In comparison to the rest of the apes, it is very thick compared with the pencil-like penis of, say, gorillas and chimps. Morris proposed that this giant organ was selected for because it stimulated the female clitoral area more than a slender simian rod would have, thereby giving more satisfaction to the female and further strengthening the pair-bond. This may be partially true, but I suspect the inordinate size of the male genitals is rather a result of selection pressures originating among males, because of its influence on social stature. There are a number of pieces of evidence for this. Male Miamin of New Guinea, and some tribes in South America, even wear artificial penis extensions to further increase their masculine appearance.
Penis size varies among different human subgroups, as do other social organs.
For example, some Negro groups maintain a flaccid penis size nearer the erect state than do Caucasians. It was a tradition among the Australian aboriginals to split the penis down the urethral slit so that it ended up as a broad flat band - kind of like cutting a garden hose along a seam and flattening it out. It used to be done with a sharpened flint, but now the leaders prefer straight razors. Like the widespread rite of circumcision, these leave the glans exposed so even a flaccid penis carries some status - as a weapon permanently unsheathed. Unlike the rest of the primates, we have an erect posture which provides an increased exposure of the penis to public view, continually reminding other individuals of its size.