A hundred years ago, men didn't leave the house without a hat. Example: the image above, a rally in Union Square in NYC, full of people, full of hats.
One hundred years later: Union Square in NYC, full of people, very few hats.
What happened? NPR's science correspondant Robert Krulwich explores what happened in the century between, when men stopped wearing hats in the mid-twentieth century. He muses on the import of John F. Kennedy, who had great hair, the Beatles, who had great hair, but ultimately decides to side with his father:
"But I am the son of a hat designer. And my father, Allen S. Krulwich, had a different explanation. The president who de-hatted America, he thought, was Dwight Eisenhower."
It's a fascinating discussion, and well worth the read. (Hint: there are cartoons!)