Everyone wore a hat and then, quite suddenly, nobody did

A garment with several jobs at once

For most of recorded history a hat did three things simultaneously, and it is worth separating them. It protected the head from sun, rain and cold, which is a straightforward functional requirement. It signalled social position, trade and sometimes religion with considerable precision. And it participated in a dense grammar of etiquette about when it came off and to whom.

That third function is the one that has disappeared most completely. Removing a hat indoors, raising it in greeting and taking it off in specific circumstances were rules people learned as children and applied without thinking, and the rules were sufficiently detailed that getting them wrong was noticeable.

The first function never went away. People still wear hats in strong sun and hard cold. What collapsed was the default of wearing one in ordinary conditions simply because one was dressed, and that default had been strong enough that going out bare-headed was itself a statement rather than a neutral choice.

The scale of the change

Photographs of urban crowds from the earlier twentieth century show near-universal headwear on adults in public. Photographs of comparable crowds a few decades later show almost none. Whatever caused it, the change was fast by the standards of clothing convention, and it happened across many countries at broadly similar times.

It also took an entire manufacturing sector with it. Hat making was a substantial industry with its own trades, materials and towns, and the collapse of everyday demand left a much smaller specialist business behind.

One useful caution: the change was less complete than it looks. Headwear persisted strongly in religious contexts, in specific occupations, in cold and hot climates, and among women in some settings long after men had stopped. The universal narrative is a simplification of an uneven process.

The explanations, and how well they hold up

The most repeated explanation is the motor car, on the grounds that a hat is awkward under a low roof. It is plausible, the timing is suggestive in some countries, and it does not explain why the change also occurred among people who walked or took public transport.

A second explanation points to wartime service, arguing that men who spent years in issued headwear were happy to abandon it afterwards. Also plausible, also not obviously sufficient, since earlier conflicts had not produced the same effect.

A third points to the general and continuing decline of formality in dress, of which the hat is simply one casualty among many. This has the advantage of fitting a much broader pattern, and the disadvantage of explaining everything and therefore not much. The honest position is that the cause is genuinely uncertain and probably plural.

What the hat was doing that nothing replaced

A hat completed an outfit in a structural sense. It provided a top to the silhouette, and older clothing was proportioned with that in mind, which is part of why period tailoring can look slightly unfinished when photographed without headwear.

It also carried an enormous amount of legible information cheaply. The shape and material of a hat announced trade, income and formality at a distance, and there is no obvious modern equivalent doing that job with the same efficiency.

And it gave people something to do with their hands and a physical vocabulary for greeting, deference and acknowledgement. Etiquette systems tend to be more useful than they look from outside, and losing the object removed the gestures with it. Nothing was designed to take over those jobs, which is the ordinary way conventions end: not replaced, simply dropped.

What survived, and why

The headwear that remains in general use is almost entirely functional or affiliative. Warm hats in cold weather, sun hats in strong sun, safety helmets where they are required, and caps carrying a badge of some kind persist because they are doing a job that has not gone away.

The formal hat survives in ceremonial settings and in a handful of professions and observances, which is the usual fate of a garment that loses its everyday role: it retreats into occasions where the point is precisely that things are done as they used to be.

There is a periodic revival of everyday hat wearing, and it has never restored the default. That is probably the more interesting fact. Clothing conventions can be abandoned very quickly, and once the shared understanding behind one has dissolved, an individual wearing the garment is making a statement rather than following a rule.