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Black men have big penises and Asians have small penises
Actually this isn't totally a myth. The average size of black men is between half an inch and an inch bigger than whites, and Asians are on average about half an inch shorter in length. (But blacks come in all sizes. See Images of Size, which has a black men penis size gallery.) Penis girth seems to vary in proportion. And it appears that the vagina of each race is built to match! Here are some research findings:
We averaged the data on erect penis size and found the averages to be approximately:
Orientals, 4 to 5.5 inches in length and 1.25 inches in diameter;
Caucasians, 5.5 to 6 inches in length and 1.5 inches in diameter;
Blacks, 6.25 to 8 inches in length and 2 inches in diameter.
Women were proportionate to men, with Orientals having smaller vaginas and blacks larger ones, relative to Caucasians.
And apparently there are differences in testicle size as well:
Measurements taken from living subjects as well as those at autopsy, show the size of testicles is twofold lower in Asian men than Europeans (9 grams versus 21 grams), a difference too large to be accounted for entirely in terms of body size.
(Adapted from "Race differences in sexual behavior: Testing an evolutionary hypothesis." Journal of Research in Personality 21(4): pp. 536-7.)