Vitamin D
You get vitamin D from either exposure to the sun, or from eating lots of red meat. Scandinavian countries where people have traditionally got blond hair are like that because that's as far north as you can grow grain. Diets consisting of mainly vegetables and grain need to be supplemented with vitamin D. As people's bodies are covered most of the times at these latitudes the most exposed place to the sun is their heads. Blond hair manages to transmit more light to the scalp than dark hair, while still being insulating from the cold. Further south the weather is warmer, fewer clothes are worn which allows more of the body to get the rays and make vitamin D, so blond hair isn't needed and dark hair reigns as a way to stop the top of your head burning in the midday sun. Further north you get diets consisting largely of meat and you then have enough vitamin D entering the body through the food, and you get dark hair people once again, like the Inuit.

Scotland is far enough north for the people to grow grain and need help with vitamin D, but not far enough to generate blond people. Hence the red hair, which is sort of half way between dark hair and blond hair.
(robbersdog, Wed 3 Mar 2010, 13:03, Reply)