In science's relentless effort to prove that we're all just extras in a sprawling Quest for Fire remake produced by aliens, anthropologists would like to remind us that men prefer women with blonde hair because blonde hair is rare and that's why cavemen liked blonde hair, which is how blonde hair became more of a thing in northern and eastern Europe 10,000 to 15,000 years ago, way before any of the other animals even knew what was going to happen.
According to Peter Frost, an anthropologist at Laval University in Quebec City, blonde hair had a novel, eye-catching appeal for our cavehuman progenitors because most pretty much everyone back in the day had dark hair, and then, when blonde hair showed up, everyone was all like, "Whoa, wtf?" Which is how people spoke to each other in prehistoric times. Anyway, Frost explained to Discovery News that the sudden appearance of blondes in the human population helped women compete for mates among a limited pool of suitably virile cavemen, and the continued contemporary appeal of blonde hair is rooted in the simple fact that it's rarer than dark hair:
The more common a hair color becomes, the less often it is preferred. It's a kind of novelty effect. The moment you become ordinary, you no longer have the same appeal. There's selection for being a bit different and eye-catching.
Frost proposes the following explanation for the emergence and propagation of blonde-haired women: During the Ice Age, men had to travel longer distance to find animals with just that right mix of edibility and vulnerability. This mean that more men died trying to make their more arduous hunting excursions, leaving women with a much smaller pool of grunting, inarticulate bachelors to choose from. Polygamy, as a result, fell out of vogue because it was harder to support large families with such limited resources. In order, then, to attract male attention, women had to bring their game to the next level, which, in terms of evolution, meant evolving octopus limbs or being able to fly, anything, really, to catch a potential mate's attention. Since there are no flying people around today, we can all assume blonde hair was evolution's eye-catching innovation.
Blonde hair is still a thing today, it seems, for a lot of the same reasons, namely, Frost insinuates, that women want to attract male attention by having conspicuously-colored hair (this is why, for instance, blonde hair dye is really popular with women in Latin America, because natural blonde hair in that part of the world is a rarity). Conversely, in Nordic countries, where there are more natural blondes, women want to darken their hair. There's also this sort of creepy reason that men like blonde hair: blonde hair is more prevalent in younger people (and sticks longer in girls than boys), and the desire to go blonde might represent an unconscious desire to seem perpetually fecund and youthful.
Jena Pincott, author of the 2008 book Do Gentlemen Really Prefer Blondes?, unearthed many of these same anthropological goodies from the primordial muck that subsumes our ancestors, drawing basically the same conclusion — blonde hair is eye-catching in places where blonde hair is rare and dark hair is eye-catching in places where dark hair is rare. The moral of this scientific interlude, everyone is that people are just trying really hard to do it with each other, but they don't want doing it to seem trite — they want it to be special, like that time they saw a Wes Anderson movie before any of their friends knew who he was.