Whenever you look at photographs from the 1920s and before, it is common to find that all of the people in the photo are wearing some kind of hat. However, nowadays, wearing hats are quite uncommon, or even decried. What happened to make this the case?
I saw one interesting and somewhat plausible answer to this question on NPR from a person who is the "son of a hat designer". Short answer: the automobile.
Until cars became the dominant mode of personal transport, there was no architectural reason to take your hat off between home and office. With Dwight Eisenhower's interstate highway system came cars, and cars made hats inconvenient, and for the first time men, crunched by the low ceilings in their automobiles, experimented with hat-removal, and got to like it.
In other words, before automobile travel became ubiquitous around the middle of the 20th century, there was no place in public life where you actually needed to take your hat off. Your hat went from your hook by the front door, to your pomaded pate, to the hat tree in your office without any compelling cause for removal in the interval. Plenty of room in a bus or commuter train to remain hatted for the length of the ride. You'd remove your hat in church, or when visiting someone's house. But the advent of the car meant that every day, twice a day, you needed to remove your hat and set it somewhere where it wouldn't interfere with the steering wheel and the stick shift... a slight inconvenience which, multiplied x400 each year, slowly eroded the custom.
I feel like this is a partial and simplistic answer, and a fuller answer would have to take into account the evolving cultures and fashions, religious beliefs, living conditions etc. that led people to feel they needed to wear hats, and then, later on, to feel that they didn't.
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What role does the decline of religion have for the prevalence of wearing hats?
Where does the rise of regular hair washing during the past 60 years fit into this equation?