Wallen and Llyod has compared variability in the penis and clitoris and found that variability in clitoral length far exceeds penile and vaginal length; they conclude that this is because there is an absence of selective pressures on clitoral length relative to penile and vaginal size, providing further evidence for the genital coevolution hypothesis.
All that being said though, if there are selection pressures for the human penis to deposit sperm 'as close as possible' to the cervix for optimal placement, the question arises: Why isn't the human penis any bigger? The female vagina is between 15-18cm in length when aroused, so why is the penis only 13-15cm erect? The answer is that it is not optimal to place sperm as close as physically possible to the cervix and that, in the case of humans, the optimal length is between 13-15cm and not 15-18cm. Levin (2019) can provide further illumination as to why this is. The author summarizes the reproductive functions of the clitoris (in contrast to the claim that the clitoris is merely for pleasure); namely the capacitation of spermatozoa to become fertile and to restrict the number of sperm that reach the ovum in order to prevent polyspermy. Upon being stimulated, the clitoris causes what is known as vaginal tenting/ballooning where the length of the vagina extends upwards into the false pelvis; this is in order to delay sperm transport so as to give enough time for the incoming spermatozoa to be capacitated (by both Vaginal and Seminal secretions) so that they can become fertile. This would explain why the average penis is between 13-15cm long and not 15-18cm long, the man with a 13-15cm penis will allow more time for his sperm to be capacitated (thus becoming fertile) as he does not deposit his sperm directly beside the cervix but rather slightly further away from it, whereas the man with a 15-18cm long penis will place his sperm to close to the cervix and does not allow an optimal amount of time for his sperm to become capacitated. It turns out Chads with big dongs are actually less fertile than men with average penises, who knew.
Sperm competition[edit | edit source]
It is useful to examine how sperm competition has influenced the morphology of the penis in other primates where sperm competition in known to have existed. What we find in these multi-male/multi-female mating systems is that they have larger testes and have much more complex penis morphologies than what is typically found in polygynous and monogamous species. Penises also have a tendency to be longer (the length of the human penis can be explained via genital co-evolution and does not require the invocation of sperm competition) and more distally complex. Considering that the Human penis is unapologetically simple is evidence against sperm competition having influenced human penis morphology. To once again refer to Alan Dixson and his book on human mating systems, he compared and rated the penile complexity (length, distal complexity, size of baculum and penis spines) of roughly 48 primate genera. Multi-male/multi-female mating systems scored higher in penile length, distal complexity, baculum length and penile spines than polygynous or monogamous mating systems.
Human penis size was, overall, not exceptional when compared to prosimians, monkeys and apes; scoring highly only in penile length (which can be explained via genital co-evolution). Humans scored similarly to a number of monogamous and polygynous species like Leontopithecus, Callimico, Erythrocebus and Theropithecus. Distal complexity (the shape and size of the glans or at least its equivalent) was found to be as complex as 21 of the 48 genera and less complex than 20 genera, leaving only 7 of the 48 genera to be rated as less distally complex as the human penis.
Gallup et al. (2003) tested Baker and Bellis's (1995) hypothesis that the shape of the human penis was specifically designed to scoop out human semen. Important to note is that these experiments utilized fake penises, fake vaginas and fake cum (these people literally extrapolated functionality of the human penis from dildos). They contend that the 'large' diameter of the human glans and corona have been selected for due to sperm competition. Outside of this study, there is no other evidence to support the view that the shape of the human penis has evolved to displace sperm. 'Acorn-like' glans (like seen in humans) is extremely common in old world monkeys regardless of whether or not they have monogamous, polygynous or multi-male/multi-female mating systems. In fact, the human penis is remarkably similar to the gorilla (whom experiences virtually no sperm competition) and differ only in size. Gallup and Burch (2004) have even noted that semen displaced by rival males might be transferred to other females and thus fertilize her. If we compare Humans to more promiscuous Chimpanzee and Bonobos, it becomes very obvious that the human penis has not evolved to adapt to sperm competition. In both the bonobo and the chimpanzee a glans is lacking and the penis is filiform; the bonobo in particular has a distinctive Y-shape. Measurements of penile and vaginal lengths in the chimpanzee indicate that the long and filiform penis probably co-evolved in association with the development of the large sexual skin swelling which characterizes this species.
It seems to be the case that there is a trade off between immune function and sperm quality, as sperm cells are Gametes and not Somatic Cells they are/can be subject to attacks from the male's own immune system in the reproductive tract; certain physiological mechanism protect the sperm (albeit incompletely) as a result of high immune function sperm quality suffers. High quality ejaculate requires suppression of the immune system. Thus fertile men, or men with good quality sperm, will likely have inferior immune systems to men with lower quality sperm.
Finally we have genetic evidence to show that the evolution of Homo Sapiens involved little to no sperm competition. Comparisons between the Y Chromosome genes involved in sperm production of Bonobos and Chimpanzees shows that bonobos exhibit much lower variation (in these genes), thus indicating that (compared to Chimpanzees) Bonobos have reduced sperm competition. Important to note that Gorillas and Orang-utans do not have much variation in the same genes either. Seven regions on the Y Chromosomes involved in the sperm production are identical (remember that Gorillas have a single male/polygynous mating system); this indicates that Humans have evolved out of a polygynous mating system and that, since no significant mutations have affected these regions, have since and always have been socially monogamous/polygynous. However these regions on the Y Chromosomes in Chimpanzees are not identical to that of Humans and Gorillas. This indicates that promiscuous mating system of Chimpanzees only evolved after they'd split from our lineage; this essentially means we did not evolved out of a promiscuous mating system and we always 'monogamous'.
On a more humorous note, bat that have promiscuous females have larger testes because of sperm competition but smaller brains when compared to bats that have faithful females. So not only do Chads with big nutsacks have the immune systems comparable in strength to the French Army (no offense meant) but they also have brains not much larger than their own nutsacks.
Sexual selection[edit | edit source]
It is certainly tenable, if not unlikely, that sexual selection has influenced the morphology of the human penis. It has been shown cross-culturally that females have a preference for larger non-erect penises. An Australian study has found a preference for larger non-erect penises using 3D models. However a preference for larger non-erect penis, which isn't very strong anyway, does not necessarily mean that female choice has driven human penis morphology. Namely because penis size wasn't a priority for ancestral females who wanted children and hunting ability was deemed more important. Humans also wear clothing and despite our intuitions, have been wearing clothing for most of the time that we've existed. Considering that we have evidence that clothing has possibly been around since 700 000 years ago and the Homo Sapiens is only about 200 000 years old, it is unlikely that females had visual access to a males penis and thus select based on penis size. Height, for example, was a lot easier to identify and could more easily be selected for. It is, however, conceivable that information about penis size is transmitted via female gossip without each female having direct visual access for making comparisons. Gossip is a human behavioural pattern that can be observed cross-culturally.
The are also some criticisms to be made about the studies that show a female preference for larger non-erect penis. Namely, how does the subconscious female mind differentiate between a partially erect and growing penis and a particularly large flaccid penis; partially erect penises do not stand upright like erect penises and thus are more difficult to distinguish from non-erect penises. The higher rating of larger non-erect penises might be because a larger penis in general would be a sign of arousal. The study conducted in Australia seems to indicate this, though the authors do not seem to notice. It was shown that larger non-erect penises were rated as more attractive up until 8cm (3") after which any increase in size would result in diminishing returns (note that the average non-erect penis is around 8cm). A non-erect/partially erect penis larger than 8cm (3") is probably an indicator of arousal. This would nicely explain why we find diminishing returns on non-erect penises larger than 8cm (3") as the larger a penis is, the more likely/obvious it is that its owner is aroused; after a certain point it becomes extremely likely that the greater size of the penis is an indicator of arousal and thus result in a plateau of attractiveness ratings.
To illustrate, if I told you that a penis was 3 inches long without giving you any information as to whether or not said penis is erect or flaccid and asked you to estimate the likelihood that that 3 inch penis is erect, you would most likely answer that the penis is flaccid. If I asked you to do the same with a 5 inch penis you would most likely answer that the penis is erect; the higher the number goes the more likely it is that we would guess that a penis is in a state of erection. Ergo, the chance that a penis is in a flaccid state diminishes the larger the penis becomes and would thus explain why we see diminished returns on flaccid penises larger the 8cm (3").
We can work out the chance a penis within a certain range is erect using the following equation:
(Percentile of Upper Erect Length - Percentile of Lower Erect Length) / (Percentile of Upper Flaccid Length - Percentile of Lower Flaccid Length + Percentile of Upper Flaccid Length - Percentile of Lower Flaccid Length)
For example a penis of an absolute size between 4-4.25 inches has a 68% chance of being erect. The equation looks like this (15.72-8.72) / (98.14-94.98 + 15.72-8.72) = 0.68
|Range of Penises (")||Percentage of Penises that are erect|
|2.25>||Less than 0.01|
|5.00<||Greater than 99.99|
As we can see, there is a rapid increase in the percentage of penises that would be erect after an absolute length of 3 inches (which explains the diminishing returns on attractiveness after that point) and that the percentage of penises that are erect plateaus at 5 inches (roughly before the same area where peak attractiveness plateaus) further lending credence to the idea that larger penises are found to be more attractive because they are an indicator of arousal.
There doesn't seem to be any indication that a partially erect penis would be rated as any less attractive than a flaccid penis at the same length. Thus contrary to the author's premature assertion that our Female Ancestors preferred to mate with long schlongs, their data suggest the opposite, that females prefer mating with more moderately sized penises. The authors tested for attractiveness up until 13cm and noted that the turning point in attractiveness ratings likely falls outside what was tested (note that the average penis is 14cm). Future studies must test the effect of a penis's rigidity before we can make a conclusive statement as to whether or not the preference for larger non-erect (which is not the same as flaccid) penises is due to the fact that a larger penis is an indicator of arousal overall or a 'just so' explanation were women just prefer larger penises.
Though to be charitable to the authors of the study, the preference for a penis size of an absolute length of about 14cm cannot be a reflection of a bone pressed measurement (as the women obviously cannot see through the fat pad above of the penis) and thus must be a reflection of a non-bone pressed measurement. A 14cm penis is situated a roughly the 80th percentile and thus you might say "Aha! See? Women really do and always have preferred larger than average penises." however this conclusion must be taken with a grain of salt. We cannot simply draw the conclusion that penis size has been influenced by sexual selection because the most aesthetically pleasing non-bone pressed penis lies at the 80th percentile because of the following reason... fat. The average American male sits at body fat of approximately 28.2%, that's not very far from being morbidly obese and it is almost certainly true that your average male across cultures and through time does not have an average body fat percentage of 28.2%. In fact in contemporary Western Culture a person is more likely to die from being too fat than from undernourishment whereas the opposite would of been true for our ancestors; many of whom would've had body fat percentages so low that they would've been at risk of starvation (and many have died from starvation). Thus we conclude that the average male would've had a much lower body fat percentage than your contemporary western male. The lower body fat percentage would then mean that his penis is less obscured by his body fat. It is under these conditions that our ancestral females would have to decide whom to mate with and under these conditions that an innate aesthetic preference for a particular penis size would've developed (and thus be reflected in contemporary women). Thus a non-bone pressed preference of 14cm which lies at the 80th percentile in Western and other Contemporary countries cannot be used to demonstrate that ancestral females preferred larger than average penises as, in terms of body fat percentage, contemporary men are very much an outlier and will likely skew the results. From these premises, we can conclude that ancestral females preferred a non-bone pressed length of 14cm, which would've been much closer to average at the time as men had much lower body fat percentages, and that the contemporary preference for an above average non-bone pressed penis of 14cm in women is less a reflection of a preference for a larger penis and more a reflection of overweight men. Considering that a bone pressed penis of 14cm sits at the 50th percentile this seems likely. This also demonstrates that your average male would do well to lose the extra fat on top of his penis so as to make it more aesthetically appealing.
Interestingly to note, however, is that the authors also tested for the effect of height and muscularity on attractiveness. Whilst (assuming the authors are correct on their assumptions on penis size) penis size and height each accounted for about 5% and 6% respectively in the variance on attractiveness, muscularity (specifically the V-Shape and lower fat around the belly) accounted for the remaining 80% of attractiveness (though note that the authors did not control for facial attractiveness). Thus gymmaxxing, would actually significantly increase one's attractiveness, with muscularity being roughly 13 times more important than height. And, presumably, 16 times more important than penis size. A quick note on muscularity, the average American male has 28.2% body fat, almost double the ideal body fat of between 10%-15% (for the purposes of attractiveness. Considering that a male with a body fat percentage of 10% and 15% sits at the 13.85% percentile and 18.24%, it seems that (unless a male is severely disadvantage in a multitude of areas) your average male can expect to attain alpha status (i.e the top 20%) if they were between a body fat percentage of 10%-15% and had a large V taper on their upper torso irrespective of how tall they were (or what their penis size was).
For a calculator on body fat percentage: See Here
Female orgasm[edit | edit source]
A paper publish in 2012 claimed that women who preferred larger penises were more likely to achieve orgasm from vaginal stimulation than women who didn't (prefer longer penises). From this they conclude that larger penises are superior for eliciting orgasm via the vagina. Important to note is that the study is correlational in nature and that they draw their data through a survey (not the worst means of data, but certainly not the best; see ovulatory shift hypothesis). The amount of Penis-Vagina Intercourse reported was 8.76 and 6.78 times per month for women with a large penis preference and women without respectively. Orgasm from PVI was 7.28 and 4.68 times per month for women with a large penis preference and women without respectively. This gives us a rate of 0.88 orgasm per PVI for women with a large penis preference and a 0.69 orgasm per PVI for women without a large penis preference; this leads us with an absolute difference on 0.19 orgasm per PVI and a relative difference of 27.52% increase in rate of Orgasm for women with a preference for larger penises. First to note is the suspiciously high rate of orgasm during PVI, almost no studies report that women orgasm 69% of the time during intercourse (let alone 88% percent of the time).
Although a 2015 study did examine the effects that the phrasing of questioning had on female reports on orgasm during intercourse. Their sample included 1569 men and 1478 women; they found that women's reports of orgasm were highest during assisted intercourse (51-60%), intermediate when clitoral stimulation was unspecified (31-40%) and lowest in response to unassisted intercourse (21-30%). Note that the study on penis size and vaginal orgasm only had a sample size of 321, which was then halved to 160 after screening out 'outliers', where as the 2015 study has a just over 9.2 times greater sample size. It seems highly suspect that Costa et .al claim that , supposedly, what they report is orgasm achieved by PVI without clitoral stimulation, yet we find authors that report a rate significantly lower (with a sample size that is 9 times as large) than what is reported by Costa et .al; to be specific, the control group reports a rate of orgasm just over 2-3 times more often than what was found in Shizari et al.; calling into question the reliability of Costa et al.'s data. It might be that the phrasing of questioning was ambiguous and thus produced a higher rate of orgasm, however Shizari et al. report that when questioning on clitoral stimulation is not specified (and is thus left up to the interpretation of the reader) females only report a rate of 31-40% (no where near the 69% given).
Costa et al. argue that "One might argue that more generally erotophilic or less inhibited women rate larger penises as more important and also have higher rates of all sexual activities and all kinds of orgasms." (as this would be a control for how orgasmic they are) and that "However, our differential results argue against that interpretation because the preference for a longer penis is associated only with vaginal orgasm frequency, not with frequency of other forms of orgasm or other sexual activities." However inferring controlling how orgasmic a woman is from her masturbatory practises is not a sufficient (nor the best) possible control for inferring how orgasmic a woman is.
Shizari et al. report that whilst men almost always orgasm during intercourse, most women do not and that both men and women report a greater than 95% success rate whilst masturbating. They report that the reason for this is because the preferred medium for attaining orgasm during masturbation is similar during intercourse for men but not for women; important to note that they also report that less than 5% of women report by masturbating using vaginal stimulation alone (indicating that it is the non-preferred method of achieving orgasm) Inferring how orgasmic a women from rate of orgasm vaginal masturbation and/or clitoral masturbation is not reliable as:
1. When masturbating, females are not limited by how long their partner lasts and can more easily reach orgasm by taking their time to reach orgasm.
2. Rate of vaginal orgasm from masturbation might not necessarily reflect how orgasmic a woman is nor might it reliably predict rate of orgasm from intercourse as it is not the preferred method of masturbation; (as only 5% of women report only using vaginal stimulation to achieve orgasm from masturbation we can assume that most women will be using vaginal stimulation concurrent with clitoral stimulation) thus we cannot use vaginal masturbation as a means for inferring how orgasmic a women is from purely vaginal stimulation as most women will do so in conjunction with clitoral stimulation.
3. Because most women will use vaginal stimulation and clitoral stimulation concurrently when masturbation, vaginal stimulation (in the case of Costa et al.) is not comparable to unassisted intercourse as they have not specified that these women masturbated exclusively using vaginal stimulation.
4. Because vaginal stimulation is the non-preferred method of masturbation, females might not be as motivated to use it for an entire session whilst masturbating and might only use it as an accessory to her preferred clitoral stimulation (i.e she might stimulate herself vaginally for a bit just for the sensation, stop, then continue with clitoral stimulation; as attempts to reach orgasm via vaginal stimulation during masturbation are not as motivating as clitoral stimulation, this will skew the rate of orgasm and will not be reflection of how orgasmic she is really is).
Thus inferring how orgasmic a woman is from her masturbatory methods and rate of orgasm when masturbation is unsound. To list a few factors that might influence female orgasm: Women with vestibulodynia and vaginismus have increased nociception (propensity to feel pain) and levator ani hyperactivity (tightness in the pelvic floor muscles) respectively. Obesity is seems to affect partnered sexual experiences, but not masturbatory ones(these are the same author as the 2012 study). It might be that increased incidence in orgasm during partnered sex might be due to psychological reasons rather than anatomical ones, as suggested by the obesity study. Most notably however is that the closer the clitoris is to the urethral meatus, the higher the frequency of orgasm. Future studies examining the effects of penis size should use this as a means for controlling the propensity for orgasm in females instead of inferring it from masturbatory methods and rate of orgasm. Wallen and Llyod (2011) that whilst the distance between the urethral meatus and the clitoris affected orgasm during intercourse, it did not affect orgasm during masturbation; this further invalidates Costa et al.'s findings.
References[edit | edit source]
See also[edit | edit source]
- Female orgasm
- Macropenis dilemma