It is easy to see why dark skin is an advantage in the tropics as a partial defence against skin cancer. It is much less easy to see why a white skin is an advantage in colder climates. Researchers have suggested two reasons, and both probably play a part. The first is that, at a given temperature, a white skin radiates less heat than a black skin. The second is that in weaker sunlight the body tends to produce less vitamin D. There is a hypothesis that the selection for lighter skin is due to the need for higher vitamin D production. The change started as soon as populations moved north out of Africa.