The first stop sign was erected in Detroit, Michigan in 1915. It had black letters on a white background printed on a sheet of metal. In 1922 Detroit established a committee to discuss the usage of the traffic signs. They came up with the idea to make the sign an octagon shape so it would stand out more. Thus marks the beginning of the psychological formations we associate with signs based on shape. Once America chose the octagon, Europe soon decided to use the same shape for their stop signs.
The only country that's defied the octagon shape is Japan. That's not surprising since they've always been known to have simpler shapes to their traffic signs. In their case, their stop sign is an upside-down red triangle (the same shape as our "Yield" signs) and the "Tomare" in the middle for "Stop" in white letters.
