Penis size is positively correlated with increasing testosterone levels during puberty.
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.
Some of the identifiable causes are deficiency of pituitary growth hormone or gonadotropins, mild degrees of androgen insensitivity, a variety of genetic syndromes and variations in certain homeobox genes.
Some environmental factors in addition to genetics, such as the presence of endocrine disruptors, can affect penis growth.
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.
While results vary slightly across reputable studies, the consensus is that the mean human penis, when erect, is in the range 12.9–15 cm (5.1–5.9 in) in length with a 95% confidence interval of (10.7 cm, 19.1 cm) or, equivalently (4.23 in, 7.53 in) — that is, it is 95% certain that the true mean is at least 10.7 cm but not more than 19.1 cm. 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 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.