...AND NOW FOR THE THEAD KILLING ANSWER!!!
Biologically speaking, a longer flaccid length would make sense for people who have adapted to life in very hot countries. Blood circulation to the extremity of the penis would help control the body temperature in the wind.
A long flaccid penis in cold countries would be more of a biological disadvantage since it might get frostbite and fall off in freezing weather, which wouldn't be good for procreation.
However because ambient temperatures don't affect the size and depth of a woman's vagina, a bigger penis isn't neccessary for human survival.
Surely if black men all had 18% bigger erections, black women would have evolved to accept to this larger size and would have a reputation for having bucket cunts! This isn't the case so the story can only be applied to flaccid size.
The Kinsey size survey DID record a flaccid size difference between African men and other races but erection size differences were negligible.
THE END