Human penises vary in size on a number of measures, including length and circumference when flaccid and erect. Besides the natural variability of human penises in general, there are factors that lead to minor variations in a particular male, such as the level of arousal, time of day, ambient temperature, anxiety level, physical activity, and frequency of sexual activity. Compared to other primates, including large examples such as the gorilla, the human penis is thickest, both in absolute terms and relative to the rest of the body. Most human penis growth occurs in two stages: the first between infancy and the age of five; and then between about one year after the onset of puberty and, at latest, approximately 17 years of age.
Measurements vary, with studies that rely on self-measurement reporting a significantly higher average than those with a health professional measuring. As of 2015[update], a systematic review of 15,521 men, who were measured by health professionals rather than themselves, concluded that the average length of an erect human penis is 13.12 cm (5.17 inches) long, while the average circumference of an erect human penis is 11.66 cm (4.59 inches). A 1996 study of flaccid length found a mean of 3.5 inches (8.9 cm) when measured by staff. Flaccid penis length can sometimes be a poor predictor of erect length. An adult penis with an erect length of less than 7 cm (2.8 in), but otherwise formed normally, is referred to in medicine as a micropenis.
Limited to no statistically significant correlation between penis size and the size of other body parts has been found in research. Some environmental factors in addition to genetics, such as the presence of endocrine disruptors, can affect penis growth.
A 2015 systematic review published by Veale et al. of medical research on the topic over the previous 30 years published in BJU International showed similar results, giving mean flaccid, stretched non-erect, and erect lengths of 9.16 cm, 13.24 cm, and 13.12 cm respectively, and mean flaccid and erect circumferences of 9.31 cm and 11.66 cm respectively. Erect lengths in the included studies were measured by pushing the pre-pubic fat pad to the bone, and flaccid or erect girth (circumference) was measured at the base or mid-shaft of the penis.
The penis and scrotum can contract involuntarily in reaction to cold temperatures, anxious or nervous level and participation in sports. This decrease of flaccid penis size is referred to by the slang term "shrinkage", due to action by the cremaster muscle. The same phenomenon affects cyclists and exercise bike users, with prolonged pressure on the perineum from the bicycle saddle and the straining of the exercise causing the penis and scrotum to contract involuntarily. An incorrect saddle may ultimately cause erectile dysfunction (see crotch pressure for more information). Individuals with hard flaccid syndrome or other pelvic floor disorders may temporarily have an abnormally small penis.
Scientific studies have been performed on the erect length of the adult penis. Studies that have relied on self-measurement, including those from Internet surveys, consistently reported a higher average length than those that used medical or scientific methods to obtain measurements.
Similar results exist regarding studies of the circumference of the adult fully erect penis, with the measurement usually taken mid-shaft. As with length, studies that relied on self-measurement consistently reported a significantly higher average than those with staff measuring. In a study of penis size where measurements were taken in a laboratory setting, the average penis circumference when erect was 11.66 cm (4.59 inches).
The average stretched penile length at birth is about 4 cm (1.6 in), and 90% of newborn boys will be between 2.4 and 5.5 cm (0.94 and 2.17 in). Limited growth of the penis occurs between birth and 5 years of age, but very little occurs between 5 years and the onset of puberty. The average size at the beginning of puberty is 6 cm (2.4 in) with adult size reached about 5 years later. W.A. Schonfeld published a penis growth curve in 1943.
Authors of a paper reviewing research on area of penis sizes conclude that "flaccid penile length is just under 4 cm (1.6 in) at birth and changes very little until puberty, when there is marked growth."
Age is not believed to negatively correlate with penis size. "Individual research studies have... suggested that penis size is smaller in studies focusing on older men, but Wylie and Eardley found no overall differences when they collated the results of various studies [over a 60 year period]."
One study investigated the relationship with digit ratio and found that men with longer ring fingers than index fingers had slightly longer penises. However, the common misconception that hand size predicts penis size has been widely discredited.
One study, Siminoski and Bain (1988), found a weak correlation between the size of the stretched penis and foot size and height; however, it was too weak to be used as a practical estimator. Another investigation, Shah and Christopher (2002), which cited Siminoski and Bain (1988), failed to find any evidence for a link between shoe size and stretched penis size, stating "the supposed association of penile length and shoe size has no scientific basis".
There may be a link between the malformation of the genitalia and the human limbs. The development of the penis in an embryo is controlled by some of the same Hox genes (in particular HOXA13 and HOXD13) as those that control the development of the limbs. Mutations of some Hox genes that control the growth of limbs cause malformed genitalia (hand-foot-genital syndrome).
A study of 253 men from Tanzania found that the average stretched flaccid penis length of Tanzanian males is 11.5 cm (4.53 inches) long, smaller than the worldwide average, stretched flaccid penis length of 13.24 cm (5.21 inches), and average erect penis length of 13.12 cm (5.17 inches).
A study of 115 men from Nigeria found that the average flaccid stretched penis length of Nigerian males is 13.37 cm (5.26 inches) long, which is near identical to the worldwide average, stretched flaccid penis length of 13.24 cm (5.21 inches) and average erect penis length of 13.12 cm (5.17 inches).
In a 1994 cover story by Psychology Today, 1,500 readers (about two-thirds women) were surveyed about male body image. Many of the women were not particularly concerned with penis size, and over 71% thought men overemphasized the importance of penis size and shape. Generally, the women polled cared more about width than men thought, and less about length than men thought, although the strength of caring for either among women showed a similar pattern.
A study published in 2002, conducted at Groningen University Hospital, asked 375 sexually active women (who had recently given birth) the importance of penis size. The results showed that 21% of women felt length was important and 32% felt that girth was important.
A study conducted at the Australian National University, published in early 2013, showed that penis size influences a man's sex appeal, and the taller the man, the bigger the effect. The study showed life-sized 3D computer-generated images, altering the height and other physical attributes, with women typically registering preferences in under 3 seconds. A preference for taller men's larger penis size was indicated.
A US study published in 2015 of the stated preferences of a panel of 75 women using 3D-printed models as scale references showed a preferred penis length of 16 cm (6.3 inches) and a preferred circumference of 12.2 cm for long-term sexual partners, with slightly larger preferred sizes of a length of 16.3 cm (6.4 inches) and circumference of 12.7 cm for one-time sexual encounters.
According to the study, however, when asked to estimate the length of their partner's penis, most women would say a size significantly smaller than what their partner was recorded to be. This suggests that perception of size is not entirely accurate. The visual impression of the size is not necessarily in correlation with the feeling in the vulva and vagina. A very long penis can cause dispareunia, if the man doesn't understand how to use it carefully.
One Australian study of 184 men looked at penis length and circumference in relationship to condom breakage or slippage. 3,658 condoms were used. The study found that when used correctly, condoms had a breakage rate of 1.34% and a slippage rate of 2.05%, for a total failure rate of 3.39%. Penile dimensions did not influence slippage, although penis circumference and broken condoms were strongly correlated, with larger sizes increasing the rate of breakage.
Androgens like testosterone are responsible for penis enlargement and elongation during puberty. Penis size is positively correlated with increasing testosterone levels during puberty. But after puberty, administration of testosterone does not affect penis size, and androgen deficiency in adult men only results in a small decrease in size. Growth hormone (GH) and insulin-like growth factor 1 (IGF-1) are also involved in penis size, with deficiency (such as that observed in growth hormone deficiency or Laron syndrome) at critical developmental stages having the potential to result in micropenis.
There are certain genes, like homeobox (Hox a and d) genes, which may have a role in regulating penis size. In humans, the AR gene, located on the X chromosome at Xq11-12, may affect penis size. The SRY gene located on the Y chromosome may have a role to play. Variance in size can often be attributed to de novo mutations. Deficiency of pituitary growth hormone or gonadotropins or mild degrees of androgen insensitivity can cause small penis size in males and can be addressed with growth hormone or testosterone treatment in early childhood. 041b061a72