Up to the 1930s people used to make it very easy to guess how they voted. If you went outside you would stick a piece of material on your head, which would let everyone else know which social class you belonged to and, therefore, almost certainly how you voted. If you look at any photograph of a crowd of British people between the wars, regardless of age or affluence, they will be wearing a hat. The same was true internationally.
The reasons for the decline in popularity of hats are many and varied. Limited headroom in cars, Brylcreem, wartime shortages of material, the desire to get away from military uniforms, delicate hairstyles, rejection of conformity and the influence of role models all played a part, although the full reasons seem to be under-researched. Among the role models considered to be particularly influential in bareheadedness was President Kennedy. However, he did wear a hat to his inauguration, contrary to popular belief.
The trend to bareheadedness has its origins much earlier. The peak year for hat manufacturing in the US was 1903, but by then there was already a growing trend for hatlessness. Newspaper reports from New Zealand in 1905 refer to the ‘hatless craze’.
Today, if you look at a photograph of a crowd of British voters, it would be very difficult to distinguish a Liberal Democrat from a Conservative or a Labour voter on the basis of their appearance. Opinion polling started as hat-wearing died out. The pollsters have had to replace the milliners.