fair skinned ppl have less melanin in their skin.. melanin is what causes the darkening of the skin when we are exposed to UV rays.. and it is what causes the dif color of skin in different races of humans :) Humans who's ancestors come from hot sunny climates have a lot more melanin than those who lived in more northern, cold and cloudy climates because ppl in hotter sunnier climates *need* more protection from the sun than those in cold cloudy climates.. So for instance Scandinavians are going to have much less melanin in their skin than someone who's background is in Italy or Greece or somewhere close to the equator. Since it is the melanin in our skin that reacts to the UV rays making us darker it is harder for the Scandinavian to tan.
Pailer people pretty much burn, rather than tan. I honestly can say from experience from going to Mexico, is that if you're staying in the sun all day or week then u NEED to wear sunscreen. I burnt from wearing 15 or 30 SPF and my face got all puffy and gross, anyways u will still tan if your wearing sun block. Also in Mexico I didn't tan. I went swimming, boogie boarding and I was honestly to busy to tan, but it was so hot out, i basically tanned. So slap on your hat and go outside! It's time to taan.