Penis

🍆
In humans, the penis is a blood-inflatable piece of meat made for various things and activities. Males who have experienced serial rejection on the grounds of their small penis may be called dickcels.
In popular science, a theory has been popularized that the human penis would be specifically shaped by female promiscuity and sperm competition. Upon closer inspection, this turns out to be an extremely dubious claim.
==Controversy about evolution==
The human penis has been subject to many theories about how its specific morphology has been influenced by evolution. These hypotheses largely center around the idea that the morphology and size of the human penis are highly unusual compared to other great apes, especially humanity's closest extant evolutionary cousins, the apes of the genus Pan (chimps and bonobos). A 1984 paper, authored by Robert L. Smith, heavily influenced these assertions about the evolutionary history of the human penis. Smith described the human penis as "extraordinary relative to the other hominoids." https://www.researchgate.net/publication/251184509_Human_Sperm_Competition
This view has been further advanced by the likes of Robin Baker (the author of Sperm Wars) and Mark Bellis (the pioneers for the idea that sperm competition has played a major role influencing human evolution), who made dubious claims 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 bookhttps://www.amazon.com/Lucys-Legacy-Intelligence-Human-Evolution/dp/0674005406 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".https://www.amazon.com/gp/product/B006F2182G/ref=dbs_a_def_rwt_hsch_vapi_taft_p1_i1
A 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, who luridly describe humans as the "great ape with the great penis".https://www.amazon.co.uk/Sex-Dawn-Stray-Modern-Relationships/dp/0061707813
It should be interesting to note the both Miller and Ryan are evolutionary psychologists, not biologists (and they are both notorious for their highly speculative and dubious theories) as well as polyamorists. Typically to account for the purported unique status of the human penis in comparison to other primates, one of four following theories are offered as an explanation: 
#The length of the Human Penis has evolved to deliver sperm as close as possible to the cervix so as to gain an advantage during sperm competition. (Smith 1984)
# The size and shape of the human penis have evolved to displace rival sperm and thus gain an advantage during sperm competition. (Baker and Bellis 1995https://www.researchgate.net/publication/233820383_Human_Sperm_Competition_Copulation_Masturbation_and_Infidelity; Gallup et al. 2003https://www.researchgate.net/publication/228466536_The_human_penis_as_a_semen_displacement_device; Gallup and Burch 2004https://www.researchgate.net/publication/232494505_Semen_Displacement_as_a_Sperm_Competition_Strategy_in_Humans)
#The size of the penis has evolved to promote female orgasm; as female orgasm influences sperm retention and transport, larger penises will be selected in order to gain an advantage during sperm competition. (Fisher 1983https://www.amazon.com/Sex-Contract-Evolution-Human-Behavior/dp/0688015999; Fisher 1992https://www.amazon.com/Anatomy-Love-Marriage-Completely-Introduction/dp/0393285227; Small 1995https://www.amazon.com/Female-Choices-Sexual-Behavior-Primates/dp/0801483050; Baker and Bellis 1993https://www.researchgate.net/publication/251181587_Human_sperm_competition_Ejaculate_manipulation_by_females_and_a_function_for_female_orgasmhttps://www.researchgate.net/publication/229125576_Human_sperm_competition_ejaculate_adjustment_by_males_and_the_function_of_masturbation_1993 & 1995https://www.researchgate.net/publication/233820383_Human_Sperm_Competition_Copulation_Masturbation_and_Infidelity; Miller 2000)
# The human penis is more visually accessible than other primate penises and thus has evolved via sexual selection. (Short 1980https://www.researchgate.net/publication/313667387_The_origins_of_human_sexuality; Diamond 1997https://www.amazon.com/Why-Sex-Fun-Evolution-Sexuality-ebook/dp/B0054QML8E)
==Penis size==
The average human penis is between 13-15cm in length and 10-12cm in circumference when erect (measured with a bonepressed ruler):
*calcSD penis size calculators and charts
Let's see what other primates look like in comparison:
*Pan Troglodyte
**Erect Length 14.4cm (Dixson and Mundy 1994)https://www.researchgate.net/publication/15175686_Sexual_behavior_sexual_swelling_and_penile_evolution_in_chimpanzees_Pan_troglodytes
*Pan Paniscus
**Stretched length 17cm (Dahl 1988)https://books.google.co.za/books?hl=en&lr=&id=K6XZOOinwPAC&oi=fnd&pg=PA133&ots=vEIkP7x_Yy&sig=RRuhkvrZigcY6Y-J-OlYpzmTAyg&redir_esc=y#v=onepage&q&f=false
*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 than the bonobo (Stretched length and erect length are approximately the same lengths). It should also be noted that all the above measurements are greater than what was originally claimed by Smith 1984, an influential proponent of the idea that the human penis is unique compared to other great apes. Smith reported measurements of 8cm for the chimpanzee, 4cm for the orang-utan, and 3cm for the gorilla. In his book about human mating systems, Primatologist Alan Dixson has said 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."https://www.amazon.co.uk/Sexual-Selection-Origins-Mating-Systems/dp/0199559430
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.https://www.researchgate.net/publication/5690037_Why_the_Human_Penis_is_Larger_than_in_the_Great_Apes 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' during coitus. 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 losses amongst his sperm and thus lower the chance of pregnancy. If a man with an average penis with 14cm ejaculated up to 4cm inside the vagina, his sperm does not need to travel very far to reach the uterus. Thus there exists 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 co-evolution hypothesis.https://www.researchgate.net/publication/5666768_Clitoral_variability_compared_with_penile_variability_supports_nonadaptation_of_female_orgasm
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 larger? 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.https://www.researchgate.net/publication/337052115_The_Clitoris-An_Appraisal_of_its_Reproductive_Function_During_the_Fertile_Years_Why_Was_It_and_Still_Is_Overlooked_in_Accounts_of_Female_Sexual_Arousal 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. In contrast, the man with a 15-18cm long penis will place his sperm closer 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 may be less fertile than men with average penises; who knew.
==Sperm competition==
===Human penis size and morphology===
It is useful to examine how sperm competition has influenced the morphology of the penis in other primates where sperm competition is present. What we find in these multi-male/multi-female mating systems is that males have larger testes and have more complex penis morphologies than what is typically found in polygynous and monogamous species.https://www.researchgate.net/publication/264936216_Sexual_Selection_and_the_Evolution_of_Copulatory_Behavior_in_Nocturnal_Prosimianshttps://www.researchgate.net/publication/247964388_Baculum_length_and_copulatory_behavior_in_Primateshttps://www.researchgate.net/publication/230099068_Observations_on_the_evolution_of_the_genitalia_and_copulatory_behaviour_in_male_primateshttps://www.researchgate.net/publication/233820898_Primate_sexuality_comparative_studies_of_the_prosimians_monkeys_apes_and_human_beings 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. That the human penis is relatively simple in shape controverts the notion that sperm competition has has a exertedd significant influence its 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 did either 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.
===Sperm displacement hypothesis===
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 (yes, literally dildos and fleshlights). They contend that the 'large' diameter of the human glans and corona in particular 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) are 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 (who experiences virtually no sperm competition) and differs 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 the more promiscuous chimpanzee and bonobos, it becomes very obvious that the human penis has not evolved adaptations 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.https://www.researchgate.net/publication/15935821_Anatomy_and_Systematic_Significance_of_the_Penis_of_the_Pygmy_Chimpanzee_Pan_paniscus
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.
Let's assume for a moment that rampant sperm competition was a consistent feature of ancestral human mating. For there to be selection pressures that, due sperm competition, have formed -what is apparently- a long thick penis to plunge out rival sperm, how quickly after insemination does your ancestral male have to copulate with a female to scoop out rival sperm before the sperm has passed into the uterus? Depending on how much mucous the cervix is secreting, it takes about 5 - 60 minutes for sperm to reach the fallopian tubes (oviduct), which means that it takes even less time for the (motile) sperm to leave the vagina and enter into just the uterus.https://www.researchgate.net/publication/313071206_Gamete_and_zygote_transport So, for the penis to have evolved due to the need for it to suction out rival sperm, ancestral females must've had multiple gangbangs around ovulation; this level of wild promiscuity is quite unlikely especially considering concealed ovulation in females which ensures that sexual activity doesn't converge around ovulation as much when compared to Chimpanzees and Bonobos. But let's grant that gangbangs were rampant and that the penis has evolved as a plunging device. If so, why has sperm competition selected for larger penises but not larger testicles when larger testicles (greater ammunition) are far more important in sperm competition?https://www.researchgate.net/publication/273420556_Sperm_competition_and_the_evolution_of_ejaculates_Towards_a_theory_base?_sg=WxI9-QVvCgZaqwgeYZZkhXN2B_a1XJhX2fXV-vVsNjFvfrLqSTqqcDKRhDlelFpy1DvFpaWozTagOMA
Another point that controverts penis plunger is the time it takes most men to ejaculate. Contrary to current popular notions, sex within traditional cultures (i.e hunter gatherer tribes) tends to not last very long, often lasting only a few seconds; this implies that premature ejaculation, far from signifying sexual dysfunction, was the norm (or was at extremely least common) for ancestral mating and that it is rather the contemporary notion that men ought to last longer during sex for the sake of their partner that is exceptional as opposed to the biological norm of premature ejaculation. However this does not bode well for 'penis plunger' theory as, if it were true, then there would be selection against premature ejaculation as men who lasted longer during sex would be able to scoop out rival sperm more efficiently than men predisposed to premature ejaculation (whom would be done with the entire ordeal within a few thrusts). In an eviroment where it would be common for a woman to be carrying rival sperm in her vagina when mating with a man then (assuming the penis is ergonomic for the purposes of plunging out sperm) men who can effortlessly mate for long periods of time out to be selected for; that this is no the case, i.e the ability for most men to have sex for prolonged periods of time relatively effortlessly, implies that selective pressures for the ability for prolonged periods of sex in men were lax/non-existant and the corollary of which effectively debunks plunger theory as plunger theory would predict selective pressures in favour of prolonged sex.
A final point to undermines the displacement theory is the assumption that, in the absence of sperm competition, the human penis ought not to possess any capacity for sperm displacement. Indeed one has to wonder if it even possible for the penis to both perform its function of delivering sperm adequately and simultaneously be unable to displace any sperm whatsoever; were this true then a reduction in the ability to displace sperm would also result in a significant reduction in the ability to deliver sperm as the two properties cannot be divorced easilly. Under this view any capacity for the human penis for displacing sperm would thus arise as a byproduct from the intense evolutionary pressure for the penis to efficiently place sperm and need not refer to sperm competition to explain this capacity. Indeed one argument in favour of the displacement theory is the fact that men experience a refractory period after ejaculation (going limp). The question arises then, why ought this refractory phase be neccesary if the penis did not possess the capacity for sperm displacement? Surely there'd be no harm for a little romping after ejaculation if it did not? Ergo (they conclude) the penis must have the capacity for sperm displacement (thus the refractory period ensures that the male does not displace his own semen) which must of arisen due to sperm competition.
This is erroneous for a number of reasons. Firstly it assumes that the only possible (though still plausible) reason for a refractory period is to prevent the displacement of one's semen. Another possible reason could be to prevent subsequent matings. There are two reasons as to why multiple serial matings could be disadvantagous. The first being that, assuming he's mating with the same female, the male might go on to mate until he's exhausted and has no more sperm to offer. The excess sperm he's delivered to the female might put her at risk for polyspermy which will in turn lower his reproductive fitness. The second reason would be sperm depletion. Your average male can ejaculate about six times in one day before depletion.https://www.researchgate.net/publication/37713987_Promiscuity_An_Evolutionary_History_of_Sperm_Competition
 A male might benifit from have sperm reserves at all times so as to strategically mate with females should the oppurtunity arise. As we can see, the ability to displace sperm need not neccesarilly cause selection for a refractory period though it still could. The next point being that it assumes that the capacity for displacement and the capacity for sperm delivery are independant properties; if this were not the case, then the capacity for sperm displacement could of arisen as a byproduct (as previously explained) and (due to the difficulty in divorcing these two functions) a refractory period was selected for to ensure the male did not scoop out his own semen.
Unlike testes size, which is an established metric by which to assess the level of sperm competition within a species, the capacity and the extant to which sperm displacement occurs during thrusting in other primates has not been measured. So even if we grant that the shape of the penis was subject to pressures due to sperm competition, it would be impossible to tell just exactly how much sperm competition it was subject to unless between species comparisons are made. Thus the conclusion that sperm competition has played a large role in human evolution based on this is premature.
So ''penis plunger'' theory is unsound because of a number of reasons; namely: The evidence for it comes from artificial samples, not organic ones; thus, there is no evidence that 'penis plunger theory' even occurs outside the minds of evolutionary psychologists. The specific shape of the human penis is common amongst a variety of old-world monkeys (monogamous or not), and thus the specific shape of the human penis is a vestige of these penis shapes. In order for the "penis plunger" theory to be valid, gangbangs around ovulation must've been a common feature of ancestral mating, common enough at least to exert consistent selection pressure on the human penis. This is unlikely considering concealed ovulation amongst human females. Penis plunger theory is reliant on the existence of sperm competition in humans, which, across a multitude of species, demands larger testicles in species adapted to it; the lack of large testicles in homo sapiens implies little to no sperm competition, which as an extension means the penis plunger theory is invalid. The norm for most modern men is to ejaculate prematurely (and that most men have to discipline themselves to last for an adequate duration) rather than to effortlessly copulate for prolonged periods of time (and thus more efficiently scoop out sperm) effectively belies penis plunger theory as false. Finally, penis plunger theory makes the unwarrented assumption that the capacity to displace semen ought only arise due sperm competition.
===Sperm quality trade off===
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 can be/are subject to attacks from the male's own immune system in the reproductive tract; certain physiological mechanism protects the sperm (albeit incompletely). As a result, higher immune function ensures sperm quality suffers. High-quality ejaculates require the suppression of the immune system.https://www.researchgate.net/publication/46512336_Does_immunity_regulate_ejaculate_quality_and_fertility_in_humans?_sg=xHfBJm2sTb3zphkEjf2IhaqmXIoKpGj7gHsSzwMzndaWgfPrptmBYumK-zEpQibd4nqITBca_xb81kM Thus men with superior sperm will likely have inferior immune systems to men with lower quality sperm (which could lead to arguments that semen quality can function as an honest signal of a well functioning immune system). Secondly as the germline of the seminiferous tubules of a man's testicles undergoes more mitotic divisions the number of de novo (new) mutations in his sperm increases linearly.https://www.researchgate.net/publication/230722028_Rate_of_de_novo_mutations_and_the_importance_of_father%27s_age_to_disease_risk?_sg=AvntIrxSI1JJhejm_JUK8JQEZ_KYkcJUHDPsEyPnvGShJXz6slEn3vuvcFTnbep-fdKb0Cse485uiJw
https://www.nature.com/articles/nature11396 Thus we find that older males and males with greater sperm production (and thus greater ejaculates) will tend to produce lower quality offspring.https://www.researchgate.net/publication/236919261_Properties_and_rates_of_germline_mutations_in_humans?_sg=YDhr99RNhSMDUa42Y8wrMcZ0e9ulLiWfeFwXU8FA2hg-IvRqZwdbXdxVsGU_RIFnNasB-MaA7n0L7p8https://www.researchgate.net/publication/44801805_Male_Reproductive_Senescence_Causes_Potential_for_Sexual_Conflict_over_Mating?_sg=Byqzjb16GvLgyFPsq6XFIxsKWCjJAsuShRwswKcO3XI8z1kL6s2LXp0n-KHig2OqSBecE4sIY-GpWGohttps://www.researchgate.net/publication/224950855_Are_old_males_still_good_males_and_can_females_tell_the_difference?_sg=eqdOaxWkzD2GdLQ-rzfMguVPhnSlCGXw7cYcK-6FY0F0HVmQoqI-Lj6JGtcRH6mk-0sqOsi46jyXeYc Thus larger ejaculates that are advantageous during sperm competition is not a desirable trait a woman would like to have for her sons unless sperm competition was extremely prevelant (in other words when the advantages of superior ejaculates in sperm compeition out way the negatives of weaker immune system and more deleterious mutations in offspring).
=== Adaptations to sexual disease===
Sperm competition by definition requires polyandry to be present in a species. Species with high levels of polyandry and promiscuity also tend to have more sexual diseases and thus will evolve and adapt immunity against these diseases. Amongst primates, species with intense polyandry also tend to have much higher blood cell counts and humans have white blood cell counts that are similiar to the gorrila and the gibbon.https://www.researchgate.net/publication/12255121_Promiscuity_and_the_Primate_Immune_System Thus the lack of strong adaptations to sexual disease in humans is an argument against rampant promiscuity in ancestral mating systems and also thereby serves as an argument against sperm competition by extension. 
===Polyspermy and reproductive success===
Pathological polyspermy is the event whereby more than one sperm fertilize an egg simultaneously, resulting in a triploid embryo that is either inviable or sterile. Greater numbers of sperm in utero as a result of polyandrous mating will increase the risk of polyspermy.https://www.researchgate.net/publication/11012754_Dispermy--origin_of_diandric_triploidy_Brief_Communication?_sg=qYTQkAN1YbAm4r3V7W7BN34PsqatnNAOcL9vX7OBywRnc596TarNXWP9HpEGkxW76HMhmuT3fsLDYk8https://www.researchgate.net/publication/10703374_How_does_polyspermy_happen_in_mammalian_oocytes?_sg=Jriu-ZJgSs-2wR8eMnzU7PgnZzPZ5_D5yNLORRlZC2sIqclUVdcCUqb1jj3h5iF4VcOupn7qvwxWAPwhttps://www.researchgate.net/publication/44575325_Trisomy_and_triploidy_are_sources_of_embryo_mortality_in_the_Zebra_Finch?_sg=-fEWYtRD-N8PLnNyNpWYeMqOlze3BFEI9IZ01_Q4vSSqxpQ-OCE0XElgPX6fATHzKi3jT7O-WXSTWVMhttps://www.researchgate.net/publication/227528564_The_evolution_of_infertility_Does_hatching_rate_in_birds_coevolve_with_female_polyandry?_sg=h-o2cujeap994gpHpeQEzpd1i6w26DkYFYGBhErIgWeWBhqjj9mg308t2zEPJA86Odllb_x3yMaEREA Thus mating with multiple men during ovulation will result in lower reproductive success. Note that polyspermy accounts for around 20% of spontaneous abortions (miscarriages prior to 20 weeks).https://www.researchgate.net/publication/321590585_Bioethics_and_the_Fetus_Medical_Moral_and_Legal_Issues This also an argument against rampant promiscuity in ancestral females as polyspermy is much more prevalent when a woman has sex with a variety of men during ovulation as the greater amount of sperm from multiple men increases the likelihood that more than one sperm fertilizes the egg. Unlike chimpanzees and other promiscuous species who have developed adaptations to defend against polyspermy with longer, more coiled oviducts that serve to limit the amount of sperm that reach the egg, human females by contrast have a short, straight oviduct.https://www.researchgate.net/publication/279429660_Sperm_Competition_in_Mammals?_sg=9a2A4optxFPMUBqhq0q4UYr_294PNUosOZDTCkZSdoVWtreBVXCU4qja2rcdiW5j_KVIs0XXlCswPcAhttps://www.researchgate.net/publication/14810355_Coevolution_between_Male_Ejaculates_and_Female_Reproductive_Biology_in_Eutherian_Mammalshttps://www.researchgate.net/publication/229958746_Mammalian_sperm_and_oviducts_are_sexually_selected_Evidence_for_co-evolution This means that female humans do not display adaptions to prevent polyspermy in the presence of abundant sperm, which means that selection pressures for long oviducts were weak/minimal and by extension implies sperm competition was also weak/minimal. This undermines the notion that all (although a vocal minority of particularly sexually promiscuous women might be) women are sluts who have to procure sperm from a multitude in order to get the best sperm.
=== Sperm length ===
A recent meta-analysis reviewing research on sperm competition spanning over 50 years has reached the conclusion that greater sperm competition results in greater total sperm length, midpiece length, sperm head length as well as flagellum length: "In addition to sperm total length, our meta-analysis revealed that, overall, selection under sperm competition also appears to favour longer sperm heads, midpieces and flagella."https://www.researchgate.net/publication/346269633_How_sperm_competition_shapes_the_evolution_of_testes_and_sperm_a_meta-analysis They note that their findings that: "The increase in both midpiece and flagellum length is consistent with the predicted selection for faster sperm, in that the flagellum is the component propelling the sperm forward, and the midpiece generates the necessary energy."https://www.researchgate.net/publication/267043501_Sexual_selection_and_the_evolution_of_sperm_qualityhttps://www.researchgate.net/publication/23496618_Sperm_competition_Linking_form_to_function Thus it would apt to compare how Homo sapiens compare to other primates in these dimensions.
A comparison of Sperm Length, Midpiece Length and Volume of Various Primateshttps://www.researchgate.net/publication/11432422_Sperm_competition_-_Motility_and_the_midpiece_in_primateshttps://www.researchgate.net/publication/231916755_Sperm_competition_and_the_evolution_of_sperm_midpiece_volume_in_mammals
Species 	Total Length(µm)	Midpiece Length(µm)	Midpiece Volume(µm^3)
Vervet 	91.5 	12.1 	11.5
Patas 	92.8 	14.5 	6.5
Stump-tail 	78.8 	10.5 	10.2
Mandril 	45.7 	7.1 	8.0
Hamadryas Baboon	77.2 	12.7 	8.5
Gelada 	88.2 	12.3 	6.5
Siamang 	68.9 	8.2 	6.8
Orangutan 	60.5 	8.9 	3.9
Chimp 	60.4 	6.3 	7.8
Bonobo 	68.1 	8.9 	9.3
Gorilla 	52.3 	13.2 	6.9
Human 	56.9 	5.9 	3.8
As we can see, humans rank third-lowest in total length and rank lowest midpiece length and volume. This indicates low sperm competition, and the extremely low midpiece lengths in humans (even in comparison to the gorilla) is somewhat of a surprise. Though humans are generally socially monogamous and would thus experience slightly more sperm competition than the gorilla, we would expect larger midpiece length and volume. Thus it is unlikely that extra-pair paternity and sperm competition were prevalent throughout our evolutionary history (extra-pair paternity rates above 10% are likely inaccurate, though rates do differ heavily by ecology and culture).
===Ethnic differences in testes size===
Two studies from Diamond and Short provide differences in testes volume across countries. This might illuminate how differences in sperm competition would've affected different populations (though overall even countries that have large testes will still have minimal sperm competition compared to the promiscuous chimpanzee and bonobo). 
An alternative explanation for differences in testes size across different countries that doesn't require sperm competition to be the casual evolutionary factor involved is that testes size and dizygotic twinning are positively correlated. According to the out of Africa theory of human evolution, as humans migrated out of Africa towards Europe and then into Asia, women became progressively smaller, more petite, and thus wouldn't be able to handle giving birth to twins as readily. This would result in selection for smaller testes in men. Anecdotally speaking, this may be suggested by the fact that interracial pregnancies between Asians and Europeans have more complications (like more C-Sections, for example) than between two Europeans. Twinning doesn't occur exclusively in humans and is prevalent in Marmosets and Tamarins who have large testes for their size.https://www.researchgate.net/publication/15931759_Testis_Weight_Body_Weight_and_Breeding_System_in_Primates?_sg=Azp6Heikf_3ap7BhH_z5BsfxFzwo0z_Ill1WLqyBmVAPvsBwJbHnYSmDRbtxVW9LCaEaDMkGs-JwY3Y Dixson, the author of this paper, noted that: "Larger than expected testes sizes in some tamarins and marmosets may be linked to the genetic predisposition to produce larger gonads and higher levels of reproductive hormones in both sexes of these monkeys." Indeed, one paper released recently found that rates of twinning appear in a similiar hierarchy to what we can find in differences in testes size.https://www.researchgate.net/publication/350034962_Twin_Peaks_more_twinning_in_humans_than_ever_before Across continents the rates of twinning were, per 1000 births, about: 17.1 in Africa, 9.2 in Asia, 14.4 in Europe, 16.9 in North America, 14.8 in Oceania, 9.3 in the South Americas and a worldwide average of 12.0. The countries with the highest rates of twinning were all West and Central African countries and many Southern and East African countries being comparable to many European ones. Countries that had the lowest rates of twinning were South American countries, South Asian and East Asian countries.
Though a sperm competition explanation doesn't seem implausible either, seeing as Asians are more K-selected than other groups; ergo it follows that Asian women would be more faithful wives than women than other races and that West Africans (As Nigerians have the largest testes), and by extension women of West African descent, are not as faithful as other races. Considering that the most unfaithful tribe in the world also happens to be Nigerian, this doesn't seem to an impossibility. Coincidentally, rates of infidelity also appear in a similar hierarchy when compared to testes size; with Asians cheating the least, Europeans cheating moderately, and Africans cheating the most, which is in line with the predictions of .https://ifstudies.org/blog/number-1-in-2018-who-cheats-more-the-demographics-of-infidelity-in-americahttps://www.researchgate.net/publication/264486868_The_Science_of_Two-Timing_The_State_of_Infidelity_Research?_sg=bKbmmatzaE9rLYygqPcvJ3ARxk4St_oUXVkuOeIJDbIwRq7jAFxynzirp7eEqz06goKlzZbI-t5TQjghttps://www.researchgate.net/publication/14680774_Sexual_behaviour_in_developing_countries_Implications_for_HIV_control
In Western countries, men typically have an infidelity rate of between 20%-25% and women between 10%-15% (though the gap is closing among younger couples in the U.S.https://incels.wiki/w/Scientific_Blackpill#Half_of_women_in_relationships_report_maintaining_a_.27back-up.27_partner_in_their_social_circle); in Guinea Bissau (yet another West African country), 35%-40% of the men cheat, and 15%-20% of the women do; finally, in Hong Kong, 5%-10% of the men cheat, and 1%-3% of the women do. We find that West Africans have both the highest rates of infidelity and the largest testes, and in Hong Kong, men have the smallest testes and the lowest rates of infidelity. The rate of cuckoldry to infidelity in western women is about 1:8.6Promiscuity#Human female promiscuity; this predicts a rate of cuckoldry of 1.7%-2.3% in West Africans and a rate of cuckoldry of 0.1%-0.3% in East Asians as compared to Europeans with a rate between 0.5%-1.8%. This difference in cuckoldry still seems rather small when compared to testes size; sperm competition likely doesn't solely account for variance in testes size.
Neither hypothesis seems to be incorrect and might actually be compatible, i.e that sperm competition variance demanded larger testes in some countries and twinning rates were higher as a result of those higher testes and that women simultaneously become more petite as we became more K-selected in Asian countries (which would create pressure for smaller testes both because of reduced sperm competition and because women were less petite). Indeed, having many children whilst investing little into each child is a feature of r-selected species and we could thus infer that from the rate of Dizygotic twinning.
A table comparing testes size across countrieshttps://www.researchgate.net/publication/288988606_Testis_Size_Ovulation_Rate_and_Breast_Cancerhttps://www.researchgate.net/publication/20073552_Ethnic_differences_Variation_in_human_testis_Size
Country 	Combined Weight of Testes (g)*	Dizygotic twinning rates (per 1000 births)
Nigeria 	52.6 	40.0
United Kingdom 	48.3 	48.3
France 	47.2 	8.9
Sweden 	43.6/46.6/49.6 	8.6
Switzerland 	39.0 	8.1
Korea 	32.9/36.8/49.6 	5.1/5.8/7.9
India 	35.4 	6.8/7.3/8.1
Japan 	31.5/34.1/36.7 	2.3
China (Hong Kong)	19.0/17.7/16.5 	6.8
*Volumes of table 2.2 converted into weights by using a correction factor (x 1.05)
And here's a more exhaustive table covering a wider variety of studies.
Testes size across various countries
Country 	Combined Testes size(Various units of measurements)
China (Hong Kong) 	19.01ghttps://www.researchgate.net/publication/9875412_Scrotal_asymmetry_and_handedness
Japan 	30.0mlhttps://www.researchgate.net/publication/316293330_NORMAL_GROWTH_AND_MATURATION_IN_THE_MALE_GENITALIA_OF_THE_JAPANESE
South Korea 	31.1mlhttps://www.researchgate.net/publication/11212609_Factors_influencing_testicular_volume_in_young_men_Results_of_a_community-based_survey
India 	35.4ghttps://www.researchgate.net/publication/230043617_Weight_of_the_testes_in_Northwest_Indian_adults
Australia 	48.2mlhttps://www.researchgate.net/publication/222661005_Human_sperm_competition_Testis_size_sperm_production_and_rates_of_extrapair_copulations
United States of America	36.3g*https://www.researchgate.net/publication/16502471_Influence_of_age_on_sperm_production_and_testicular_weights_in_men/50.2g**https://www.researchgate.net/publication/230318893_The_relative_weight_of_testes_in_primates
United Kingdom 	46.0mlhttps://www.researchgate.net/publication/11256562_Regional_differences_in_semen_qualities_in_the_Baltic_region
France 	45.0ml
Finland 	46.0ml
Finland 	46.0ml
Denmark 	47.0ml
Sweden 	41.5mlhttps://www.researchgate.net/publication/8271258_The_Frequency_of_Mumps_and_of_Mumps_Orchitis_and_the_Consequences_for_Sexuality_and_Fertility
Switzerland 	37.2mlhttps://www.researchgate.net/publication/18330822_Testicular_volume_during_adolescence_Cross-sectional_and_longitudinal_studies
Czechoslovakia 	37.4mlhttps://www.researchgate.net/publication/18047385_Basic_morphological_data_of_external_genitals_in_177_healthy_Central_European_men
Nigeria 	50.1mlhttps://www.researchgate.net/publication/19136185_Anthropometric_study_of_male_external_genitalia_of_320_healthy_Nigerian_adults?_sg=6TmP4OEGVZc0Q9vCVsv2PS5c2ulrLswiSxUs9vnm31KT6SC_IvqP9KTgT96OkyHMZSqvRrzSuhN5H9M
*Mainly white
**Mainly black; sample size of 3
===Testes size comparisons amongst great apes===
Comparisons of populations within our species might illuminate as to how sperm competition, amongst other factors like twinning, would affect testes size. Humans, as has been shown, can fall anywhere between 20-50 grams in testes size which is consistent with the testes sizes, relative to body mass, of many monogamous and polygynous species as we shall soon see with between species comparisons of humans and other great apes.
Testis size and body weights of various great apes.https://www.researchgate.net/publication/229679901_Observations_on_sexual_development_in_male_Orang-utans_Pongo_pygmaeus?_sg=h-WebA4iZnOereVSJpCSJ3WuyyuxW8FUpWghc7rlVwoGRRIVOMP2iN4QW3oGSoHiTieWPEdLaRaPKLghttps://www.researchgate.net/publication/14768065_Testicle_size_of_orang-utans_in_relation_to_body_size?_sg=9rPt3ruViR47nUDOuvwaNXacaODt-rSl41FqygYdr-DGxd93iSJMEbR1mjoeN-G-_0cxamep_4dn3YMhttps://www.researchgate.net/publication/230318893_The_relative_weight_of_testes_in_primates?_sg=tciK0xbc5j-FIXGLL_899rxTIQemxvSweglASMGodpsomG-_h0jZSjvSI3-2D1ibwQlPogrMXxkJsHwhttps://www.researchgate.net/publication/15175686_Sexual_behavior_sexual_swelling_and_penile_evolution_in_chimpanzees_Pan_troglodytes?_sg=3oSZJ5ZCtGeXbRV94uNBkRaYOPPL4o4gmRnoURtt6e9XbYNSZwto-6Anz-VIHs3pMl87sknE9zh01Ikhttps://www.researchgate.net/publication/8227398_Sexual_behavior_reproductive_physiology_and_sperm_competition_in_male_mammals?_sg=8fwCCPTSQ1kcnUK-xyNhciQXvHzCohsJR8v7PD_bYLQO8Wnx63y2bSLZ83EF7YDuwfIWLOj50Xpxe3chttps://www.researchgate.net/publication/230093431_The_testis_of_Gorilla_gorilla_beringei?_sg=iDCLToquhvp-82lR20AA9DKda-nQ_VwZh2knTVIKf5TpGp4Fx3taYq1CYdNk5Y2WgHa9J9aqB3oVzichttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC3408371/
Species 	Testis size (g)	Body Weight (kg)	Testes to body weight ratio
Pongo pygmaeus 	36.53 	101.3 	1:2773
Pongo abelii 	34.87 	105.7 	1:3031
Pan troglogytes 	148.89 	53.22 	1:357
Pan paniscus 	167.7 	47.95 	1:286
Gorilla g. beringei	28.96 	164.66 	1:5696
Homo Sapiens 	20-50 	60-80 	1:3000/1:1600
As we can see, humans, when compared to gorillas, at least 1.9 times as large in relative testes size and at most 3.6 times as large in relative testes size; whilst human females obviously aren't as close being to as faithful as female gorillas (the fidelity of female gorrilas is in large part due to lack of oppurtunity) they are still comparitively very faithful; especially when compared to more promiscuis species like the bonobo whom at have testis that are relatively larger at most 10.6 times larger and at least 5.6 times larger. The relative difference between humans and chimps compared to between humans and gorillas in testis size is anywhere between 19.8 or 6.4 times larger; in other words we're much closer to gorillas in terms of sperm competition that we are to chimpanzees; making it very doubtful that sperm competition was extremely prevalant in our ancestral enviroment. It is most likely that ancestral females where for the most part faithful with on average 10%-25% of females cheating (if contemporary statistics are reflective of our ancestral enviroment); going by the extremely low rate of non-paternity in humans (between 1%-2%), almost all infidelity on the part of females likely wasn't about the sex in and of itself per se (i.e to aquire good genes to cuckold her mate) but rather to aqquire resources and to secure a back up mate should her current, possibly prefered mate, leave or die.
===Sperm pleiomorphism===
Much of the sperm that human males produce tend not to be functional; most deviate from typical model of human sperm (i.e a single oval shaped head with a long slender tail and midpiece connecting the two) by having two or more heads, having oddly shaped or bent heads, multiple or crooked tails etc. These deviations from the typical model of human sperm are known as pleiomorphisms. This is not only true for humans and has been observed in gorrilas as well.https://www.researchgate.net/publication/16265348_Chromosomes_and_spermatozoa_of_the_African_great_apes?_sg=wdvdCN-Wd6lbU_Kdayp_2abQ7i-6GdqNhb9CwFkQfrViL5KxSJed7q0LhwZf8EifWlm0nWqtkY5Dtu4https://www.researchgate.net/publication/22577822_Morphological_abnormalities_in_spermatozoa_of_man_and_great_apes?_sg=z0JUjPIBJvDPEWuWi7F2dxf2AImSPiukUWtDLVyeGzti_BA68ZzjwkoleBlvVCc9M13Y6RgRRTQ8saY In his book, Sperm Wars, Robin Baker wrongly assumed that these abnormalities were something more than just abnormalities. For example, sperm with multiple heads were identified as 'blockers' whom would prevent other rival sperm from passing by them and that other oddly shaped 'killer' sperm are meant to bump into rival sperm to incapacitate and kill them by releasing chemicals from their acrosomes (tip of the sperm). However, studies mixing the ejaculates of multiple men (to test if the so called killer sperm would start killing rival sperm) have not found any evidence for the Kamikaze Cum hypothesis.https://www.researchgate.net/publication/12671631_No_evidence_for_killer_sperm_or_other_selective_interactions_between_human_spermatozoa_in_ejaculates_of_different_males_in_vitro?_sg=wVcLWU_rWoQU_q5KQbMHEO3LD8C05zlR8E5cWaiMGBiD1XeoLtwvcv4O07GPAZ9URKsMCbAMa4eEbsw Amongst the great apes, pleiomorphism is greatest in Humans and Gorrilas and weakest in Chimpanzees (whom experience far greater sperm competition).https://www.researchgate.net/publication/314680528_Deformed_Sperm_are_Probably_Not_Adaptive?_sg=nz1LTD4WNVhESroavqRw6ZWyyA2BDOD6YmTGmeuaZ-9WgOMhhid4DcH5DbXJqRorKofbaSR0rzv_RbQ This demonstrates that Sperm Competition doesn't select for high levels of Pleiomorphism but rather lower levels of Pleiomorphism (Pleiomorphism is the results of errors during spermatogenesis; selection pressures will thus elimate the genes responsible for these errors as the sperm need to be more efficient to succesfully compete with rival sperm) and that high levels of Pleiomorphism is due to lax selective pressures for more efficient sperm which by extension implies sperm competition is minimal or absent. So much for the Kamikaze Cum. Human and Gorrilla ejaculates have roughly 26%-30% of their sperm suffering from some form of pleimorphism as compared to Chimpanzees and Bonobos in the 2%-5% range. In fact the author writes:"The modal cell types in the ejaculates of men and gorillas were also morphologically identical. The less frequent cell types defined as morphologically abnormal spermatozoa were also very similar and occurred in similar proportions. It is therefore impossible to distinguish between man and the gorilla by a simple examination of the ejaculate, although it is possible to distinguish between man and the chimpanzees or between the gorilla and the chimpanzees. Both species of chimpanzee produced identical spermatozoa."
===Genetic evidence===
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 show that bonobos exhibit much lower variation (in these genes), thus indicating that (compared to chimpanzees) bonobos have reduced sperm competition. It is important to note that gorillas and orangutans do not have much variation in the same genes either.https://www.researchgate.net/publication/46170901_Y_Chromosomal_Variation_Tracks_the_Evolution_of_Mating_Systems_in_Chimpanzee_and_BonoboSeven regions on the Y Chromosomes involved in the sperm production are identical in humans and gorillas; 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 the promiscuous mating system of chimpanzees only evolved after they'd split from our lineage; this essentially means we did not evolve out of a promiscuous mating system, and we always socially monogamous or polygynous' https://www.researchgate.net/publication/6582334_The_Evolutionary_History_of_Human_and_Chimpanzee_Y-Chromosome_Gene_Loss
On a more humorous note, bat species that have promiscuous females have larger testes because of sperm competition but smaller brains when compared to bats that have faithful females.https://www.researchgate.net/publication/7170875_Mating_system_and_brain_size_in_bats 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 ==
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.https://www.researchgate.net/publication/6638728_Studies_of_human_physique_and_sexual_attractiveness_Sexual_preferences_of_men_and_women_in_Chinahttps://www.researchgate.net/publication/6661420_Human_Physique_and_Sexual_Attractiveness_Sexual_Preferences_of_Men_and_Women_in_Bakossiland_Cameroon An Australian study has found a preference for larger non-erect penises using 3D models.https://www.researchgate.net/publication/236140654_Penis_size_interacts_with_body_shape_and_height_to_influence_male_attractiveness 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.https://www.researchgate.net/publication/292671435_Size_did_not_matter_An_evolutionary_account_of_the_variation_in_penis_size_and_size_anxiety
Penis size likely wasn't a priority for ancestral females who wanted children and hunting ability was deemed more important (and much mating was arranged and penis size likely also wasn't exactly a priority for a woman's parents). 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 had possibly been invented 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 could even select based on penis size.
Height, for example, is a lot easier to visually 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 behavioral pattern that can be observed cross-culturally.http://joelvelasco.net/teaching/2890/brownlisthumanuniversals.pdf Some selection pressure for penis size could also be exerted after the formation of the relationship and first intercourse, as some evidence does suggest a substantial minority of women are willing to dissolve a relationship with a man if his penis is "too small" (21%) compared to it being "too large" (7%).https://incels.wiki/w/Scientific_Blackpill#Women_most_prefer_penises_longer_than_84.8.25_of_all_men.27s So, particularly in societies that did not place a high emphasis on pre-marital chastity and post-marital fidelity, it is clear to see how a weak sexual selection pressure for larger penises could have been exerted (likely being largely driven by a general female aversion against very small penises rather than a preference for very large penises), perhaps explaining the existence of likely racial/national difference in penis size to some extent.https://calcsd.netlify.app/datasets
===Erect vs. flaccid penises===
There 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. Indeed this is in line with Weber's law of cognitive bias in signal processing, which states that a unit increase in a signal (like how bigger penises signal attraction) will have less of an effect on the reciever of the signal as the signal gets progessively larger.https://www.researchgate.net/publication/330018898_A_Taste_for_the_Beautiful_The_Evolution_of_Attraction?_sg=339CLcyr2mn0SnguIcpGcfB4dZ8EyTDqWEljNae4WkCVL_3mnVxd02NFPp27IcqPcBKMEPok7kJeBH0 To put it crudely a women might interpret a 5 inch penis as "Oh you're horny?" but an 8 inch penis as "Ok man, you're horny; I get it." 
To show that larger penises are rated as more attractive due to them being an indicator of arousal (as opposed to a preference for larger penises due to just a preference for larger penises), we can look at one study that measured the genital responses of women when exposed to non-preponent images (flaccid penises) and preponent images (erect penises).https://www.researchgate.net/publication/264086102_Gender-specific_genital_and_subjective_sexual_arousal_to_prepotent_sexual_features_in_heterosexual_women_and_men Preponent images were elicited a substantially greater sexual response than non-preponent images. Two features that distinguishes erect penises from non-erect penises are size as well as, well, erectness. Thus that longer non-erect penises are found to be more attractive because they are an indicator of arousal is sound because it can be shown that indicators of arousal (erect vs. flaccid) are attractive to women.
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 a 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 diminishing returns in the favorable effect of penis length on attractiveness ratings after 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
Percentage of penises erect at differing ranges
Range of Penises (")	Percentage of Penises that are erect
2.25> 	Less than 0.01
2.25-2.50 	0.26
2.50-2.75 	0.32
2.75-3.00 	0.96
3.00-3.25 	2.34
3.25-3.50 	6.24
3.50-3.75 	16.74
3.75-4.00 	42.15
4.00-4.25 	68.62
4.25-4.50 	88.97
4.50-4.75 	96.25
4.75-5.00 	99.25
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 at roughly the 80th percentile. Thus you might say: "Aha! See? Women really do and always have preferred larger than average penises."
===Abdominal fat and judgements of penis length===
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, body fat. The average American male sits at a body fat percentage of approximately 28.2%, which'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've 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 indeed died from starvation). 
One can conclude from this information 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 penis 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). 
So 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, which is 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 disadvantaged in a multitude of areas) your average male can expect to attain alpha status (i.e., be placed in the top 20% of physical attractiveness) 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
=== Innervation of the Vagina ===
If females were to sexually select for longer penises in mates because the additional length is more sexually stimulating than shorter penises, then we should expect that most of the innervation of the vagina to be situated further away from the entrace of the vagina (with the bulk of the nerves at the back). This will ensure females will not select males with shorter penises as their penises will not reach the pleasure regions in the vagina and will instead select males with longer penises whom will able to pleasure them. However, that is not what we find; instead most of the nerves are situated near the entrace and on the anterior wall (upper part of the vagina when a woman lays on her back) of the vagina.https://www.researchgate.net/publication/24182142_Innervation_of_Vagina_Microdissection_and_Immunohistochemical_Study?_sg=6RPK0zn5s9qVoXNqbA-x5CABA9XMuPpFTkPPlKZNvXJDmAHyFUpDGQtXlcRl6k-ixT7mTo2XZlSMM_8
Number of Terminal Nerve Branches to the Vaginal Wall
Row*	Right Lateral Wall	Anterior Wall	Left Lateral Wall	Posterior Wall
5 	0.14 ± 0.2 	0.03 ± 0.06 	0.07 ± 0.14 	0
4 	0.33 ± 0.25 	0.33 ± 0.36 	0.37 ± 0.14 	0.07 ± 0.08
3 	0.57 ± 0.09 	0.84 ± 0.18 	0.64 ± 0.32 	0.14 ± 0.11
2 	0.78 ± 0.33 	1.49 ± 0.51 	0.61 ± 0.16 	0.19 ± 0.61
1 	0.75 ± 0.35 	0.89 ± 0.46 	0.66 ± 0.47 	0.14 ± 0.11
*Rows are situated such that each number represents an equal partition of the vagina, with 1 being the closest partition to the entrance and 5 being the furthest
As we can see, most of the innervation is within the first 3 fifths of the vagina (roughly the first 4.2" or 10.7cm when aroused) with some innervation in the 4th partion (which is roughly situated inside the vagina at the 4.2-5.6 inches or 10.7-14.2 centimeters) and with almost none in the deepest parts of the vagina. Accounting for about a half inch of fat situated atop the penis which might obstruct thrusting, an increase in penile length beyond 6.1" or 15.5cm (the top 20% of penises) will essentially provide no marginal increase in stimulation as there is nothing to stimulate that far in the vagina (this is assuming that thrusting as deep as possible is the prefered method of thrusting amongst the majority of women, which it is not). An increase in penile length beyond 4.7" or 11.9cm (the top 90% of penises) will only ensure a small increase in marginal pleasure (still assuming that thrusting as deep as possible is prefered). Thus, for the vast majority of penises will be able to (length wise) provide similiar amounts of pleasure; with larger penises possibly providing somewhat greater pleasure with any length beyond 6.1" being obsolete. This is not to deny that penis size can never be an issue, men with penises smaller than 4.7" (roughly 10% of men) may not be able to effectively stimulate the most sensitive areas closest to the entrance of the vagina, but it does controvert notion that females will actively seek men with gargantuan pornstar penises and supports the notion that average penises are optimal (as large penises might drub the cervix and possibly cause pain as the cervix itself might be sensitive). 
=== Evidence from Dildos ===
One might look at the average Dildo length (7.5" or 19.1cm), compare it to the male average of 5.5" or 14.0cm, and go on to conclude that this is evidence against women preferring average penises.https://archive.fo/HfWlw However, if this is the case, then why is it that the average dildo circumference is almost identical to the average penis circumference? This is quite odd, especially considering the overwhelming majority of women (90%) agree that girth is more important than length. Clearly there is more than meets the eye at an initial reading of the data. The first thing to note is that approximately and inch of most dildos are unusable by design due to the fact that a woman needs somewhere to grip; this reduces the average usable length down to 6.5" or 16.5cm. However, as explained in the previous section, lengthwise there's almost no innervation past the first 5.6" of the vagina and thus an increase in (useable) length past that point ought not increase pleasure. This is despite the fact that the ratio of dildos sold that are below 7" in length (and thus 6" in usable length which can reach the first 5.6") to the ratio of dildos sold above 7" is 1:3.https://thethruster.com/blogs/news/the-most-popular-dildo-sizes (self calculated) So the question arises, why the overkill? Why the overkill in length but not girth (which is more important according to women)? The correct way to accord these two seemingly contradicting data points is not to say that "Well obviously women prefer larger penises otherwise women would buy 5" dildos and not 9" ones," and that therefore "I should tell my wife that she's allowed to fuck Pornstars and Chads with megalo schlongs because I can't match up."https://www.humpstump.com/penis-size-average/ See final two sections, you can't make this stuff up
Rather, one might consider that the dissimilarity in the mechanics of a man thrusting into a woman and a woman thrusting a dildo into her vagina. For one, the man's penis is anchored to him whereas the dildo has to be gripped (which neccesarily reduces usable length by an inch). Secondly a man can insert his penis into a woman rather easily whereas with a dildo this is done at an acute to the vagina; in to insert a dildo parallel into one's vagina, one's arm must extend past it a number of inches equal to the length of the dildo (which for most dildos and women is impossible). Thus, the requirement for a grip at the base of the dildo and the angle at which the dildo must be inserted into the vagina by the woman will result in and increase in length due to ease of use rather than an increase in stimulation/pleasure. If an increase in stimulation were the motivating factor, then we should find that women will buy thicker, not longer, dildos. Thus we can eliminate the hypothesis that "Dildos are longer than organic penises because women want greater pleasure" on these grounds which will leave ease of use as a better explanation for their inflated length. The fact that the circumference of the average dildo is identical to the average penis supports, rather than contradicts, the notion that average penis length and girth are the prefered measurements for inducing stimulation.
=== Male and Female Perception of Penis size ===
One paper also found that women whom reported having partners with small penises reported lower levels of satisfaction than when they reported their partner as being average or large.https://www.researchgate.net/publication/232595653_Does_Size_Matter_Men%27s_and_Women%27s_Views_on_Penis_Size_Across_the_Lifespan
Women's attitudes toward their partner's penis size
Perceived Size	%Very satisfied with size	%Desire Larger	%Desire Smaller
Small 	32 	68 	.04
Average 	86 	14 	.04
Large 	94 	1 	5
Due to its large sample size, all differences were statistically significant at the (p<0.001) level. However, though the disparity of satisfaction between women reporting partners with small penis sizes for their partner and women reporting larger penis sizes for their partner likely reflects actual anotomical difficulties with smaller penises simply due to the large disparity, we should be more skeptical of concluding that anotomical reasons are behind the disparity between women with Average and Large partners, namely because what counts as average and what counts as large is based off of self report and not actual measurement. 
Women might associate larger penises as being more pleasurable than average penises even though they might not be; thus we might some see women report their average partner as large because sex with them is very satisfying rather than because they are very satisfied from sex with a large penis per say. Evidence for this comes from the fact that 6% of women report their partner being small, 67% reporting average and 27% reporting large despite the fact that penis size follows a normal distribution. A normal distribution being that the distribution of data is symmetrical at the mean (which should mean that there should be equal amounts of people reporting small and large) and that 68% of the data fall within 1 standard deviation of the mean. If we define small as a standard deviation below the mean, average as a standard deviation within the mean and large as a standard deviation above the mean: we should find 16% reporting having small partners (as opposed to 6%), 68% reporting average (close enough to the 67% reported) and 16% reporting large (as opposed to the 27%).
That this is not the case can mean one of two things, either mean in relationships have much larger penises than the male population as a whole (something which still hasn't been measured to this day) or that women overestimate penis size qualitatively. Anecdotal evidence from men with above average penises suggests that women tend to overestime penis size significantly. For example, this one woman reports that:"I’ve been with just over a dozen men whose penises ranged from 5 inches to 14 inches and were of various girth. Size does matter when it comes to having an orgasm through intercourse, and length is less important than girth. My personal preference would be 9 to 10 inches and thick enough that I can’t quite close my hand around it. This size affords me the ability to have vaginal orgasms in multiple positions and isn’t so large that fellatio is a challenge." -JoyStatistically speaking, having a little more than 12 partners and encountering a man with a 14 inch penis (Jonah Falcon is only 13.5") but never encountering a man with a penis smaller than 5 inches is so statistically unlikely that she's either lying or overestimating the penis sizes of her partners by quite a large margin. The following table illustrates the number of partners (rounded to the nearest whole number) a woman would need to sleep with in order to encounter a penis of a given deviation from the mean or greater (i.e a penis a small/large or smaller/larger).
Number of partners required to have a 50% of encountering any given Penis Size
Penis Size(")	Number of partners required
<3 	6931
<3.5 	433
<4 	50
<4.5 	9
<5 	3
5.5 	1
>6 	3
>6.5 	9
>7 	50
>7.5 	433
>8 	6931
As we can see, as one moves further away from the average the number of partners required to have a 50% of encountering a penis that large/small or larger/smaller increases exponentially. The average number of lifetime partners for most women is between 4-7; thus this makes it unlikely that the majority of women will not encounter penises larger than 6.5" or smaller than 4.5" (unless she's really looking). Considering that one will genrally not encounter problems with one's penis unless one has a penis shorter than 4.7" (because of the innervation of the vagina) we can understand why most women will remark that 'size doesn't matter', most women will not likely encounter a partner with a penis short enough to be problematic as penises that short are uncommon. Thus when a woman states that size doesn't matter, she isn't neccesarily attempting to be duplicitous but has likely never encountered a partner to discover that size can, at times, be problematic.
Though this is not to say that men do not overestimate their penis size either, they do but less egregiously than women do; according to the same study 12% of men reporting having a small penis, 66% report average and 22% report large. Possible reasons that men overestimate their penis size amounts mostly to self esteem. Men who reported larger penises also reported being more attractive as well as greater facial satisfaction (seemingly unrelated variables) however this can be remediated if we account for the fact that more confident men that are higher in self esteem might regard themselves as more attractive and have larger penises even though they aren't/don't. Thus it isn't unreasonable to assume that satisfaction with one's partner might cause a women to report her partner as being larger than he actually is. Thus we might find that actual measurements of the penis size of a woman's partner might dissipate the disparity between women reporting an average or large partner.
=== Crude Re-estimation of Partner Size and Sexual Satisfaction ===
We can make an educated guess as to how Sexual Satisfaction might change for partner size by making a few adjustments to the data. Going by our definition of what constitutes Small, Average and Large, we know that 11% percentage points of the 27% percentage points whom rate their partner as large (or 40.74%) are likely average or upper average; we also know that 10% percentage points of the 67% percentage points of women whom rate their partner as average (or 14.93%) are likely small. The n's for Large, Average and Small are 7138, 17713 and 1586 respectively. 
Of the 7138 women who rate their partner as Large, roughly 2908 are average or upper average. If we make the assumption that the 1% (71) of women in the large group who desire large penises likely all have average partners; we can similiarly assume that the 5% (356) of women who desire smaller penises likely have large partners. Thus we can calculate that 91.58% (compared to the previous 94%) remaining women (4230) must be very satisfied with their partners.
Of the 17713 women who rate their partner as Average, roughly 2644 are small. If we assmue that the percentage of very satisfied women in this group is the same as the percentage for women in the self rate small group is the same (32%) which amounts to 846 women, then we can subtract 846 women from the (86%) 15233 self-rated average-partnered women who are very satisfied with their partners to get 14387. Similiarly we can arrive at the number 681 for the desire larger group; we then have to add the 71 women incoming from the large group to arrive at a final 752 desire larger. Simliarly we add the very satisfied women to arrive at final 18070 very satisfied. Thus we can calculate that 96.01% (compared to the previous 86%) of the new average group are very satisfied with their partners (the small group stays at 32% because of the assumptions).
So as we can see, a crude re-estimation of the Very Satisfied groups (using very reasonable assumptions) finds that women with partners who are average should actually be more slightly more likely to be very satisfied with their partners than women with partners who are large. This is consistent with the view that a marginal increase from average penis size add little to sexual pleasure.
===Arranged Marriages===
Most marriages in our evolutionary history were arranged.https://www.researchgate.net/publication/51111510_Evolutionary_History_of_Hunter-Gatherer_Marriage_Practiceshttps://journals.sagepub.com/doi/full/10.1177/1474704919887706 Secondly, when parents did decide whether or not a particular mate will be a good match for their daughter, they cared less about looks and more about resources and status when compared to the mate preferences of the child.https://www.researchgate.net/publication/225066511_Meet_the_parents_Parent-offspring_convergence_and_divergence_in_mate_preferences?_sg=ZXw3yNGJfkUg0P-etOGucAf4pfoCiw4m_6mzd2LguNtq53y0NQKjgSw6bR9AJ9R0lwisUw86-4QEG0khttps://www.researchgate.net/publication/40729210_Parental_choice_What_parents_want_in_a_son-in-law_and_a_daughter-in-law_across_67_pre-industrial_societies Although mate preferences are similar between parents and child (they tend to be correlated with about 0.83), it is quite unlikely that parents are going to reject a potential candidate because the amount of inches on his shaft is smaller than the amount of zeroes in his bank account.
== Female orgasm ==
A paper published 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, the authors conclude that larger penises are superior for eliciting orgasm via the vagina.https://www.researchgate.net/publication/233815146_Women_Who_Prefer_Longer_Penises_Are_More_Likely_to_Have_Vaginal_Orgasms_but_Not_Clitoral_Orgasms_Implications_for_an_Evolutionary_Theory_of_Vaginal_Orgasm 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-in-vagina intercourse reported in this study was 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 0.83 orgasm per PIV intercourse for women with a large penis preference and a 0.69 orgasm per PIV intercourse for women without a large penis preference; this leads us with an absolute difference of 0.14 orgasm per PIV intercourse and a relative increase of 20.29% increase in the rate of orgasm for women with a preference for larger penises. First to note is the suspiciously high rate of orgasm during PIV intercourse; almost no studies report that women orgasm 69% of the time during sexual intercourse (let alone 88% percent of the time). 
Important to note as well is that the study only found a statistically significant difference (p<0.05) in the absolute amount of vaginal orgasms between women with a large penis preference and women without a large penis preference. Orgasm per PVI had to be calculated by the reader and was not supplied by the authors; due to the fact that the standard deviation in Orgasm per PVI in women with a large penis preference and women without a large penis preference was not supplied, it becomes impossible to know if the increased rate of orgasm in women with a large penis preference is statistically significant (p<0.05). 
Women with a large penis preference report on abolute amount of vaginal orgasms 2.07 standard errors above women without a large penis preference, making it statistically significant (p<0.05; p=0.0358). However, women with a large penis preference also report an absolute amount of PVI 1.19 standard errors above women without a large penis preference; which although isn't large enough for it to be statistically significant (p=0.23), it is large enough to cast doubt on the assertion that the difference in the rate of Orgasm per PVI between women with or without a large penis preference will be statistically significant. To determine if larger penises are superior for eliciting vaginal orgasm, then it is the difference in the rate of orgasm, not the absolute amount of orgasms, that is to be considered, which is exactly what the authors (suspiciously) haven't done. 
Although a 2015 study did examine the effects that the phrasing of questioning had on female reports on orgasm during intercourse.https://www.researchgate.net/publication/320679171_Women%27s_Experience_of_Orgasm_During_Intercourse_Question_Semantics_Affect_Women%27s_Reports_and_Men%27s_Estimates_of_Orgasm_Occurrence?_sg=7kPbPsCjwwC5WlGbLYpZFHCR0W-8i0W60EU-7mXEM2jIKqyp3OewG9lruM3VlCJChBjqXJ0gHaz1fXo 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,' whereas 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 orgasms achieved by vaginal intercourse without clitoral stimulation, yet we find authors that report a rate significantly lower (with a sample size that is nine 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% (nowhere near the 69% given). 
Costa et al. argue that "One might argue that more generally erotophilic or less sexually inhibited women regard 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). The study, however, concluded: "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." 
For evidence of this, 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 masturbating using vaginal stimulation alone (indicating that it is the non-preferred method of achieving orgasm).
Another problem with this is that women with a 'large' penis preference might not even prefer actually large penises and may actually prefer average or slightly above average penises due to misperception in penis size. As stated in a previous section, women (likely) qualitatively overstate the size of their partners as the number of women reporting their partner's as large is greater than the number of women reporting their partner's as being small when they should be equivilant. Going by this article's definition of what qualifies as average, small or large then we should find that roughly 1-(0.16/0.27)= 41% of women who report their partner as being large are actually average or slightly above average and that 1-(0.10/0.67)= 15% of women who report their partner as being average are actually small (the 0.10 value are the 10% of men who ought to be classified as small as only 6% instead of 16% are classified as being small).
Costa et. al do not measure directly the penis sizes of the partner's of the women in the sample; rather they ask them whether or not they prefer to have sex with a man with a large penis. What constitutes what a large penis is from appearance and sensation alone will be subjective as a woman's past experiences will determine what will be considered large. This subjective measure is problematic for a number of reasons. The first being that women qualitatively overestimate penis size (as we've already established); not only do women qualitatively overestimate penis, they do so to a greater degree than men (using the same definitions and calculations, only about 27% of men claiming to be large are actually average compared to 41% of women making the same claim for their partner's). The next reason being that a qualitative assessment of a partner being large is associated with (slightly) greater sexual satisfaction than with a partner of average size. For reasons discussed previously, unless direct measurements are made, we cannot ascertain whether or not this is because women actually derive greater pleasure from intercourse with larger penises or if women associate larger penises with greater satisfaction and will thus believe that lovers have larger penises because of their sexual prowess despite being average. The latter conclusion seems more likely considering the degree to which women qualitatively overstate the penis sizes of their partners.
Thus, even if women with a large penis preference did in fact exhibit a greater rate of orgasm (which is something Costa et. al did not prove mind you; they only demonstrated a difference in the absolute number of orgasms) we cannot be certain that the penises in question were even large at all and that the effect might simply come from the fact that women may consider sexually able lovers as having larger penises even though they might not.
===Issues with the research on female orgasm===
However, inferring how orgasmic a woman is from her masturbatory practices is likely not a sufficient control for inferring how orgasmic a woman is, 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 the 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 woman 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 a reflection of how orgasmic she is really is). It can be concluded from this that 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.https://www.researchgate.net/publication/12709781_Neurochemical_Characterization_of_the_Vestibular_Nerves_in_Women_with_Vulvar_Vestibulitis_Syndromehttps://www.researchgate.net/publication/41486737_Botulinum_Neurotoxin_Type_A_Injections_for_Vaginismus_Secondary_to_Vulvar_Vestibulitis_Syndrome Obesity seems to affect partnered sexual experiences, but not masturbatory ones(these are the same author as the 2012 study).https://www.researchgate.net/publication/266380199_Orgasm_and_women's_waist_circumference 
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.https://www.researchgate.net/publication/49718166_Female_Sexual_Arousal_Genital_Anatomy_and_Orgasm_in_Intercourse 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 & Lloyd (2011) noted 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. As there are anatomical structures that affect success with orgasm during intercourse but not during masturbation, their "differential results" that "argue... 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." is essentially meaningless as the differential result can instead be explained by the clitoral-urethral meatus distance [CUMD] without refering to penis size. The authors write:
"When orgasms from masturbation were considered there was no meaningful relationship between CUMD and whether or not a woman experienced autosexual orgasms. Thus the influence of CUMD on women's orgasms is likely limited to orgasms solely from sexual intercourse."
Percentage of women experienceing Orgasm during intercourse or Autosexual stimulation in women with a CUMD less that or greater than 2.5cm
Sample and Type of orgasm 	CUMD<2.5cm	CUMD>2.5cm
Bonaparte sample - Orgasm during Intercourse	84% 	0%
Landins sample - Orgasm during Intercourse* 	78% 	35%
Bonaparte sample - Auto sexual orgasm 	75% 	89%
*orgasm in women defined as experienceing orgasm more than 66% of the time
As we can see, the difference of a centimetre or two could mean potentially experiencing orgasm during intercourse and never experiencing it at all. Secondly, even though women with a greater CUMD report a large difference in orgasm from intercourse, autosexual (self-stimulated) orgasms are much more similiar. Thus the contension that greater than average penis size affects orgasm because a preference for big penises predicts higher vaginal orgasm in women but not masturbatory orgasms can very easily be explanied by anatomical differences in women that affect vaginal orgasms in women but not masturbatory ones. As we can see, wether or not the CUMD is less than or greater than 2.5cm produces anywhere between a 43%-84% difference in women ever experiencing orgasm; the difference in merely that rate of orgasm reported by Costa et. al (a difference of about 19%) can easily be explained away by the CUMD. Secondly, the fact that CUMD affects wether or not a woman will achieve orgasm via intercourse dispells the myth that 'Vaginal orgasms' are distinct from Clitoral ones and that all orgasms in women are merely just clitoral orgasms achieved through different means.
===Penile length and sexual satisfation===
A relatively recent study provides insights into how penile length could affect sexual satisfaction.https://bjui-journals.onlinelibrary.wiley.com/doi/10.1111/bju.15416 The study utilized 29 couples and to test the effect of penile length on sexual satisfaction put penile rings of various widths and had the women report the difference sexual satisfaction they recieved when having sex with their partners when swicthing from one ring to another. The men in the study reported measuring, on average, 17cm (about 4cm away from the average); however it should be noted that measurments were not taken by the authors and were recieved via self reports. Although the participants did recieve instruction on the proper method of measurement, the authors also write: "The measurements of penis size were self‐reported, and these are likely to be biased towards overestimation. This may have been biased across all participants so that the analysis of the impact of restricting the depth of penetration by penis length on sexual pleasure still stands.
...
Other reasons for the bias could be measurement error or a self‐selection bias for men who had a larger than average penis size. Measurement errors have been previously found even between clinicians who have received training for such procedures, suggesting that this would be the most plausible explanation for the size overestimation."There were 4 different types of rings used in the study; ranging from slimmest to thickest: A rings were 0.5cm thick, B rings were 2.5cm thick, C rings were 3.8 cm thick and D rings were 5cm. The table below reports the number of women whom, upon switch from the control ring (A) to either the B,C or D rings reported reduced, increased or equivalent amounts of pleasure from purely sexual intercourse.
Number of women whom reported increased, decreased or equivalent amounts of pleasure when swithing penile ringes
Type of Pleasure 	Switching from Ring A -> B (n=43)
- / = / +	Switching from Ring A -> C (n=45)
- / = / +	Switching from Ring A -> D (n=39)
- / = / +
Overall Experience 	12 (28%) / 30 (70%) / 1 (2%) 	20 (45%) / 24 (53%) / 1 (2%) 	19 (49%) / 19 (49%) / 1 (2%)
Intercourse alone 	14 (33%) / 22 (55%) / 7 (16%)	19 (42%) / 20 (45%) / 6 (13%)	26 (66%) / 10 (26%) / 3 (8%)
Emotional connection	16 (37%) / 21 (49%) / 6 (14%)	15 (33%) / 26 (58%) / 4 (9%) 	21 (51%) / 12 (31%) / 6 (15%)
What we see for the most part isn't very surprising; most of the time there is no effect or a negative effect; the couples that experience the negative effect are likely the ones where the man already has an average penis and then because of the rings goes to below average. It is interesting to find that, whilst overall sexual experience didn't increase for many, sexual intercourse did for a significant amount of women; this implies that the largest penises aren't the most pleasurable.
Table comparing length of penis to overall sexual pleasure when using one of the four rings
 	13cm	14cm 	15cm	16cm	17cm	18cm
Overall Sexual Pleasure with the A ring	85% 	84.5%	84% 	83% 	82% 	80.5%
Overall Sexual Pleasure with the B ring	45% 	52% 	62% 	69% 	78% 	86%
Overall Sexual Pleasure with the C ring	44% 	52% 	58% 	67% 	75% 	83%
Overall Sexual Pleasure with the D ring	37% 	45% 	53% 	62% 	70% 	79%
*The values were eyeballed from the graphs given in the study so there might be some errors by a percentage point or two
What we see here is quite surprising; with the A ring (the control, which is the closest to sex as nature intended it) men with average penises are rated as most pleasurable; but the most pleasurable were penises at 18cm (or 7") that had been shortened by the rings. Taking into account that reported length had likely been overstated then the 18cm penises might actually be 17cm or 16cm penises; which would mean they'd be effectively reduced down to 13.5-14.cm with the B rings. Secondly, longer penises were least affected by increasing ring width but shorter penises were most affected. This implies that somewhat shorter than average penises aren't as pleasurable; that average penises are most pleasurable and that any increase in length after that is useless (or even slightly detrimental) to determing overall sexual pleasure.
We might guess how pleasurable penises shorter than 13cm might be rated for overall sexual pleasure by subtracting the width of the rings from the lower measurments and assume that the resultant length caused by the shortening of the ring will be rated equivilantly to a natural penis of that length. For example, the D ring subtract 5cm of length; 15cm penises report and overall satisfaction of 53%; it follows then natural 10cm penises will also induce overall sexual pleasure at 53%. However this is fallacious as the rings very likely will affect how the man would thrust; the authors state that:
"The male partner reported that they changed their behaviour either ‘very much’ or ‘extremely’ during intercourse because of the experiment."
This implies that 10cm penises will likely be rated as much higher than the 53% estimate. Secondly 16cm penises with B rings reported an 69% overall sexual satisfaction; but that means that only 13.5cm worth of effective length which was not reported for penises of 14cm whom instead reported 84.5% (which because of the A ring would have 13.5cm of effective length). So it might be the case that 10cm penises might acutally be rated as 78.5% sexually satisfying overall (something that seems plausible considering how the other penises with A rings were rated). Overall, this estimation predicts that penises will be rated as more sexually satisfying at a moderate inclince untile around 13cm-14cm at which point sexual satisfaction plataues and any increase results in a less steep decline. One anomaly to be acounted for would be that why men with 18cm penises reduced by 2.5cm (by the B ring) are rated as more sexually pleasurable that penis that are 13cm, 14cm or 15cm long. This might be because prior to the application of the ring, the male thrusted too deep and that the rings caused him to change the speed and depth of his thrusting and would thus be rate as just slightly more pleasurable.
So it appears that the evidence on female prefence for certain penile lengths corraborate; with the most aesthetically pleasing and sexually pleasing penises found to be at the 13cm-14cm mark.
==Genital Coevolution==
Copulation requires direct contact between males and females in most mammalian species and is in fact the most direct activity that males and females participate in.https://www.researchgate.net/publication/279730721_Mechanisms_and_Evidence_of_Genital_Coevolution_The_Roles_of_Natural_Selection_Mate_Choice_and_Sexual_Conflict?_sg=XV3XL_gJrpIF_MC0QDCuej0XBjTmpzt9Ko2ZcM2nKXNKaSBdGlVnQRHByLZv7yid6iJKUh8eTclPuQw Your average penis thus needs to be suitable to achieve its function in fertilizing your average female and to do so it needs to be large enough. It has to be large enough that the vaginal walls can adequately stimulate it so that it can ejaculate, large enough to efficiently place sperm and large enough that in can fit into the greatest number of females that one can successfully copulate with (but not so large as to reduce mating oppurtunities). Thus as the male's penis has to fit into the greatest possible number of females it can possibly mate with, as the structure and shape of the vagina (where the penis rubs itself against) changes the structure of the penis will as well to ensure successful mating; this called genital coevolution.
=== The Big Vaginas Hypothesis ===
Human penises are large because human vaginas are also quite large. Human vaginas are larger due our bipedilism which caused the pelvis to grow shorter but much wider (especially in females); this wide pelvis allowed females to give birth to children with extremely large heads which then elongated the female vagina. Homminids usually had sex from behind and thus would require a longer penis for efficient mating. Dixson made comparisons for 9 species for which both the vaginal lengths and erect penis length were known and found that they were highly correlated (r=0.85); in other words, by squaring 0.85, vaginal length accounted for approximately 72.25% of the variation of penile length between species.
==References==
==See also==
*Female orgasm
*Dickcel
*Dickpill
*Macropenis dilemma
*Macrophallism
*Bonepressed
*Jelqing
*Dickmog
*Bloxers
*Vagina