The size of the penis is decided for men while they are still embryos floating in the confines of the female uterus.

One may say that the size of the penis can actually determined even before conception by forecasting the effects of the combination of genetic material from the two parents.

However, there is no known reliable method for such forecasting. At the physical level, the penis size is handled by the HOX genes.

limbs and that of the penis.

Many people believe that the size of the penis can be estimated from the size of hands or feet and the fact that the same genes handle the formation of both genitalia and limbs could provide the missing clues.

Mutations in these genes could trigger malformations to the external genitalia, such as the micropenis (when the penis is less than half an inch in size) or the macropenis, which is a penis of abnormally large size.

The size of the penis can also be influenced by the level of exposure to various hormones of the developing embryo. One study based on data provided by professor Alfred Kinsey found that homosexual men have statistically larger penises than heterosexuals, which could be explained by exposure to estrogen.