The average size at the beginning of puberty is 6 cm (2.4 in) with adult size reached about 5 years later. In humans, the AR gene located on the X chromosome at Xq11-12 which may determine the penis size.
The SRY Gene located on Y chromosome may have a role to play.
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). 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. 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]." A 2015 review of the literature found two studies finding height and stretched or flaccid length to be moderately correlated, seven studies finding weak correlation for flaccid, stretched, or erect length, and two studies that found no correlation between flaccid length and height.
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. There are certain genes like homeobox (Hox a and d) genes which may have a role in regulating penis size.
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.
The penis and scrotum can contract involuntarily in reaction to cold temperatures or nervousness, referred to by the slang term "shrinkage", due to action by the cremaster muscle.
The same phenomenon affects cyclist 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.
The most accurate measurement of the size of a human penis can be derived from several readings at different times since there is natural minor variability in size depending upon arousal level, time of day, room temperature, frequency of sexual activity, and reliability of measurement.
When 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.