When and why did having long hair become associated with women, and short hair with men?

Because of a traditional practice in which a woman became the life-long partner of a man if he should cut and possess a lock of her hair, Igbo girls and unmarried women in southeastern Nigeria kept their hair cut very short (and tended to travel in groups, beating any would-be perpetrators).

Also, some Masai women and, as far as I know, Kikuyu women, of what is now known as Kenya, also tend to keep their hair cut very short.

Unfortunately, now that many African societies are being influenced by American media images, more and more African women seem to consider long hair, even in the form of wigs and weaves, far more feminine than their own relatively short hair, and therefore are beginning to spend a great deal of money on acquiring such aids to "femininity".

According to a Reuters report of August 6, 2014, African women from South Africa, Cameroon and Nigeria were spending a whopping $6 billion per year on the "dry hair" industry, which markets and sells weaves, extensions and wigs.

Add to this the quote from 1 Corinthians 11:14-15, suggesting that, in order to please God, women should have long hair, and men (unlike Maasai warriors and many North American native men) should have short hair, to the mix, and one can see how deeply religious (now Christian), once-colonised people might be additionally influenced.

"Does not the very nature of things teach you that if a man has long hair, it is a disgrace to him, but that if a woman has long hair, it is her glory? For long hair is given to her as a covering." — 1 Corinthians 11:14-15 (NIV)

I guess most women of African descent don't have much in the way of "glory", according to this passage. "The very nature of things" only teaches me that adult male lions have manes, and adult female lions have very short fur all over their bodies. However, a human male and female within the same family will have a similar length of hair on their heads, unless one of them cuts it, and cutting it has nothing to do with nature. That's a human intervention.

As I think we've all realised by now, hair length, for both men and women, is only a cultural construct.

Just a thought but before World War One men generally had longer hair and beards. However, short hair on men has often been enforced as a mean of control, in police, military and other forces that require obedience and discipline. Slaves and defeated armies were often required to shave their heads. There may be some sort of connection there.

As the men in control sporting short haircuts became the norm, society eventually followed.

This is much older than the first world war. Paul wrote in 1 Corinthians 11:14 "Does not even nature itself teach you, that, if a man has long hair, it is a shame unto him?" This shows that at least in the Hebraic world in the Roman period it was expected that men have short hair.

It's obvious that having short or long hair is an identity sign for men and women respectively, more or less worldwide.

No, it's not obvious, especially not in history.

You may be mistaking a Western, Roman Catholic, modern behavior for something universal.

The Romans were a little strange in their belief that men should shave and wear their hair short(ish). They bequeathed this to the Roman Catholic Church, which then constantly tried to impose it on European culture. This was passed down to the Protestant churches, mostly rather puritanical, who felt that long hair on a man was Vanity when he should be working as hard as possible to be ugly and practical. From there it reaches the Victorians, who additionally were obsessed with making women a different creation, and finding or creating as many differences as possible between the two sexes (women, you see, by nature breathe thoracically, while men breathe abdominally -- a difference only found when women wear corsets and men don't, but touted by medicos of the time as part of the basic anatomy of the sexes).

IN NATURE

This is about the level of "natural" in men having shorter hair. Nature, in fact, teaches us that a man and his sister will grow hair about the same length, shaft thickness, and mass. That she averages 4" shorter results in her hair seeming just a trifle longer. But women don't somehow magically grow much longer hair.

What you need to consider in nature is that humans are totally weird in being near-naked over the rest of the body but having this very long mane on top of the head (and men, on their faces).

Elaine Morgan, in The Descent of Woman, points out the theories of other scientists deriving a number of human traits, starting with the diving reflex, to a time when our pre-human ancestors seem to have lived on the beach and considered going marine mammal, as the elephants nearly did, too. This adaptation shows in the streamlining of hair on the body, or its near loss (the adaptation is more advanced in women), while developing the head of hair as tow ropes for infants. Anyone with long hair who has had a little baby clamp on knows how strongly they can grip it, and with a look of pure delight: I think it makes them feel happy and secure.

Male pattern baldness is irrelevant to this. Even as a man is thinning on top, or totally chrome-domed, he can grow the rest quite long enough to tow a child by. (Note: it affects about 70% of men and 40% of women by the time they are old.)

The average head hair grows 1/2" a month, and lives about 3 years, giving a max length of 18". People with knee-length hair grow it faster and their follicles hang in there longer. Until my later 40s, my hair grew 1" a month, but only lived about 2-2.5 years. As a result, I could grow it sittable, but not longer, but it was all very healthy and bouncy, because none of it had much wear and tear. Ill health will always result in nails and hair growing more slowly and the product being more fragile.

In most periods, anyone with hair over their shoulders had it "long." Braids that hung down onto the bosom were notably long hair of the femme fatale in Burnt Njal's Saga. I have met women with knee-length hair, or longer (most men will cut it shorter), but when you see it as a fashion, as in the 12th C, you can bet the braids are fake. Very few people can grow it this long. (This always reminds me of the mid Victorian fashion for piling up so much attached hair that it was larger than the wearer's head.)

Hair length, texture, and mass are all hereditary. People ask me how to get a thick head of long hair, and I tell them, "Choose your ancestors carefully."

More averages: among Europoids, the thickest number of hairs per area is usually found in brunets, followed by redheads, with blonds having the least. That's relative. East Asians tend to have coarse hair, more spaced, that gets thinner with age, so that if they wear it short you can see a lot of scalp. Older Asian women often get nearly as thin as their brothers, which is very visible if they insist on wearing Western-style short hair. The use of equine mane and tail shampoos has become popular in Japan and China because of their hair texture: most people shampoos are aimed at European hair.

Women's hair usually reaches maximum thickness during their first pregnancy. It's shocking how much comes out in the brush after the baby is born. There's another thinning at menopause, from hormonal shifts.

Brunette women with really thick manes are often fighting sideburns and light mustaches, I might point out. It's not the most feminine trait.

It's almost always men who have the half-inch long thick eyelashes, too. What's often marked as desirable is not what's common to the sex, but what's rare.

IN SOCIETIES

We'll take this in all cases as "before Westernization." Also, assume women are wearing hair pretty long unless otherwise noted.

That long beards and hair were somehow dangerous in battle seems to have influenced no one at all. Some of the most notable warrior cultures wore long hair.

In East Asia, both sexes customarily wore long hair in most societies. Male and female Sikhs must wear it uncut, for their religion.

In the basins of the Orinoco and Amazon, it is usual for men and women both to wear short hair. On the Great Plains, the Sioux and the Cheyenne, in both sexes wear hair in long braids.

Steppe tribes, men of the Scythians, Sauromatians, Sarmatians, Sakas, &c, wore long hair, with or without mustaches or beards. (https://en.wikipedia.org/wiki/Scythians#/media/File:Scythian_Warriors.jpg) They often bundled it up at the back of the head. Women were differentiated by matrons wearing tall tarbooshes, one sign by which they should have known that the Golden Prince burial was actually a Golden Princess (weapons and all).

The ancient Egyptians in both sexes cropped the hair very short, and women wore cloth headdresses or wigs, as men might also. (https://en.wikipedia.org/wiki/Wig#/media/File:Egypte_louvre_286_couple.jpg)

The ancient Minoan men wore their hair very long. (https://en.wikipedia.org/wiki/Prince_of_the_Lilies#/media/File:Feather_prince.jpg)

You can always spot a Spartan in art by his long locks (I refuse to see 300 as being obvious modern machismo nonsense) and lack of mustache, though wearing a beard. A Spartan bride cut her hair very short the day of her wedding. We aren't certain they didn't keep it short as a mark of matronhood. Xenophon notes that they felt long hair made a handsome man more beautiful and an ugly one more fearsome. (http://035fe62.netsolhost.com/t.d/1917.jpg) Archaic Greek statues show men with very long hair.

Other Greeks are usually shown in Classical art as having shortish hair and a beard. Slaves, on the other hand, were required to have their heads shaved regularly, and their beards at the same time. It was unmanning them. Also, you knew anyone shaven was a slave, and not a free man. Slave women usually had shortish hair.

Persians and Medes also wore collar-length hair and beards, but are shown with them very carefully curled. (https://en.wikipedia.org/wiki/Darius_I#/media/File:Darius_In_Parse.JPG) This is pretty much the Mesopotamian fashion after the Sumerians. Thing is, the ladies appear to have worn their hair the same. (https://en.wikipedia.org/wiki/Achaemenid_Empire#/media/File:Iranian_queen.jpg)

As it happens, I was just reading The Anglo-Saxon Home: A History of the Domestic Institutions and Customs of England from the Fifth to the Eleventh Century by John Thrupp.

"Long and flowing hair was, at first, evidence that the wearer was a noble, and always, that he possessed, unforfeited and unimpeached, all the rights of an Anglo-Saxon freeman. It conferred dignity on the wearer; and the highest and most illustrious were proud of it. It was the distinction in which the Merovingian kings of France most gloried; and Harold Fair-hair and Cnut the Great considered that the length and beauty of their hair added to their lawful claims to popular admiration."

Shaven heads and short growth were both marks of the slave, the criminal, and the outlaw.

In the early period, Saxon maidens wore their hair long, and cut it short on their wedding day, and kept it that way. Later, it was worn trimmed and bound up on marriage.

Long hair comes and goes as a European male fashion throughout the Middle Ages and Renaissance.

From 1624, Louis XIII supported the use of the peruke or male wig, worn very long, because his hair no longer looked so hot worn naturally this way, as he was greying and balding. This led to an exaggeration of length and curliness and mass in male hairstyles under his son, Louis XIV. Charles II brought this style to England, as the periwig. So while men actually had their hair quite short, wearing a cap to bed or for lounging, the fashion or look was that of a huge mane not equaled even in the 1980s hair bands. (https://en.wikipedia.org/wiki/Hairstyle#/media/File:George_I_Elector_of_Hanover.jpg) (https://en.wikipedia.org/wiki/Wig#/media/File:Ex-voto_a_sainte-genevieve_-Detail-Largilliere.jpg)

In the 1700s, powdering the wig became popular. Then men began wearing their own hair long in wig-like styles, powdered or not, which resulted in much more realistic-looking styles we know from the American Revolution. In the 1790s both wigs and powder were out of fashion. Benjamin Franklin is usually shown with long hair loose. (https://en.wikipedia.org/wiki/Benjamin_Franklin#/media/File:BenFranklinDuplessis.jpg)

In the 1790s, men began to sometimes choose short hair, but so did women, in the fashions of the Directoire.

In the early 1800s, Beau Brummell wore his hair quite long, so we know it was fashionable, though shorter hair was coming in. (https://en.wikipedia.org/wiki/Beau_Brummell#/media/File:BrummellEngrvFrmMiniature.jpg)

However, Brummell himself converted to collar-length hair, to accomodate the proper crisp cravat and curved collar he came to favor. After this, while some men wear long hair (especially where barbers may be few and far between), it is not fashionable again in the West until the late 1960s.

(Then just add Semaphore's great answer)

Do note that by the 20th C women are cutting their hair short, so that the "boyish bob" again put the genders about equal on hair length. You can call the 20th C "The Age of the Beauty Shop," when women had to go in monthly or so to get their hair cut again (and permed/dyed/relaxed) because short hair was now the norm for women. Women who wore it long (shoulder-length) and loose were considered either too youthful or too sexy: it really wasn't "proper" for clubwomen and matrons.

SOURCES

The Costumer's Manifesto is always a good quick check for In/Out for an era (http://www.thecostumersmanifesto.com/index.php/Costume_History)

Boucher, 20,000 Years of Fashion (good for very early dress -- like, Neolithic)

Gernsheim, Victorian and Edwardian Fashion: A Photographic Survey

Gorsline, What People Wore: A Visual History of Dress from Ancient Times to 20th Century America (notable for covering American West)

Hill & Bucknell, * The Evolution of Fashion: Pattern and Cut from 1066 to 1930*

Houston & Hornblower, Ancient Egyptian, Assyrian, and Persian Costumes and Decorations

Houston, Medieval Costume in England and France, The 13th, 14th and 15th Centuries

Morgan, The Descent of Woman

Price, Dame fashion: Paris - London, 1786-1912

Thrupp, The Anglo-Saxon Home: A History of the Domestic Institutions and Customs of England from the Fifth to the Eleventh Century

Waugh, The Cut of Men's Clothes 1600-1900

The First World War is often identified as a turning point in men's hair length.

Prior to the war, both men and women commonly kept long hair, at least in western societies (and the Far East). This became problematic during the Great War, where armies encountered severe hygiene issues fighting in the trenches. Under the unsanitary conditions of the front, soldiers adopted short hair to mitigate the scourge of lice or fleas.

[T]he men who fought in that war were numerous (65 million) and the 56 million survivors lived a further half century and had an overwhelming influence on those younger ... Men for the next half century had to measure themselves against the war veterans, to try to give themselves and others the impression that they too could have endured the trenches. That became the measure of manhood. And so the style of the veteran of the trenches - short hair, suntan, dangling cigarette, the taciturn, unfeeling, nonchalant post which does not flinch when someone a few yards away gets shot - becomes the male style of the twentieth century.

_{- Antony, Michael. The Masculine Century: A Heretical History of Our Time. iUniverse, 2008.}

Due to the prestige attached to the veterans, their characteristic hair cut was retained and emulated as the armies demobilised. This was reinforced by the adoption of crew cuts for servicemen during WWII.

Obviously there's a great deal of generalisation going on when we speak in such terms as the hair length of entire societies. Some men, even today, kept their hair long. Meanwhile, many women also cut their hair short, especially during the inter-war years. The bob cut for instance became popularised after 1920.

This wasn't the first time people cut their hair short, though. As the Industrial Revolution progressed in the 18th century, short hair became attractive to those working with machinery for safety reasons (many women likewise cut their hair short for the same reason when they entered the war factories during the two World Wars).

The long, wavy or curly hair portrayed in early centuries thus became a rarity during the 19th century. Nonetheless long hair was not out of the ordinary, and middle to upper class men typical hair lengths remain much longer than would be the norm after the World Wars. Some examples of men with longer hair during the 19th century include:

^{From left to right: 1. British Jurist Charles Hay Cameron. 2. German composer Johannes Brahms. 3. American ornithologist, John James Audubon. 4. Italian violinist Niccolò Paganini. 5. American writer Edgar Allan Poe when young. 6. Anglo-Irish poet Oscar Wilde. 7. Confederate General A. P. Hill. 8. Hungarian composer Franz Liszt. 9. British novelist Charles Dickens in youth. 10. French scientist Henri Victor Regnault. 11. American officer George Armstrong Custer. 12. French composer Erik Satie before going bald.}

In fact, typical hair lengths has been very variable throughout history. What you call "obvious identity signs" are not constant and has never been universally true in history. Although WW1 and WW2 may be said to have kicked off the current fashion trend, it should not be considered either permanent or unprecedented.