Why do redheads tend to sunburn more easily?
You should have a look at wikipedia > melanin.


  Melanin is a broad term for a group of natural pigments found in most organisms. [..] Melanin is an effective absorber of light; the pigment is able to dissipate over 99.9% of absorbed UV radiation.1 Because of this property, melanin is thought to protect skin cells from UVB radiation damage, reducing the risk of cancer. Furthermore, though exposure to UV radiation is associated with increased risk of malignant melanoma, a cancer of the melanocytes, studies have shown a lower incidence for skin cancer in individuals with more concentrated melanin, i.e. darker skin tone. Nonetheless, the relationship between skin pigmentation and photoprotection is still being clarified.


Most of the variation in skin color among humans is caused by differential levels of melanism. People with red hair have a low level of melanism (almost by definition) and therefore suffers more from sunburn than people with more pigmented skin.