A: This is a tough question to answer because the topic has not been well-studied. Two studies of male and female identical twins did find a link between wearing hats and increased hair loss for the men, but not the women. In another study, wearing hard hats or military head wear had no effect on hair loss.
However, we think it’s possible that wearing hats that are very tight — or very hot — could decrease blood flow to the hair follicles, stressing them and causing them to fall out. The hair loss would be temporary at first, but it could progress to permanent hair loss over time. So a loosely fitting fedora may be a safer bet than a tight knit cap or baseball hat.
—Dermatologist John Anthony, MD.