Short hair. The modern men’s cut and the even women’s hair is born from the French Revolution.

Notice how spikey her hair is in the back? It’s because she’s chopped off her hair. Here’s another lady with a similar look:

Men’s hair went from being nice, long, and pulled back like this:

To the more familiar short cut like this:

Almost overnight! The short hair style that was very popular in the early 19th Century was called "à la victime". The "victime" meaning those that were sentenced to die via the Guillotine during the French Revolution. Those that were victims of the Revolution were mostly the nobility who were accustom to wearing wigs.

To wear a wig properly in the 18th C, both ladies and gentlemen shaved their heads to be completely bald. The wig was meant to fit perfectly and sort of be suctioned to the bald head (you can read a bit more here).

Since the nobility all had bald heads and couldn't take their wigs with them in prison, they would have a short crop of hair by the time Madame Guillotine came a-callin' a few weeks later.

To have the "à la victime" look, both men and women in the 1790's-1800's would chop their hair super short. Men's hair never recovered from this fashion. After this, men were encouraged to keep their hair off their collars. Women's hair did recover - but only for about 100 years.

So, the reason men wear their hair short is all due to looking like a noble about to get his head chopped in the late 18th/early 19th century!