USA – According to Kinsley Institue, Durex, and Definitive Penis Internet (DPI) surveys, the median erectile length of men’s penises is between 5 to 7 inches. Yet, regardless of one’s erectile length, it is still larger than their primate friends.
Kinsley institute reviewed its founder’s data 30 years after it was published, and concluded that 1 in 100 men reaches beyond the 5 to 7-inch erectile median to 8 inches, 7 in 1,000 men go beyond 8 inches, and only 1 in 1,000 touches 9 inches. They also revealed that only 18 in 1,000 men have an erection over the median.
Although Durex and DPI maintained that their findings were similar to Kinsley, their data suggested that there were four to eight times more men who have erectile lengths beyond the median. Durex and DPI’s internet surveys suggested that there were between 4 and 7 in every 100 men reaching 8 inches, between 30 and 40 in every 1,000 reaching 9, and between 10 and 30 in 1,000 reaching beyond 9 inches.
Kinsley also stated that erections above 9 inches are rare enough to be statistically immeasurable, but Durex and DPI have suggested that 1 in 100 men have erectile lengths of double digits.
However, having to rely on voluntary participants that submitted their own measurements may have resulted in a less accurate representation of the population. Durex and DPI have taken measures to ensure a more accurate result – Durex rejected lengths under 3 inches as well as those over 10 inches while DPI rejected the bottom 1% as well as the top 2% of submissions. DPI also asked participants to submit a photo online which included a tape measure.
While the average of these three studies is at 6.25 inches, other studies done by various institutions averaged the erectile length to be almost an inch lesser. The varying results suggest that a wider sample size with better methods of obtaining these lengths is needed for a more accurate representation of the population.
Between 2007 and 2010, at least 15 studies regarding penises were published. Through these studies, it became clearer that men who perceive their erectile lengths to be lower than average are less likely to volunteer as participants for these studies. This means that the average and median lengths from these studies could be lower in reality.
Regardless of one’s erectile length, the penises of humans are still significantly larger than the other 192 primates. When erect, the gorilla and orangutan penises only measure up to 1.5 inches. While the chimpanzee doubles that, it still falls short of the average human being. Although there has been no scientific evidence to explain why this is so, the consensus from various disciplines is that a man’s penis began the process of “runaway selection” when the evolution of human beings from crawling to upright walking shifted the sexual focus from rear to front of both sexes, according to archaeologist Timothy Taylor in The Prehistory of Sex.
Meanwhile, feminists are leaning towards the view that the angle of the vagina moved forward and down, deeper into the body when females became erect, forcing the penis to follow the same principle as the giraffe’s neck: “it grew in order to get something it could not otherwise reach”, according to Rosalind Miles in The Women’s History of the World.
Other theories include the penis’s evolution for attraction of mates and as a warning signal for rivals, as well as increasing chances of reproduction by becoming closer to the cervix.
While man’s erectile length has yet to be explained scientifically, the reason for the varying sizes of testicles has been attributed to one’s promiscuity. In the early 1980s, evolutionary psychologist David Buss hypothesised that the more promiscuous the primate, the larger the size of his testicles. Following that, British scientists measured the weight of 33 primates’ testes. The chimpanzee’s weighed 113g, three times higher testes-to-body weight ratio than humans. The gorilla fared poorer than man, but Buss explained that this was because the gorilla faced lesser competition than chimpanzees and bonobos and was thus less promiscuous.
While man may have the largest penis amongst all primates, his sperm production per gram of tissue is also significantly lesser than chimps and gorillas. As aptly put by Lynn Margulis and Dorion Sagan in Mystery Dance: On the Evolution of Human Sexuality, man once, when the business of insemination was a contest, had a bigger “testicular engine”.