Although age is certainly a factor, most presidents’ hair changes grey after they have been in office for several years. Most have believed it is the stress of the job that makes this happen, but now scientists have some evidence to back that theory up. In an experiment with mice, scientists injected a stress-causing drug into a mouse. Within a few days, the mouse’s hair turned white. The added stress hormones somehow affect the way in which hair cells manufacture pigment. Stress causes the cells to steal the pigment in order to manufacture other enzymes or proteins.