Penis length

The most accurate measurement of human penis size 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.

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 the best research to date on the topic has concluded that the mean length of an erect human penis is approximately 13.12 ± 1.66 cm (5.17 ± 0.65 in). Flaccid penis length is generally a poor predictor of erect length.

Most human penis growth occurs between infancy and the age of five, and between about one year after the onset of puberty and, at latest, approximately 17 years of age.

A statistically significant correlation between penis size and the size of other body parts has not been found in research. 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.

Penis size is positively correlated with increasing testosterone levels during puberty. However, after puberty, administration of testosterone is not capable of affecting 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.

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