According to a 1979 study by Kinsey Institute researchers Gebhard and Johnson, the average erect penis length is between 5 and 6 ½ inches, with a circumference between 4 and 5 inches [Source: Kinsey Institute]. About 0.6 percent of men have a condition called micropenis, defined as having an erect penis length of less than 2 inches (or 2.5 standard deviations below the mean).
Some of the conditions that cause micropenis are the same as those that cause XY intersex, such as androgen insensitivity syndrome (AIS). However, men with micropenises generally don't have genital ambiguity. Micropenis can also be caused by other genetic problems such as growth-hormone deficiency. If the condition is detected in childhood, hormone therapy can lead to some growth. However, it's unlikely that the penis will ever reach average size. Medications and supplements meant to enlarge the penis usually don't do so permanently, but some men have had success with surgical enlargement.
Micropenis can cause both social and psychological problems in men. Men may also have difficulty having sexual intercourse. In the 1960s and 1970s, some doctors advocated reassigning the gender of boys with micropenis to girls, with the belief was that gender identity is learned. Some researchers still advocate raising these boys as girls, but men who underwent reassignment as children have since spoken out against it.
In the last section, we'll examine one final unusual sexual condition: supernumerary sex organs.