While some women proudly sport a silver mane, most of us face the arrival of new gray hairs with dread. The good news: Scientists are hard at work on how to prevent the salt-and-pepper look. So what do researchers know that you don't? 1. Normal aging is the biggest culprit. Okay, no surprise here. Dermatologists call this the 50-50-50 rule. "Fifty percent of the population has about 50 percent gray hair at age 50," says Dr. Anthony Oro , professor of dermatology at Stanford University. And like skin, hair changes its texture with age, says Dr. Heather Woolery Lloyd , director of ethnic skin care at the University of Miami School of Medicine. 2. Your ethnicity makes a difference. Caucasians tend to go gray earlier - and redheads earliest of all. Then Asians. Then African-Americans. Scientists haven't figured out why yet. 3. Stress seems to play a role. "Stress won't cause you to go gray directly," says Dr. Roopal Kundu , associate professor in dermatology at Northwestern University's Feinberg School of Medicine. "But stress is implicated in a lot of skin and hair issues." During an illness, for example, people can shed hair rapidly. And hair you lose after a stressful event - like getting chemotherapy - may grow back a different color 4. Your lifestyle makes a difference. Smoking, for example, stresses your skin and hair. "Low vitamin B12 levels are notorious for causing loss of hair pigment," says Dr. Karthik Krishnamurthy , director of the Dermatology Center's Cosmetic Clinic at Montefiore Medical Center in the Bronx, N.Y. By contrast, eating foods such as liver and carrots may be helpful for holding off going gray, says Dr. Wilma Bergfeld , a senior dermatologist at the Cleveland Clinic. 5. Hair and its color are separate things. Hair stem cells make hair, and pigment-forming stem cells make pigment. Typically they work together, but either can wear out, sometimes prematurely. Researchers are trying to figure out if a medicine, or something you could put in your scalp, could slow the graying process. (Hair dye simply coats your hair in color but doesn't alter its structure.) 6. Your hair doesn't turn gray - it grows that way. A single hair grows for one to three years, then you shed it - and grow a new one. As you age, your new hairs are more likely to be white. "Every time the hair regenerates, you have to re-form these pigment-forming cells, and they wear out," says Oro.