No direct scientific evidence suggests that stress causes hair to turn gray, states WebMD. There does seem to be a correlation of prolonged stress and the acceleration of grey hair in some people, but the process of "going gray" is part of a multivariable equation.
A type of cell called melanocytes is responsible for creating melanin, which in turn is responsible for adding pigment to hair. As people age, the ability for the melanocytes to produce hair color pigments begins to deteriorate, and gray hairs appear. Because stress can have numerous effects on the body, including advanced aging, it is speculated that stress is a factor in hair turning grey, but there is no direct evidence, notes the Scientific American.
Huffington Post argues that stressful events can lead a person to eat poorly and possibly suffer various nutritional deficiencies that alter the individual's natural hair color, however, hair graying is largely controlled by genetics. During the first stages of graying, it is possible for gray hairs to become repigmented and regain their original color as they grow out.
The belief that stress contributes to hair graying is commonly based on the evidence that U.S. presidents seem to age rapidly during their four or eight years in office, explains HowStuffWorks. This is likely due to their age, as most gray hair begins appearing between 30 and 35 years old, and many presidents are well into middle age by the time they take office. It is normal for men to turn gray during this phase, and white males tend to gray earlier than black men, notes Edmonds. People usually begin to get gray hair around the same age as their parents did, so that is the best predictor of when a person's hair color will change.