lao ai had the largest penis in history and that he had an affair with queen dowager zhao (c. 280–228 bc), the mother of qin shi huang, by pretending to be a eunuch. ancient koreans admired large penises and king jijeung (437–514 ad) of the silla dynasty is said to have had a forty-five-centimeter penis that was so large his subordinates had to search for a woman that fit him. traditional japanese erotic paintings usually show genitals as exaggeratedly large. the oldest known painting of this type, found in the hōryū-ji temple in ikaruga, dates to the eighth century ad and depicts a fairly large penis. the ancient indian sexual treatise kama sutra, originally written in sanskrit, probably between the second and fourth centuries ad, divides men into three classes based on penis size: "hare" size (about 5–7 cm, or 2–3 inches, when erect), "bull" size (10–15 cm, or 4–6 inches), and "horse" size (18–20 cm, or 7–8 inches). the treatise also divides women's vaginas into three sizes ("deer", "mare", and "elephant") and advises that a man match the size of the vagina of the woman he is having sex with to the size of his own penis. it also gives medically dubious advice on how to enlarge one's penis using wasp stings. post-classical portrait of lodovico caponi (painted c. 1550–1555) by angelo bronzino shows a codpiece peeking from underneath the young man's jacket. larger codpieces were regarded as more fashionable. in medieval arabic literature, a longer penis was preferred, as described in an arabian nights tale called "ali with the large member". as a witty satire of this fantasy, the 9th-century afro-arab author al-jahiz wrote: "if the length of the penis were a sign of honor, then the mule would belong to the quraysh" (the tribe to which muhammad belonged and from which he descended). the medieval norsemen considered the size of a man's penis as the measure of his manliness, and a thirteenth-century norse magic talisman from bergen, a wooden stave inscribed with writing in runic script, promises its wearer: "you will fuck rannveig the red. it will be bigger than a man's prick and smaller than a horse's prick." a late fourteenth century account of the life of saint óláfr from the flateyjarbók describes a pagan ritual which centered around a preserved horse's penis used as a cult artifact which members of the cult would pass around in a circle, making up verses in praise of it, encouraging it and the other members of the group to behave in sexually suggestive ways. during the renaissance, some men in europe began to wear codpieces, which accentuated their genitals. there is no direct evidence that it was necessarily worn to enhance the apparent size of the wearer's penis, but larger codpieces were seen as more fashionable. contemporary perceptions sign in south africa advertising penis enlargement male self-perception males may quite easily underestimate the size of their own penis relative to those of others. a survey by sexologists showed that many men who believed that their penis was of inadequate size had average-sized penises. another study found sex education of standard penile measurements to be helpful and relieving for patients concerned about small penis size, most of whom had incorrect beliefs of what is considered medically normal. the study found that almost all of their patients that were concerned about their penis size overestimated the average penis size. the perception of having a large penis is often linked to higher self-esteem. fears of shrinking of the penis in folklore have led to a type of mass hysteria called penis panic, though the penis legitimately can shrink in size due to scar tissue formation in the penis from a medical condition called peyronie's disease. marketers of penis enlargement products exploit fears of inadequacy, but there is no consensus in the scientific community of any non-surgical technique that permanently increases either the thickness or length of the erect penis that already falls into the normal range. shrinking and enlarging widespread private concerns related to penis size have led to a number of folklore sayings and popular culture reflections related to penis size. penis panic is a form of mass hysteria involving the believed removal or shrinking of the penis, known as genital retraction syndrome. the penis can significantly shrink due to scar tissue formation from a condition called peyronie's disease which affects up to 10% of men. products such as penis pumps, pills, and other dubious means of penis enlargement are some of the most marketed products in email spam. at present there is no consensus in the scientific community of any non-surgical technique that permanently increases either the thickness or length of the erect penis that already falls into the normal range (4.5" to 7"). among male homosexuals a study undertaken at utrecht university found that the majority of gay men in the study regarded a large penis as ideal, and having one was linked to self-esteem. one study analysing the self-reported kinsey data set found that the average penis of a homosexual man was larger than the average penis of their heterosexual counterparts (6.32 inches in length amongst gay men versus 5.99 in in heterosexuals, and 4.95 inches circumference amongst gay men versus 4.80 in in heterosexual men). evolution the human penis is thicker than that of any other primate, both in absolute terms and relative to the rest of the body. early research, based on inaccurate measurements, concluded that the human penis was also longer. in fact, the penis of the common chimpanzee is no shorter than in humans, averaging 14.4 cm (5.7 inches), and some other primates have comparable penis sizes relative to their body weight. the evolutionary reasons for the increased thickness have not been established. one explanation is that thicker penises are an adaptation to a corresponding increase in vaginal size. the vaginal canal is believed to have expanded in humans to accommodate the larger size of a newborn's skull. women may then have sexually selected men with penises large enough to fit their vagina, to provide sexual stimulation and ensure ejaculation. other evolutionary hypotheses to explain humans' relatively large penis length and girth include a sperm competition hypothesis and a mate competition hypothesis. the sperm competition hypothesis does not have much support as in other mammals where sperm competition is present, larger testes evolve, not larger penises. the mate competition hypothesis involves the prediction that a human with a larger penis would be able to displace the sperm of another. studies have found that larger penises do not displace other sperm more effectively than smaller penises, but rather longer penises may ejaculate sperm inside the vagina in places that would be harder for a following penis to displace. the depth of pelvic thrusting was correlated to the displacement of competing sperm.