For about 15 years I have been teaching the conventional wisdom of evolutionary psychology that hair is a primary signal of youth and health in women (cues of youth and health being equated with female beauty).
So that thick and lustrous hair was, under human ancestral conditions, an un-fake-able signal of youth and health - such that only healthy young women could possess it.
In other words, significant systemic disease or age would make hair less thick and/ or duller.
The result was that human men evolved to find thick lustrous hair attractive in women.
Of course, modern technology has made it possible for older and unhealthy women to mimic - more or less accurately - the thick lustrous hair of youth by using hair dye, hair conditioners, and even hair extensions and wigs. Nonetheless, although these may be convincing from a distance, from close range the special qualities of youthful and healthy hair are unmistakable.
So, from this evolutionary psychology perspective, and assuming that women are wanting to make themselves more attractive to men, it would make sense for older and less healthy women to dye their hair as carefully as possible such that it simulates the appearance of the hair of healthy young women.
But it would not make any sense for young, healthy women to dye their hair, since this would make it less attractive.
And indeed, twenty years ago sensible young women virtually never dyed their hair - except when they wanted to signal their membership of some 'tribe' (such as punks or goths, or being 'arty' ) and this was 1. the less sensible women, and 2. a distinct minority. A proportion of young women also signalled their sexual availability by bleaching their hair blond - although this made their hair very obviously artificial and less attractive.
This habit was first broken by the improvements in the technology for making hair blond. A very high proportion of young women with brown (aka. brunette, 'mousy') hair then 'went blond - around here it was about a quarter to one third of all young women who became artificially blond. Nonetheless, even about ten years ago many young women had natural, uncoloured hair.
But now essentially all young women have dyed hair.
The most striking aspect of universally dyed hair is the naturally brown-haired young woman who has her hair dyed brown.
Essentially, the hair is re-dyed almost exactly the same colour it was to begin-with.
So that her hair ends up the same as it was before, only less attractive - because it has been dyed.
Now, an 18 year old with naturally lustrous hair ends up with hair which looks the same as a thirty five year old with dyed hair.
What is going on?
From a perspective of evolutionary psychology I believe that this implies that modern women are clearly not primarily aiming to be attractive to men (or why else would they go to such effort and expense to make their hair - a major cue of their youth and health - less attractive?).
In a nutshell the reason is fashion. But what a strange thing this thing 'fashion' turns-out to be!
Women are dying their hair the same colour it was before because dying your hair is currently fashionable, and women do not want to go against fashion because... well, nothing really, except that it is apparently a fundamental drive among women (much more than it is among men) to conform to the perceived peer group behaviour of other women.
It is the 'perceived' peer group which is crucial, not necessarily the actual current behaviour of women in the real world: fashion is a product of the mass media which provides a powerful, pervasive, high status and seemingly-authoritative signal that 'fools' evolved human psychology into believing that what is depicted in the mass media is reality (in fact more real than actual reality) - and therefore what the peer group want.
(This point concerning the 'reality' of the mass media - especially to women - is so important, and so neglected, that it can scarcely be over-stated.)
A relatively very small number of women - and the (mostly homosexual) men, who substantially dominate the fashion industry - actually 'decide' what nearly all women do, by means of their large influence over the content of the mass media.
If the mass media depicts and endorses - explicitly or (even more effectively) implicitly that it is first 'OK' then 'in' to do something: then that thing will be done, by almost all women eventually; almost without regard for the real-world consequences, because the actual real world is perceived as less real than the mass media world.
And there are no obvious limits to what the peer group of other women can make individual women do to themselves: historical examples include genital mutilation and foot-binding; modern examples include tattooing and piercing - dying hair the same colour being a relatively mild example.
What differs about the modern situation is that fashions change so quickly and frequently that their absurdity is blatant. (One can imagine that in the past, when 'fashions' changed so slowly and infrequently that they became characteristic of cultures, that it would not be obvious what was an arbitrary fashion and what was simply intrinsic to human behaviour. Indeed, fashion actually is an aspect of culture: i.e. culture which changes rapidly, within a lifetime.)
hence the blatantly arbitrary and ugliness-producing effects of fashion make no difference to the willingness of women to inflict damage on their attractiveness - the prime evolutionary directive is apparently to observe and learn fashion/ culture - and do it ASAP.
So brown hair dyed an almost identical brown in young women is a small but prime exhibit in the case for the fundamental psychosis of modernity - that modern humans inhabit a society where strong, basic, evolved behaviours are subverted and over-turned, even where this behaviour causes immediate and obvious damage to an individual's probable reproductive success.