At the beginning of Barack Obama's second term, many magazines and newspapers are looking back at how the president aged over the first four years.One time-lapse video, according to Gawker, shows the president "Age 10 Years in 5 Years in 2 Minutes."The Washington Post, meanwhile, shows photos of Obama "Then & Now," with former White House physician Connie Mariano describing presidents as looking like they "fast-forwarded eight years in the span of four years," presumably because of the stress of the job.Can chronic stress really cause early wrinkles and gray hair?It's not clear.While psychological stress can lead to DNA damage associated with aging, it's not clear whether this damage manifests itself visibly, such as in crow's feet around the eyes or a dusting of silver on the crown.That hasn't stopped some news outlets from heralding a link between stress and graying hair. When a 2011 study showed a mechanism through which stress could cause DNA damage, articles in the Daily Mail, Yahoo and elsewhere touted the study as proof that stress can cause visible aging. "Stress Really Does Make Your Hair Go Gray, Scientists Find," proclaimed the headline in the Telegraph.However, the study had little or nothing to do with gray hair, with the words "gray" and "hair" never even appearing. Furthermore, the study used adrenaline, not stress, and it was conducted on mice, not humans.As William Saletan pointed out in Slate in 2009, many other studies have found no relationship between early graying and aging. For example, a study of 20,000 men and women in Copenhagen could find no relationship between deaths from heart disease and outward signs of aging, such as balding, wrinkly skin and gray hair.Instead, most graying seems to be determined by genetics and has little to do with one's health or proximity to the grave. If presidents tend to go gray while in office, it may simply be because most normal graying happens during the same years in which presidents serve in office."Overnight" graying from stress is extremely rare even if it exists at all.Though traumatized fictional characters have had their hair suddenly turn white since at least the days of Shakespeare ("Thy father's beard is turned white with the news," Falstaff tells the Prince in Henry IV, Part 1), the phenomenon, if real, remains unproven.Some supposed historical instances of "Marie Antoinette syndrome" (for women) or "Thomas More syndrome" (for men) appear to be apocryphal, the work of melodramatic historians, while others might be explained simply by the sudden deprivation of wigs or hair dye.When reports of canities subita, as dermatologists call it, surface today, they're usually explained by "alopecia areata," a condition of sudden hair loss, though alopecia areata is not usually linked with stress.Photo selection can also exaggerate the appearance of aging.While Obama's sprinkles of gray started making headlines just 44 days into his presidency, an article this week from the Huffington Post actually complimented the president's complexion. He had an "amazing glow," noted Huffington, appearing "refreshed, rested and really glowy."