When we start getting gray hair, we tend to blame stress -- a high-pressure job, an illness, even our unruly children.

But the link between gray hair and psychological stress is little more than folk wisdom, unsupported by numerous scientific studies. That didn't stop widespread speculation last week, in The New York Times and elsewhere, that the newly noticed salt and pepper above President Obama's temples were the first physical manifestations of some of the highest job pressures on the planet.

A more likely explanation is that Mr. Obama is starting to turn gray for the same reasons other people do. He's getting older. The age of graying seems to be determined by heredity, The Journal of Investigative Dermatology reported in 2005. Whites tend to gray first, often as early as their mid-30s, followed by Asians and then Africans. About half of 50-year-olds are at least 50 percent gray. So it would seem that Mr. Obama, at 47, is a little late to the graying game.

But while the arrival of gray hair is relatively predictable, how and why hair ages this way is not well understood. Unlocking those secrets could have potential well beyond vanity, leading to a better understanding of the aging process at the cellular level. Scientists even hope that by identifying the mechanism that kills hardy hair-pigment cells and leaves us awash in gray, they can develop new treatments for shutting down more troublesome cells -- like those that cause skin cancer.

Last month, a team of European researchers achieved something of a eureka moment. They had been studying a genetic defect called vitiligo, which results in patches of skin that have no pigment. People with vitiligo (vit-uh-LYE-go) have low activity of catalase, an enzyme that breaks down hydrogen peroxide, resulting in elevated levels of hydrogen peroxide in the skin.

Given that graying hair results from an absence of pigment, it occurred to the scientists that hydrogen peroxide and catalase might play a critical role in the process. Every hair cell makes a little hydrogen peroxide, but over time the amount builds up. The European team discovered that this buildup ended up blocking the normal synthesis of melanin, the natural pigment in hair.

Our hair, it turns out, bleaches itself from the inside out. And by identifying the chemicals involved, researchers may be closer to understanding if the graying is influenced by stress.

''Now it becomes possible to understand whether stress is involved in the process; before this, we didn't know what to look for,'' said Dr. Gerald Weissmann, research professor at the N.Y.U. School of Medicine and editor in chief of The FASEB Journal, which published the study last month. (The initials stand for Federation of American Societies for Experimental Biology.)

But even if scientists find a link between psychological stress and graying, it's quite likely that genetic factors ultimately determine who is susceptible.

''Clearly our genetic makeup comes into the game,'' said the European study's lead author, Dr. Karin U. Schallreuter, professor of clinical and experimental dermatology at the University of Bradford in England. ''Some are more susceptible and some are not. There are people around who still have black hair in the late periods of life.''

The role of hydrogen peroxide is likely to be only a piece of the gray-hair puzzle. What's intriguing about skin and hair cells is that while most of the body's cells are programmed to turn themselves off when exposed to harmful stresses, certain skin and hair cells are much tougher. A few years ago, Harvard researchers reported that graying occurred when the stem cells that govern hair coloring lost their hardiness and shut down. The hope is that research into those cells may ultimately lead to treatments for melanoma, a skin cancer that can be fatal.

One of the many mysteries of gray-hair research is why some people have salt-and-pepper hair. ''If it was purely based on one's antioxidant system or the ability to handle oxidative stress, then you still have to explain why some follicles can produce perfectly pigmented hairs in a sea of white hairs,'' said Desmond J. Tobin, associate dean of research and knowledge transfer at the University of Bradford.

Dr. Tobin notes that the skin is the only major organ that is directly exposed to environmental stress outside the body and the changing environment inside the body.

''Skin and hair follicles are very important as an accessible test tube to look at for aging,'' he said.

Notably, scientists haven't found a link between signs of aging in hair and real aging in the body. A major study of 20,000 men and women in Copenhagen looked for any links between heart-disease mortality and physical signs of aging like gray hair, baldness and facial wrinkles. They found none.

''People with premature graying of the hair don't die any sooner than anybody else,'' said Dr. Leo M. Cooney, professor and chief of geriatrics at Yale University School of Medicine. ''I think the study shows that gray hair has something to with your genetics and very little to do with premature aging.''